Direct interaction of FtsZ and MreB is required for septum synthesis and cell division in Escherichia coli

Direct interaction of FtsZ and MreB is requiredfor septum synthesis and cell division inEscherichia coli

Andrew K Fenton and Kenn Gerdes*

Centre for Bacterial Cell Biology, Institute for Cell and MolecularBiosciences, Baddiley-Clark Building, Medical School, NewcastleUniversity, Newcastle upon Tyne, UK

How bacteria coordinate cell growth with division is not

well understood. Bacterial cell elongation is controlled by

actin–MreB while cell division is governed by tubulin–

FtsZ. A ring-like structure containing FtsZ (the Z ring) at

mid-cell attracts other cell division proteins to form

the divisome, an essential protein assembly required for

septum synthesis and cell separation. The Z ring exists at

mid-cell during a major part of the cell cycle without

contracting. Here, we show that MreB and FtsZ of

Escherichia coli interact directly and that this interaction

is required for Z ring contraction. We further show that

the MreB–FtsZ interaction is required for transfer of cell-

wall biosynthetic enzymes from the lateral to the mature

divisome, allowing cells to synthesise the septum.

Our observations show that bacterial cell division is

coupled to cell elongation via a direct and essential

interaction between FtsZ and MreB.

The EMBO Journal (2013) 32, 1953–1965. doi:10.1038/

emboj.2013.129; Published online 11 June 2013Subject Categories: cell & tissue architecture; microbiology &pathogensKeywords: cell division; cell elongation; FtsZ; MreB; septum

Introduction

Actin and tubulin homologues are essential for the coordina-

tion of internal processes in all cells. These common and

conserved proteins form structural elements that make up the

cytoskeleton, a dynamic protein network that orchestrates

many cellular processes, including protein recruitment and

trafficking, cell motility and cell division in both eukaryotic

and prokaryotic organisms (Erickson, 2007; Fletcher and

Mullins, 2010). To gain a deeper insight into the cellular

functions of cytoskeletons, it is important to understand

how activities of both actin and tubulin families of proteins

are coordinated. In eukaryotes, actin is regulated by tubulin

microtubules that carry protein factors able to alter

actin filament activities (Basu and Chang, 2007), while in

prokaryotes such interactions through additional protein

factors have not been described. The concept of a

prokaryotic cytoskeleton arose from the discovery of two

essential proteins: FtsZ and MreB, which are homologues of

tubulin and actin capable of forming macro-molecular

structures (Lutkenhaus et al, 1980; Wachi et al, 1987;

Bork et al, 1992; De Boer et al, 1992). FtsZ and MreB are

essential for cell division and shape maintenance respectively

(Bi and Lutkenhaus, 1991; Jones et al, 2001). After a decade

of research on MreB and over two on FtsZ in many bacterial

species, it is clear that a general theme of organising cell

morphology via an essential underlying cytoskeletal structure

is an approach taken by most bacterial cells (Michie and

Lowe, 2006; Typas et al, 2012). However, it is uncertain how

the cytoskeleton interacts with the different components of

the cell to carry out this function.

MreB is widely conserved among rod-shaped bacteria and

interacts with the cell-wall elongation machinery through a

membrane-spanning complex comprising of MreC, MreD

and RodZ (Divakaruni et al, 2005; Kruse et al, 2005; Shiomi

et al, 2008; Bendezu et al, 2009; White et al, 2010). This

complex acts together with the peptidoglycan (PG) cell-wall

biosynthetic enzymes to produce lateral PG, thereby giving

bacterial rods their characteristic shapes. Pioneering work

suggested that MreB formed a continuous helical pattern on

the inside face of the cytoplasmic membrane (Jones et al,

2001). However, higher-resolution microscopy has indicated

that MreB forms short filamentous structures in vivo

(Domınguez-Escobar et al, 2011; Garner et al, 2011;

Van Teeffelen et al, 2011). Biochemical characterisation of

MreB has been difficult; although this family of proteins

can form filaments in vitro, their dynamics are far from

fully understood. However, solved crystal structures have

provided insights into the biochemical properties of MreB

including its establishment as a true actin homologue and

defining the MreB–RodZ interaction surface (Van den Ent

et al, 2001, 2010). MreB proteins from different species are

known to directly interact with the cytoplasmic membrane,

though the exact mechanism of binding varies between

species (Salje et al, 2011). In E. coli, MreB has an

N-terminal amphipathic helix that directly anchors MreB

filaments to the inner leaflet of the cytoplasmic membrane

(Salje et al, 2011).

FtsZ polymers, in conjunction with FtsA, ZapA, ZapB and

other condensation factors, form a ring-like macro-molecular

complex at mid-cell called the Z ring (De Boer, 2010).

The regulated assembly of this structure is responsible for

the spatial and temporal coordination of bacterial cell

division. Recruitment of additional cell division factors to

the Z ring occurs in an ordered mostly linear pattern, each

protein requiring the localisation of upstream factors for its

localisation forming a mature divisome complex (De Boer,

2010). Interestingly, the Z ring forms early in the cell cycle

and persists without contracting until late in the cell

cycle, indicating that maturation of the divisome occurs in

*Corresponding author. Centre for Bacterial Cell Biology, Institute forCell and Molecular Biosciences, Baddiley-Clark Building, MedicalSchool, Newcastle University, Richardson Road, Newcastleupon Tyne NE2 4AX, UK. Tel.: þ44 (0)191 2225318;Fax: þ44 (0)191 2227424; E-mail: [email protected]

Received: 15 February 2013; accepted: 10 May 2013; publishedonline: 11 June 2013

The EMBO Journal (2013) 32, 1953–1965

www.embojournal.org

EMBO

THE

EMBOJOURNAL

THE

EMBOJOURNAL

1953&2013 European Molecular Biology Organization The EMBO Journal VOL 32 | NO 13 | 2013

http://dx.doi.org/10.1038/emboj.2013.129

http://dx.doi.org/10.1038/emboj.2013.129

mailto:[email protected]

http://www.embojournal.org

two steps (Aarsman et al, 2005); the first being Z ring

establishment, the second, downstream factor recruitment

and Z ring contraction. The signal or events that trigger Z ring

contraction are unknown.

When the divisome is complete, it provides the bio-

chemical and mechanical activities required for septal

PG synthesis, septation and separation (De Boer, 2010). In

addition to cell division, the Z ring has an emerging

secondary role in pre-septal PG synthesis, prior to synthesis

of the septum (De Pedro et al, 1997; Aaron et al, 2007; Varma

et al, 2007). The enzymatic requirements for pre-septal PG

synthesis are not fully understood, although this process

requires ZipA and seems to depend on the activity of

penicillin-binding protein (PBP)2 rather than PBP3

(Wientjes and Nanninga, 1989; Varma and Young, 2009;

Potluri et al, 2012).

The mreB gene is essential in E. coli under conditions that

support rapid cell growth; however, it is possible to physio-

logically suppress the requirement for mreB by overexpres-

sion of FtsZ (Kruse et al, 2005; Bendezu and de Boer, 2008).

Cells conditionally suppressed in this way grow as irregular

spheres as they have lost the mechanism ensuring lateral PG

incorporation (Kruse et al, 2005). The mechanism for this

suppression is unclear, although it has been suggested that

overexpression of FtsZ allows formation of Z rings, which

will physically reach around the diameter of spherical cells

(Kruse et al, 2005). Alternatively, additional FtsZ could help

overcome membrane perturbation events brought about by

uncoupling of membrane biosynthesis rates with cell volume

in mreB mutants (Bendezu and De Boer, 2008).

Several indirect observations have raised the possibility

that MreB plays a role in cell division. On a morphological

level, compromising MreB function in bacterial cells

gives both cell elongation and division phenotypes (Wachi

and Matsuhashi, 1989; Fenton et al, 2010). The most direct

evidence to suggest an involvement of MreB in division

comes from Immuno-Fluorescence Microscopy (IFM)

studies of Caulobacter crescentus, indicating that MreB

forms ring-like structures at mid-cell that colocalise with

the Z rings (Figge et al, 2004). In E. coli, MreB also

localises as bands or rings, often positioned at mid-cell in

pre-divisional cells (Vats and Rothfield, 2007; Vats et al,

2009). However, independent studies using IFM and GFP-

labelled MreB did not observe ring-like MreB structures

(Karczmarek et al, 2007; Swulius and Jensen, 2012).

Moreover, different fusions between MreB and fluorescent

proteins yield different subcellular patterns (Swulius and

Jensen, 2012). Thus, the localisation pattern of MreB

relative to FtsZ has not been firmly established in E. coli.

Here, we report a novel role of MreB in bacterial cell

division using E. coli as the model organism. Microscopic

observations validate previous suggestions that MreB is

recruited to mid-cell and we comprehensively describe

MreB dynamics in living cells. We show that MreB is

recruited to the septum in virtually all cells via a direct

interaction with FtsZ. We identify a mutation in MreB that

removes the interaction with FtsZ and simultaneously blocks

cell division. Remarkably, a single amino-acid (aa) change in

FtsZ simultaneously restores the interaction with and

suppresses the division defect of the MreB variant. Using

fluorescently tagged cell-wall biosynthetic enzymes, we dis-

covered that inhibition of cell division was correlated with

the lack of recruitment of PBPs 1B and 2 to the Z ring. Our

data support a model in which MreB delivers PBP1B and 2,

and perhaps additional factors to the Z ring, thereby generat-

ing a link between cell elongation and division in bacteria.

Results

MreB is recruited to the Z ring

To study MreB protein dynamics, we generated a functional

mYpet–MreB fusion protein (Supplementary Materials and

methods) and expressed it in wild-type (wt) E. coli MG1655

cells at a level that did not affect growth rate or cell morphol-

ogy (Supplementary Figure S1A). These cells had B6% of

the total MreB pool labelled with mYpet (Supplementary

Figure S1B). Addition of mYpet–MreB in this way had no

impact on wt MreB protein levels and was therefore consid-

ered a phenotypically neutral cytoskeleton-labelling method

(Supplementary Figure S1B).

Our mYpet-labelling method revealed that MreB formed

ring-like structures in addition to the punctate pattern present

along the cell periphery (Figure 1A). The ring-like patterns

only appeared at mid-cell in cells undergoing division. MreB

structures at mid-cell colocalised with Z rings labelled with

an FtsZ–mCherry fusion protein (Figure 1B). These MreB

bands were present at all stages of cell invagination and were

never observed independently of Z rings, raising the possibi-

lity that Z rings recruit MreB (see Movie in Supplementary

Figure S9). Examining and scoring this colocalisation

revealed that 75% of Z rings had overlapping MreB bands

(449 Z rings scored, n¼ 11).

To observe the localisation pattern of unlabelled native

MreB and compare it to the mYpet–MreB observations we

used IFM. E. coli cells were chemically fixed and polyclonal

anti-MreB antibodies used to detect localisation patterns (see

Supplementary Figure S1E for western blot). IFM revealed a

very similar punctuated MreB localisation pattern along

the cell periphery interrupted by MreB bands at mid-cell

(Supplementary Figure S1C). Ring-like IFM signals co-

localised with 68% of FtsZ–mCherry-labelled Z rings

(Supplementary Figure S1D) and are thus consistent with

the mYpet–MreB labelling (117 Z rings scored, n¼ 3).

We also studied a functional fluorescent MreB sandwich

fusion encoded by mreB–rfpSW, which had been successfully

introduced into the E. coli chromosome by allelic replacement

in strain FB76 (Bendezu et al, 2009). MreB–RFPsw showed

localisation patterns at mid-cell very similar to those de-

scribed above (Supplementary Figures S1F and G).

Using temperature-sensitive mutant strains revealed that

mYpet–MreB could be recruited to Z rings consisting of only

FtsZ and other essential condensation factors, without the

additional downstream septation factors such as FtsA

(Supplementary Figure S2). These findings were consistent

with previous work; however, they did not reveal anything

about interaction dynamics (Vats and Rothfield, 2007).

Time-lapse fluorescent microscopy showing mYpet–MreB

behaviour revealed that MreB was rapidly recruited to newly

formed Z rings (within 2.5 min) and frequently remained

colocalised with Z rings throughout the entire cell division

period (see Movie in Supplementary Figure S9, stills from the

time lapse are shown in Figure 1C). Additionally in cells

showing no previous Z-ring–MreB colocalisation, MreB was

recruited to Z rings at the time of ring establishment

MreB–FtsZ interaction is essential in E. coliAK Fenton and K Gerdes

1954 The EMBO Journal VOL 32 | NO 13 | 2013 &2013 European Molecular Biology Organization

(Supplementary Figure S10A). Spontaneous Z ring relocation

events also led to MreB signal migration to the new division

site (Supplementary Figure S10B). Finally, MreB bands were

seen to disperse away from established Z rings in a minority

of cells (Supplementary Figure S10C). Imaged in this way,

virtually all newly formed Z rings showed initial MreB

colocalisation; colocalisation either persisted through cell

division or in a minority of cases dispersed away from

the closing septum.

MreB interacts with FtsZ

Using a Bacterial Two-Hybrid (BTH) approach (Karimova

et al, 2005), we screened a series of known cell division

factors for interaction with two full-length MreB fusions

(T25–MreB and T18–MreB). This included all fusions

reported by Karimova et al (2005) and full-length FtsZ

labelled at both termini (Figure 2A). Initially, this approach

revealed a cryptic pattern of tagged MreB protein interaction

signals, some of which were strong (Figure 2A). The inter-

action pattern varied considerably between the two fusion

series (compare panels (i) and (ii) in Figure 2A), raising the

possibly that some of the signals were artefacts of the reporter

technique.

The BTH assay is known to be particularly sensitive

to detecting interactions between membrane-associated

proteins (Karimova et al, 2005). MreB of E. coli interacts

with the cell membrane via its N-terminal amphipathic helix.

Deletion of this helix effectively abolished the ability of MreB

to bind to the membrane (Salje et al, 2011). Therefore, we

repeated the interaction study with MreB variants devoid of

the N-terminal amphipathic helix (Figure 2A, MreBDN panels

of (i) and (ii)). A striking and similar effect was observed for

both interaction series; only the single interaction signals

between the soluble MreB and FtsZ were present in both

cases (Figure 2A). This result was consistent with previous

yeast two-hybrid studies suggesting a direct MreB–FtsZ inter-

action (Tan et al, 2011).

Using tap tagging, previous large-scale protein-interaction

studies in E. coli identified potential interaction partners for

FtsZ and MreB. Although other protein targets were found,

these studies identified a putative MreB–FtsZ interaction

(Butland et al, 2005; Hu et al, 2009). Here, we used in vivo

crosslinking to validate the MreB–FtsZ interaction suggested

by our BTH analysis. Wild-type cells expressing N-terminally

His-tagged MreB were treated with formaldehyde, a

chemical crosslinker with a very short spacer arm (2.3–

2.7 A) (Ishikawa et al, 2006). Crosslinked complexes were

purified, dissociated and subjected to western blot analysis

(Figure 2D). This revealed that His–MreB was able to pull out

native FtsZ from wt cells, consistent with previous studies.

Thus, two independent methods support that MreB and FtsZ

interact.

MreBD285Adoes not interact with FtsZ and blocks cell

division

By alignment of diverse MreB sequences, we identified a

number of conserved residues located outside the five ele-

ments that form the canonical actin nucleotide-binding

pocket (Bork et al, 1992) and changed them to alanine.

These MreB variants were tested using the BTH assay. As

seen from Figure 2B, changing the aspartate at position 285 to

alanine severely reduced the interaction signal with FtsZ,

whereas the interaction signals with wt MreB, MreC and

RodZ were unchanged (Figure 2B and C). Using both

MreBD285A fusions in the BTH assay showed that self-binding

was maintained (Supplementary Figure S3A). Mapping of

A Brightfield

mYpet-MreB

B mYpet-MreB FtsZ-mCherry Merge Brightfield

C mYpet-MreB FtsZ-mCherry Merge Brightfield

t = 00:00

t = 22:30

t = 10:00

Figure 1 MreB is recruited to the FtsZ ring. (A) Representativebright field and fluorescent micrographs of MG1655 expressingthe mYpet–MreB fluorescent protein from pAKF106 (PBAD::mYpet–mreBCD). Examples of both non-dividing and dividing cells atdifferent stages of cell division are shown. (B) Dual-label fluores-cence micrographs of MG1655 expressing both the mYpet–MreBfusion from pAKF106 and FtsZ–mCherry from pQW59 (Plac::ftsZ–mCherry). Merged images reveal signals colocalised at mid-cell. Using this technique, MreBwt–FtsZ colocalisation frequencywas 75% (499 Z rings scored, n¼ 11). (C) Selected images from atime-lapse series following a single MG1655 cell expressing thesame fluorescent proteins shown in B. Arrows indicate the locationand colocalisation of major MreB foci and FtsZ rings. The full time-lapse series and additional MreB–FtsZ colocalisation examples areshown in Supplementary Figures S9 and S10. All cells were grownin M9 glucose media with 0.05% arabinose inducer at 301C 3 h priorto imaging (see Supplementary Figure S1A and B for growth curvesand western blot). Induction of the pQW59 plasmid was notrequired to give Z rings labelled with FtsZ–mCherry. All imagesshown are representative of at least three independent repeats.Scale bars¼ 2mm.



Asp285 on the Thermotoga maritima MreB crystal structure

showed it to locate in subdomain IIA, near the predicted

proto-filament interface and away from the RodZ interface

(Van den Ent et al, 2001, 2010) (Supplementary Figure S3C).

Together, these results showed that MreBD285A could still

interact with the cell-wall elongation complex and that the

reduced BTH interaction with FtsZ was not caused by

reduced protein stability or misfolding.

To validate the BTH result, the MreBD285A variant

was included in the in vivo crosslinking assay (Figure 2D).

A

B

(i)

(ii)T18

T25

MreB

MreBΔN

MreB

MreBΔN

T25

T18

C

MreB

MreBD285A

T25

T18

D

Anti-MreB

Anti-FtsZ

+ Crosslinker – Crosslinker

4000

3500

3000

1500

1000

500

0

Mre

B-Mre

B

Mre

B-FtsZ

Mre

BD28

5A -Mre

B

Mre

BM

reB

Mre

BM

reBD28

5A

Mre

BD28

5A

Mre

BD28

5A -FtsZ –v

e+v

e

FtsZ

β-ga

lact

osid

ase

activ

ity (

Mill

er U

nits

)

FtsZ (C

)

FtsZ (C

)

FtsZ (N

)

FtsA ZapA

FtsQ FtsB FtsL FtsW FtsI FtsN FtsX +ve

–ve

FtsZ (C

)

FtsZ (N

)

FtsA ZapA

FtsQ FtsB FtsL FtsW FtsI FtsN FtsX +ve

–ve

Mre

BM

reC

RodZ

–ve

+ve

Figure 2 MreB interacts directly with FtsZ and this interaction is abolished in the MreBD285A variant. (A) Bacterial two-hybrid (BTH) screen ofE. coli cell division factors against MreB. FtsZ has been labelled in both the N-terminus (N) and C-terminus (c). An MreB variant with a 9 aaN-terminal truncation (DN) is also included in this screen. Both T25- and T18-fusion orientations are shown for each BTH combination (i) and(ii). (B) BTH plate showing full-length T18-labelled MreB and MreBD285A against T25-labelled FtsZ (c) and known MreB interaction partnersessential for cell elongation. The BTH101 strain was used for all BTH cloning, a full list of plasmid vectors used for this screencan be found in Supplementary Table SI. Cells were spotted onto NA plates containing ampicillin (50 mg/ml), kanamycin (25 mg/ml) andX-gal (40mg/ml). Plates were incubated for 48 h at 301C prior to imaging. (C) BTH b-galactosidase activity assays showing T18-labelled MreBand D285A variant against T25-labelled FtsZ and MreBwt. Error bars¼ s.d., n¼ 4. In all cases, BTH � ve control strains contained emptypKNT25 and pUT18 vectors and þ ve control strains contained the pKT25-zip and pUT18C-zip fusions. (D) Western blots of His-tagged MreBand MreBD285A formaldehyde crosslink and pull-down preparations, using both anti-FtsZ and anti-MreB primary antibodies. MC1000 cellscontaining either pTK500 (Plac::His8–mreB) or pAKF126 (Plac::His8–mreBD285A) were grown to an OD600 of E0.3 without induction, crosslinkedwith 1% formaldehyde and quenched after 10 min using 150 mM glycine. Resulting crosslinked complexes were affinity purified using theMagneHisTM suspension, crosslinks were reversed by incubation at 951C for 1 h and complexes analysed. Equal volumes of protein complexpreparations, un-crosslinked His–MreB and His–MreBD285A preparations and purified His–MreB (200 ng) and native FtsZ (100 ng) proteinswere used as controls, n¼ 4. MreB and FtsZ antibody affinity control western blots are shown in Supplementary Figure S3B.



As seen, the MreBD285A –FtsZ interaction was severely

reduced (41000-fold). We conclude that the aspartate

at þ 285 in MreB is essential for the MreB–FtsZ interaction.

To assess the phenotype of the MreBD285A variant, we took

two complementary approaches. In the first, we used

an MreB depletion system to study the MreBD285A variant

phenotype in the absence of wt MreB (Kruse et al, 2005). The

depletion system utilises a DmreBCD strain complemented in

trans by a low-copy R1 plasmid carrying Pmre::mreBCD.

Depletion of the complementing plasmid was triggered by

induction of the anti-sense RNA copA (under Plac control),

which rapidly (within E10 min) blocks R1 plasmid

replication (Supplementary Figure S4A). Both the plasmid

and the MreBCD proteins were depleted by the natural turn-

over and dilution in growing E. coli populations with full loss

of MreB signal on western blots taking E1 h (Supplementary

Figure S4B). Complementation by MreBCD or MreBD285ACD

was provided by a third plasmid carrying the test genes under

control of the PBAD promoter (see Supplementary Figure S4A

for a diagram of the experimental setup).

As expected, the R1 (Pmre::mreBCD) plasmid complemen-

ted the mreBCD deletion. Moreover, the PBAD::mreBwtCD or

PBAD::mreBD285ACD plasmids had no effect on cell morpho-

logy or growth without induction (Figure 3A, left panels).

Also as expected, depletion of MreBCD without the reple-

nishment of MreBCD from the test plasmid led to the forma-

tion of rounded spherical cells (Figure 3A, middle panels),

and rod-shape morphology was maintained by replacement

with wt MreBCD, with a minor increase in cell width

(Figure 3A, upper right panel). In contrast, cells expressing

the MreBD285ACD variant in the absence of wt MreBCD

exhibited extreme lengthening (Figure 3A, lower right

panel). These experiments demonstrated that MreBD285A

supported cell elongation but inhibited cell division. Also,

under all conditions tested, the MreBD285A variant was unable

to complement MreB depletion in liquid or on plates, and was

thus incompatible with long-term cell viability.

In the second approach, we compared ‘low level’ over-

expression of MreB and MreBD285A, together with MreCD, in

wt cells. Overexpression was carefully controlled in these

experiments to not perturb cell viability, cell width or growth

rate (Supplementary Figures S3D and S4C, D). Interestingly,

such mild expression of MreBD285A led to a 45% increase in

average cell length, suggesting that cell division was inhibited

(Figure 3B). In some 17% of such cases, this inhibition

was very strong, resulting in cells greater than 18 mm in

length (Figure 3B(i) and Supplementary Figure S4D).

Statistical analysis showed that expression of MreBD285A but

not wt MreB significantly increased the average cell length

(Figure 3B(ii)). This supported the result from the depletion

study and gave us a tractable genetic system to investigate the

MreBD285A elongation phenotype.

MreBD285Ais recruited to MreB assemblies but not

to the Z ring

To investigate how the MreBD285A variant exerted its

dominant-negative effect in cells also expressing wt MreB,

we introduced the D285A allele into our mYpet–mreB

construct. In wt cells, mYpet–MreBD285A exhibited a typical

MreB localisation pattern with lateral patchy foci,

suggesting that MreBD285A was capable of entering MreB

assemblies (Figure 3C). Double labelling by expression of

mYpet–MreBwt and mYpet–MreBD285A in FB76 (mreB–RFPsw)

showed that the fusion proteins colocalised, thereby confirm-

ing that the D285A variant entered MreB assemblies

(compare left and right panels of Figure 3D).

Next, we investigated the MreBD285A localisation pattern

relative to FtsZ using an FtsZ–mCherry fusion protein

and the same MreB-labelling strategy as shown in

Figure 1B. Statistical analysis revealed a significant reduction

in MreB–Z-ring recruitment from 75% for MreBwt to 28% for

the MreBD285A variant (414 Z rings, n¼ 7, P40.01). Although

this represents a large drop in colocalisation, the 28% pro-

portion of colocalisation exhibited by MreBD285A is probably

an overestimate of MreBD285A–Z-ring recruitment, as this

variant could be recruited indirectly to the Z ring through

its interaction with untagged wt MreB at the septum.

Mutations in ftsZ restore the FtsZ–MreB BTH interaction

signal

Using an error-prone variant of Pfu polymerase, Pfu(exo� )

D473G, we PCR amplified the ftsZ open reading frame (ORF)

and used the BTH assay to identify mutations in ftsZ that re-

established the interaction with MreBD285A (Figure 4A,

Supplementary Figure S5A) (Biles and Connolly, 2004). This

screening generated five single aa changes in FtsZ that robustly

restored the MreBD285A–FtsZ interaction signal. All five FtsZ

variants interacted with wt FtsZ and all but one (FtsZP203Q)

could still bind wt MreB (summarised in Figure 4A). When

mapped onto the FtsZ crystal structure from Methanococcus

jannaschii and Staphylococcus aureus all five aa changes

clustered to a discrete patch on one face of FtsZ (Figure 4B).

To test the functionality of the FtsZ variants in the absence

of wt FtsZ, we used the FtsZ depletion strain VIP205 (Garrido

et al, 1993). Depletion of FtsZ reproducibly generated a E106

reduction in cell viability, which could be fully

complemented by inducing wt ftsZ in trans (Supplementary

Figure S5B). Three of the five FtsZ variants: P203R, T296A

and M302L fully complemented FtsZ depletion giving the

same CFU as the wt ftsZ control. One variant, D301N,

partially complemented the depletion giving a 100-fold drop

in CFU. Only FtsZP203Q did not complement FtsZ depletion

(Supplementary Figure S5B, summarised in Figure 4A).

Strikingly, FtsZP203Q was the variant that no longer gave a

positive-BTH interaction signal with MreBwt (Figure 4A,

Supplementary Figure S5A). When used in a complemen-

tation assay, the FtsZP203Q variant elongated cells, raising the

possibility that inhibition of cell division occurred through a

mechanism similar to that of MreBD285A -induced elongation

(Supplementary Figure S5D). Most importantly, the existence

of aa changes in FtsZ that restore the interaction with MreB

showed that the MreB–FtsZ interaction was direct.

FtsZP203Q rescues the MreBD285A phenotype

Expression of mreBD285A in wt cells led to cell elongation

through blockage of cell division (Figure 3A) and reduced the

number of cells in a growing culture compared to a control

culture expressing MreBwt (compare lines 1 and 3 in

Figure 4C, see also Supplementary Figure S5C). Taking

advantage of this reduction in viable counts, we tested the

FtsZ variants for rescue of the MreBD285A phenotype in the

FtsZ depletion strain VIP205. FtsZP203Q was a particularly

important variant to test as it was the only one that interacted

with MreBD285A but not with MreBwt (Figure 4A).



mYpet-MreBD285ABrightfield Merge

D mYpet-MreB MreB-RFPSWBrightfield mYpet-MreBD285A MreB-RFPSWBrightfield

C

A

mreBD285ACD

mreBCD

Replaced Depleted Un-induced

mYpet-MreBBrightfield Merge

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

(ii)

_ + _ + mreBCD

Over expression Empty vector

control

_ + mreBD285ACD

Over expression

*

o o

Cel

l len

gth

(μm

)

B (i)

mreBD285CD

mreBCD

InducedUn-induced

Figure 3 The MreBD285A variant inhibits cell division but not cell elongation. (A) Representative brightfield micrographs showing the MC1000MreBCD depletion strain (MC1000 Dmre, pKG339, pTK554). Images show cells without MreB depletion ‘un-induced’, depleted of MreB‘depleted’ or complemented with either mreBCD or mreBD285ACD expressed from pAKF128 (PBAD::mreBCD) or pAKF129 (PBAD::mreBD285ACD)‘replaced’. Cells were grown in LB þ tetracycline (20 mM), 2 mM IPTG and 0.2% arabinose for 3 h at 301C prior to imaging. See SupplementaryFigure S4A for diagram of depletion system. (B) MC1000 cells containing pAKF128 or pAKF129. Cells were grown in M9 medium with0.000,05% arabinose inducer at 301C for 5 h. (i) Representative bright field micrographs of cells with and without induction (ii) Box andWhisker plot showing cell length measurements with mreBCD and mreBD285ACD ‘overexpression’ compared to an empty vector control(pBAD24). Central line¼mean length, box¼± s.d. and whiskers¼maximum/minimum lengths observed. *t-test¼ P40.01, o¼no significantdifference between populations. 670 cells were sampled for each condition, n¼ 3. For cell width measurements and a full series of controlimages, see Supplementary Figure S4C and D. (C) Representative brightfield and fluorescent micrographs of MC1000 carrying pAKF131(PBAD::mypet–mreBCD; left panel) or pAKF132 (PBAD::mypet–mreBD285ACD; right panel), n¼ 3. Note the septal localisation of the mYpet–MreBversus the punctate localisation pattern of mYpet–MreBD285A. (D) mYpet–MreB (pAKF131) and mYpet–MreBD285A (pAKF132) colocalise withthe MreB–RFPSW in the FB76 strain. Arrows have been used to highlight selected colocalising foci, n¼ 3. Scale bars¼ 5 mm(A, B) and 2 mm (C, D).



Cells were depleted of FtsZwt and complemented by

induction of ftsZ from a plasmid (pAKF135-Ptet::ftsZ, or

pAKF137-Ptet::ftsZP203Q) induced at a level that did not

seriously alter cell morphology. Expression of MreB or

MreBD285A was induced in these strains from pAKF128

(PBAD::mreBCD) or pAKF129 (PBAD::mreBD285ACD). After 8 h

of growth in M9 media, cells were plated on solid medium

containing antibiotics, IPTG (to replenish wt FtsZ), but

without induction of the plasmid-borne ftsZ and mreBCD

genes (Figure 4C, Supplementary Figure S5C).

Expression of FtsZP203Q plus MreBwt reduced viable

counts four-fold (Figure 4C, compare MreBwt expression

with FtsZ and FtsZP203Q complementation of VIP205).

Critically, in cells expressing MreBD285A the pattern was

reversed: FtsZP203Q expression increased viable counts by

nearly 200-fold compared to FtsZwt (Figure 4C, compare

StrainmreB alleleused for

overexpression

ftsZ alleleused for VIP205 complementation

Average CFU/ml afterMreB overexpression

Corrected foldcomplementation

VIP205/pAKF128/pAKF135 MreBWT FtsZWT 3 ×106 1

VIP205/pAKF128/pAKF137 MreBWT FtsZP203Q 7 ×105 0.22

VIP205/pAKF129/pAKF135 MreBD285A FtsZWT 4 ×104 1

VIP205/pAKF129/pAKF137 MreBD285A FtsZP203Q 8.4 ×106 193

C

AMreB

bindingMreBD285A

bindingFtsZ

bindingΔftsZ

complementation

FtsZ ++ – ++ ++

FtsZP203R ++ + ++ ++

FtsZP203Q – + + –

FtsZT296A ++ + ++ ++

FtsZD301N ++ + ++ +

FtsZM302L ++ + ++ ++

B (i)

(ii)

Figure 4 Single aa changes in FtsZ restore functional interaction with MreBD285A. (A) Table summarising the phenotypes of ftsZ point mutantsidentified in the ftsZ–mreBD285A PCR mutagenesis screen. Bacterial two-hybrid (BTH) scores represent signal strength from BTH101 doubletransformations, plated on NA plates with selective antibiotics and X-gal (see Supplementary Figure S5A for images of plates); þ þ ¼ strongbinding signal,þ ¼weaker binding signal, � ¼no detectable signal. Details of plasmids used in these assays can be found in SupplementaryTable SI. ‘DftsZ complementation scores’ are representative of CFU measurements shown in Supplementary Figure S5B; þ þ ¼ full(comparable to ftsZwt complementation control),þ ¼partial (100 drop in CFU compared to ftsZwt control) and � ¼no complementation(same CFU as � ve empty vector contol). Complementation was assayed using the VIP205 (MC1061: Ptac::ftsZ) strain complementing withPtet::ftsZ (pAKF135) constructs containing the identified point mutations (pAKF136-140), the full list of plasmids can be found inSupplementary Table SI. Note that only the FtsZP203R variant has no detectable MreB binding and cannot complement the VIP205 ftsZdepletion strain. (B) Structural model of an FtsZ dimer of M. jannaschii (i) and S. aureus (ii) (Oliva et al, 2004; Matsui et al, 2012) showing thepositions of the aas in FtsZ (purple), changes to which can restore the interaction with MreBD285A. The two monomers of the FtsZ dimer areshown in different shades of orange. GTP molecules are shown in blue. Identification of the equivalent aa positions in E. coli FtsZ were basedon a sequence alignment of FtsZ (not shown). (C) FtsZP203Q can rescue MreBD285A expression. E. coli VIP205 (Ptac::ftsZ) cells were grownwithout IPTG inducer giving FtsZ depletion. This was complemented by expression of FtsZ from pAKF135 (Ptet::ftsZ) or pAKF137(Ptet::ftsZ

P203Q) induced using 20 ng/ml chlorotetracycline. A FtsZ depletion system was used in this assay as overexpression of ftsZ canlead to cell elongation and thus CFU reduction. Strains contained either pAKF128 (PBAD::mreBCD) or pAKF129 (PBAD::mreBD285ACD) inducedwith 0.005% arabinose for controlled MreB overexpression. Cells were grown in M9 media for 8 h, serially diluted 1/10 and used forenumeration on NA plates with antibiotics and 30mM IPTG, but without induction of the plasmid-borne ftsZ and mreBCD genes. These platesgave viable counts reflecting the effect of the mreB/ftsZ-mutant combinations in the liquid media. CFU measurements are an average of threeindependent repeats (n¼ 3). Note that the expression of MreBD285A reduced CFU as cells elongate (compare CFU/ml when MreBwt orMreBD285A is overexpressed and the FtsZwt allele is used for VIP205 complementation). Using FtsZP203Q for VIP205 complementation in cellsoverexpressing MreBwt reduced cell viability four-fold (see corrected fold complementation). In contrast, FtsZP203Q expression in VIP205 cellsalso expressing MreBD285A yielded a near 200-fold higher CFU (in growing cultures). Images of serially diluted plates are shown inSupplementary Figure S5C along with plate images showing MreBD285A complementation for all the FtsZ variants identified in this study.



FtsZwt or FtsZP203Q rescue of MreBD285A expression).

Therefore, this assay demonstrated that the division-inhibi-

tion effect of MreBD285A could be counteracted by FtsZP203Q

but not by FtsZwt.

Cells elongated by MreBD285A contain regularly spaced

‘locked’ Z rings

To investigate the elongation phenotype of cells expressing

MreBD285A, we determined localisation patterns of fluores-

cently labelled cellular components. FtsZ–mCherry labelling

revealed that Z rings were spaced regularly along the lengths

of cells elongated by MreBD285A expression (Figure 5A,

Supplementary Figure S7A). DNA staining using

Hoescht showed that these Z rings formed between

segregated nucleoids. Thus, cell elongation was not due to

dysfunctional chromosome segregation (Supplementary

Figures S6A and B). Together, these observations suggested

that Z ring formation and positioning were unaffected in

elongated cells, but raised the question of what protein

activities were absent in the ‘locked’ Z rings.

Locked Z rings do not support septal synthesis of

peptidoglycan

To investigate the locked Z rings, we used a recently

developed method that specifically labels sites that are

actively synthesising PG. This technique exploits the

tolerance of bacterial PG biosynthetic enzymes to accommo-

date D-amino acids with diverse sizes into PG. Addition of a

fluorescent tag to these aas leads to progressive labelling of

the cell wall over time, through a combination of growth and

turnover (Kuru et al, 2012). In E. coli wt cells, pulse labelling

the cell wall with the fluorescent D-aa HADA gives a very

strong incorporation signal at Z rings, weaker incorporation

at the cell periphery and very little incorporation at the

relatively inert cell poles (described in Kuru et al (2012)

and shown in Figure 5B (i)). Control cells treated with the

FtsI inhibitor aztreonam show multiple sites of HADA incor-

poration colocalising with Z rings along their lengths

(Supplementary Figure S6C). In contrast, in elongated

MreBD285A-expressing cells, the cell-wall regions colocalising

with the locked Z rings failed to incorporate HADA without

affecting incorporation elsewhere (Figure 5B (ii)). This result

showed that the locked Z rings did not actively synthesise PG,

while the elongation enzymes remained functional.

Cells elongated by MreBD285A display mislocalisation

of PBPs

To gain insight into the mechanism of MreBD285A-mediated

cell elongation through inhibition of Z ring-dependent PG

biosynthesis, we used a combination of fluorescent protein

tagging and time-lapse microscopy. We focused on the loca-

lisation patterns of the PBP enzymes: PBP1A, PBP1B, PBP2

and PBP3, all of which are important for cell-wall and septum

synthesis.

In wt cells, PBP1A is usually localised to the cell membrane

and is thought to principally function in cell elongation

(Vollmer and Bertsche, 2008; Banzhaf et al, 2012). In cells

expressing MreBD285A, PBP1A–mCherry formed discrete foci

at the cell periphery in both elongated and dividing

cells, consistent with the expected pattern (Supplementary

Figure S6D compared to Supplementary Figure S6E). In time-

lapse imaging, these foci migrated along the long axis of

elongating cells, similar to the behaviour of MreB (compare

movies of Supplementary Figure S11A and Supplementary

Figure S6D and E). Thus the localisation pattern of PBP1A–

mCherry, which functions mainly in cell elongation (Vollmer

and Bertsche, 2008), was not seriously affected by MreBD285A.

Both PBP1B and PBP2 are involved in cell elongation, but

have putative roles in cell division (Den Blaauwen et al, 2003;

Bertsche et al, 2006; Banzhaf et al, 2012; van der Ploeg et al,

2013). In wt cells both proteins localise as stable foci at

the dividing septum, in addition to migrating peripheral

foci along the cell length, reminiscent of MreB localisation

(Figures 5C and D—red arrows in control cells). Discrimi-

nating between these two behaviours was technically

challenging; here septum-associated foci were defined as

those that stayed stationary at the cell periphery for more

than three frames (7.5 min) and had a ‘partner’ focus on the

opposing side of the membrane (Supplementary Figure S11

and stills in Supplementary Figures S7B and C). In MreBD285A

elongated cells, such stable septum-like behaviours of PBP1B

and PBP2 were not observed, with both proteins forming only

longitudinally migrating foci (Supplementary Figure S11 and

stills in Figure 5C and D). In contrast, using a PBP3-specific

cell division inhibitor resulted in stationary PBP1B and PBP2

bands or ring-like foci in elongated cells (Supplementary

Figures S8B and C). This suggested that both PBP1B and

PBP2 were not successfully recruited to the locked Z rings

present in MreBD285A elongated cells.

PBP3 is the penultimate essential factor known to be

recruited to the Z ring (De Boer, 2010). Thus if PBP3 is

present at the septa, it can be considered a marker for the

localisation of all upstream cell division proteins (De Boer,

2010). In wt cells, PBP3 formed bands or rings at mid-cell

immediately prior to cell division and remained associated

with the constricting ring (Figure 5D, red arrows). In elon-

gated MreBD285A cells, PBP3 rings were also detected, sug-

gesting that all upstream septal proteins were present in these

locked structures (Figure 5E, white arrows). Over time, the

PBP3 rings dispersed instead of contracting (Supplementary

Figure S11).

The presence of PBP3 in locked Z rings raised the possibi-

lity that these structures contained all the essential cell

division factors. To investigate this, we studied the localisa-

tion pattern of a FtsN–mCherry fusion, which is the last

essential cell division factor (De Boer, 2010). In wt cells,

FtsN–mCherry is recruited immediately prior to septation

(Figure 5F—red arrows in control cells, S7E). Expression of

this fusion in MreBD285A elongated cells gave bands or

rings of FtsN, similar to those observed for PBP3

(Figure 5F, white arrows).

In summary, our results demonstrated that locked

Z rings recruited PBP3 and FtsN but failed to recruit PBP1B

or PBP2. These observations indicated that the FtsZ–MreB

interaction functions to deliver PBP1B and PBP2 to the Z ring.

Discussion

The discovery of a conserved bacterial cytoskeleton consis-

ting of tubulin–FtsZ and actin–MreB was relatively recent and

no mechanisms for coordination of these factors have been

described—that is—each element has until recently been

considered largely independent of the other. Here we show

that, in E. coli, MreB interacts directly with FtsZ and that the



interaction is essential for Z ring constriction, divisome

maturation and PG septal synthesis.

Three separate MreB-labelling approaches showed that

MreB colocalised with Z rings, in addition to adopting its

previously established punctate localisation pattern. Unlike

the patterns reported in Vats and Rothfield (2007), these ring-

like patterns only appeared at mid-cell and in cells

undergoing division. Time-lapse microscopy revealed that

MreB was recruited early to the Z ring and remained

colocalised throughout ring constriction. Our time lapses

indicated that most if not all Z rings recruited MreB at the

time of formation.

A single aa change in MreB (aspartate þ 285 to alanine)

simultaneously reduced MreB recruitment to the Z ring

FtsZ–mCherry

A

PBP2–mCherry

PBP3–mCherry

PBP1B–mCherry

C

D

E

B (i) (ii)

FtsZ–mCherry

HADA

F

FtsN–mCherry

Figure 5 PBPs are mislocalised and septal PG synthesis is inhibited in MreBD285A-expressing cells. MC1000 cells elongated by expressingMreBD285A from pAKF129 (PBAD::mreBD285ACD) containing a second plasmid expressing GFP-tagged division and elongation factors:(A, B) FtsZ–mCherry (pAKF133, PBAD::ftsZ–mCherry), (C) PBP1B–mCherry (pAKF145, Plac::mrcB–mCherry), (D) PBP2–mCherry (pAKF146,Plac::mrdA–mCherry), (E) PBP3–mCherry (pAKF147, Plac::ftsI–mCherry) and (F) FtsN–mCherry (pAKF150, Plac::mCherry-ftsN). In most casescells were grown in M9 media for 5 h at 301C with 0.000,05% arabinose. For expression from pAKF146, cells were grown in M9 glucose with0.1% arabinose inducer. Empty vector and MreB overexpression control images are shown in Supplementary Figure S7; contrasting elongatedcells treated with PBP3-sepcific inhibitors are shown in Supplementary Figure S8. Elongated cells contain multiple evenly spaced Z rings (A).Fixed HADA fluorescent D-aa-labelled cells (B). MC1000 pAKF133 cells containing empty pBAD24 vector (i) or elongated through MreBD285ACDexpression from pAKF129 (ii) were incubated with 1 mM HADA for 30 s, cells were fixed using 70% ethanol and washed in PBS beforemicroscopy. Note HADA incorporation at mature Z rings in (i—red arrows) are absent from elongated MreBD285A cells (ii—blue arrows), seeSupplementary Figure S6C for FtsI inhibitor controls. Note FtsZ, PBP1B, PBP2, PBP3 and FtsN are all recruited to the septum during normal celldivision—red arrows (Supplementary Figure S7). In MreBD285A elongated cells, PBP3 and FtsN are recruited to inhibit septa (E, F—whitearrows), whereas PBP1B and PBP2 are not (D, E). The behaviours of tagged PBP–mCherry fusions are shown in Supplementary Figure S11.Images are representative of at least four independent repeats. Scale bar¼ 2 mm.



and inhibited cell division. In cells devoid of wt MreB,

inhibition of division by MreBD285A was severe, showing

that recruitment of MreB to the Z ring was essential.

MreBD285A was still capable of binding the known

MreB-interacting partners MreC and RodZ, and supported

cell elongation in an MreB depletion strain. Thus,

MreBD285A was specifically defective in supporting cell divi-

sion. The aspartate at position 285 in MreB is conserved

among evolutionarily distant bacteria, raising the possibility

that interactions through this residue could be a conserved

phenomenon.

Crosslinking experiments showed that FtsZ and MreB

interacted in vivo and that this interaction was abolished in

MreBD285A. Single aa changes in FtsZ restored the interaction

with MreBD285A, yielding strong support of a direct inter-

action. The aa changes in FtsZ were not located to the proto-

filament interface, GTP-binding site, or C-terminal FtsA/

ZipA-binding regions, suggesting that they would not disrupt

the formation of functional Z rings (Ma and Margolin, 1999;

Oliva et al, 2004; Matsui et al, 2012). One of the FtsZ variants

(P203Q) had simultaneously lost its capability to complement

depletion of wt FtsZ and its interaction with wt MreB but,

strikingly, counteracted cell division inhibition exerted by

ectopic expression of MreBD285A. This shows that

the FtsZP203Q variant, although lethal when expressed as

the sole copy of FtsZ, could support cell division in the

right context. This lends further strong support to the

finding that the MreB–FtsZ interaction is direct and essential.

Cells elongated by ectopic expression of MreBD285A formed

regularly spaced, ‘locked’ Z rings, segregated their nucleoids

and formed mature divisomes containing PBP3 and FtsN.

However, these mature Z rings did not incorporate the

fluorescent D-aa HADA, showing that no septal PG biosynth-

esis had taken place (see Figure 5B(ii) compared to

Supplementary Figure S6C). PBP3 and FtsN are recruited

late to the divisome, just before the onset of division

(Addinall et al, 1997; De Boer, 2010) (Figure 6). Strikingly,

even though the locked Z rings contained PBP3 and FtsN they

did not recruit PBP1B and PBP2. This result was somewhat

unexpected since PBP1B interacts directly with PBP3 (and

FtsN) (Bertsche et al, 2006; Muller et al, 2007) and PBP2

seems to interact with PBP3 in vivo (Van der Ploeg et al,

2013). FtsN is the last known essential cell division protein

recruited to the divisome (Muller et al, 2007; de Boer, 2010).

Since FtsN has been proposed to function as a trigger

of septation (Corbin et al, 2004; Gerding et al, 2009;

Lutkenhaus, 2009), it was interesting to learn that locked Z

rings contained FtsN, yet still did not divide. Taken together,

our results show that PBP1B, PBP2 and possibly additional

factors must be recruited to the divisome by MreB for cell

division to occur. Our results therefore provide a mechanism

for recruitment of essential PBP enzymes to the divisome

from the elongation complex, namely PBP1B and/or

PBP2 (Figure 6).

Based on these observations, we propose a model for the

elongation to division transition of cell-wall biosynthesis. In

the model, a proportion of the MreB pool is recruited to the

Z ring to deliver protein factors from the cell-wall elon-

gation complex (Figure 6). This recruitment depends on

the MreB–FtsZ interaction that mediates transfer of protein

factors between elongation and divisome complexes, in

particular PBP2 and PBP1B identified in this study.

However, other factors, such as MurG and MraY, are known

to be present in both the elongation and divisome complexes

(Aaron et al, 2007; Mohammadi et al, 2007). Further studies

are required to determine if they are also recruited to the Z

ring by an MreB-dependent mechanism.

MreB is a conditionally essential protein: during rapid

growth, the cells become spherical, inflate and lyse.

However, during slow growth, the cells propagate as spheres

(Bendezu and De Boer, 2008). Thus our observations raise

the question of how the spherical cells are able to divide in

the absence of MreB that is clearly needed for division of wt

cells. Spherical cells cannot synthesise lateral cell wall due to

the lack of MreB; thus their cell wall is synthesised though the

FtsZ-dependent septal and pre-septal cell-wall synthesis

machinery (Varma and Young, 2009). Since, in the absence

of MreB, there is no lateral cell wall, we infer that PBP1B and

2 by default are associated with the septal and/or pre-septal

synthesis machinery, thus allowing the enzymes to perform

their essential function. Alternatively, the absence of MreB

changes the cell-wall machinery so dramatically that

the requirement for PBP1B and/or 2 is bypassed.

This study has shown that MreB plays an essential role in

cell division of E. coli under rapid growth conditions; pre-

vious work revealed that FtsZ plays a role in cell elongation

through pre-septal synthesis (De Pedro et al, 1997).

These observations expanded upon the established roles of

FtsZ

Z ring

MreB

FtsA

ZipAFtsQ

FtsL

FtsB FtsI(PBP3)

FtsN

MreCRodZ

MreDPBP2RodA

Z ring + cell division factors = mature divisome

FtsW

PBP1B

FtsK

PBP1APBP1B

PBP2

‘Elongation’ complex Septal-PG biosynthesiscomplex

MreB–FtsZ interaction delivers PBP1Band PBP2 to the septum

(1)

(2)

(3)

Figure 6 Model of the interaction network of cell elongation with cell division factors in E. coli. (1) FtsZ polymers at mid-cell assemble into theZ ring. (2) The direct interaction between MreB and FtsZ (this study) allows the downstream transfer of PBP1B and PBP2 into the maturedivisome (3) (hatched arrow). This provides all the biosynthetic enzyme activities required for septal PG synthesis. Ordered assembly of the 10essential cell division proteins was adapted from De Boer (2010). Binding structure of the elongation complex information was adapted fromKruse et al (2005); Bendezu et al (2009); White et al (2010); Banzhaf et al (2012). FtsI-, FtsN- and PBP1B-binding information was adapted fromMuller et al (2007). FtsI-, PBP2-binding information was taken from Van der Ploeg et al (2013).



both proteins and their associated protein complexes.

The understanding that MreB and FtsZ interact during

cell division to coordinate cell elongation with division opens

a new window to study how cell division is controlled in

bacteria. We suggest that the PBP1B and PBP2 proteins play

essential roles in septal and/or pre-septal PG synthesis,

respectively. It could be argued that MreB may not be

involved in pre-septal PG synthesis because spherical cells of

an mreB deletion indeed synthesised pre-septal PG (Potluri et al,

2012). However, as discussed above, the round cell morphology

may bypass the requirement of MreB to deliver PBP 1B and 2 to

the divisome, thereby resolving any such apparent paradox.

Materials and methods

Strains and growth mediaFor details of all strains and plasmids used in this study seeSupplementary Table SI. All microscopic observations and growthcurves were carried out using M9 growth media (Na2HPO4 0.6 g/l,KH2PO4 0.3 g/l, NaCl 0.05 g/l, NH4Cl 0.1 g/l, 0.2% w/v casaminoacids, 0.001% w/v thiamine, 1 mM CaCl2, 1 mM MgS04, pH¼ 7.4)made fresh from autoclaved components. Glycerol (0.5% v/v) wastypically used as a carbon source; however, glucose (0.4% w/v)was occasionally used where indicated.

Epi-fluorescence microscopyEpi-flourescence microscopy was carried out using an Olympus 1X71inverted microscope through a 100X Zeiss Plan-NEOFLUAR oil objec-tive (NA 1.3), held at 301C using an environmental chamber.

For mYpet–MreB detection, MG1655 pAKF106 (PBAD::mYpet–mreB) stationary-phase cultures were diluted 1/100 into 25 ml M9glucose media with 0.05% arabinose in a 125 ml conical flask. Underthese conditions, induction of the PBAD promoter was minimal anddid not affect growth rate or cell morphology (SupplementaryFigures S1A and B). Resulting mixtures were incubated for 3 h at301C, typically giving OD600 values between 0.29 and 0.33. Culturesamples were spotted onto pre-set 1% M9 glucose agarose pads,which had been pre-equilibrated to 301C and imaged immediately.The same conditions were used for the MreB/FtsZ double-labellingstrain: MG1655, pAKF106, pQW59 (Plac::ftsZ–mCherry) and allmicroscopy involving the FB76 (mreB–RFPSW) strain.

IFM was carried out using a standard protocol, the full details ofwhich are given as Supplementary Data. Cells were fixed in 1%formaldehyde and 0.1% glutaraldehyde. MreB localisation wasdetected using rabbit polyclonal anti-MreB antibodies first reportedin Kruse et al (2003), an antibody specificity western blot is shownin Supplementary Figure S1E. FITC-conjugated anti-rabbit-goat IgGantibody (Sigma-Aldrich) was used for secondary antibody bindingand fluorescent detection.

Full details of the microscopic apparatus and software used forimage acquisition, media recipes, IFM methods and details of themYpet–MreB tag development can be found in the SupplementaryMaterials and methods.

MreBD285A ‘overexpression’ microscopyNative mreB and mreBD285A was ‘overexpressed’ from plasmidspAKF128 and pAKF129 in MC1000 using empty vector (pBAD24)as a control. MC1000 (Dlac, DaraD) was used for microscopicstudies as it allows fine control of expression from Plac and PBADpromoters. Stationary-phase cultures were diluted 1/100 into 25 mlM9 media with 0.000,05% (33.3mM) arabinose. Resulting mixtureswere incubated shaking at 301C for 5 h giving OD600 values between0.3 and 0.4. Cells were imaged and their dimensions measuredusing the MATLAB plug-in MicrobeTracker (Sliusarenko et al,2012). The experiment was repeated in triplicate with at least 200cells measured per repeat. A 670 cell sample cutoff was chosen asthis equalled the smallest sample size collected.

For detection of PBP-fluorescent protein tags in mreBD285A-expressing cells the same protocol was used; except in the case ofpAKF146 (Plac::mrdA–mCherry) where M9 glucose media with0.1% arabinose inducer was used to repress expression. Forchromosomal DNA staining 1mg/ml Hoechst 33342 (Invitrogen)was used to stain cells immediately prior to imaging.

BTH analysisPlasmids used for BTH analysis (Karimova et al, 2005) were co-transformed into BTH101 (cya-99). Five microlitre of stationary-phase culture of representative transformants was spottedonto nutrient agar (NA) plates containing selective antibioticsand 40 mg/ml 5-bromo-4-chloro-3-indolyl-b-D-galactopyranoside(X-gal). After incubation at 301C for 24–48h, plates were scannedon an Epson perfection V700 photo flatbed scanner. Measurementsof b-galactosidase activity in liquid media were performed on 1 mlaliquots of growth-phase cultures as described in Miller (1972).

MreB–FtsZ in vivo formaldehyde crosslinkingE. coli MC100 cultures expressing His-tagged MreB from pTK500 orHis–MreBD285A from pAKF126 were crosslinked using 1% form-aldehyde for 10 min and quenched using 150 mM glycine.Crosslinked His–MreB complexes were purified under denaturingconditions as described in Ishikawa et al (2006). For full details seeSupplementary Materials and methods. Equal volumes ofprotein were loaded onto SDS-PAGE gels and individual proteinsidentified by western blot using anti-FtsZ and anti-MreB antibodies(both used at a 1/10 000 dilution) (Kruse et al, 2003; Galli andGerdes, 2012).

BTH ftsZ point mutagenesis screenAn error-prone PCR using a modified Pfu (exo� ) D473G polymeraseand primers: AKF_FtsZ_pKNT25_F and AKF_FtsZ_pKNT25_R wereused to amplify the ftsZ ORF from pKNT25–ftsZ exactly as describedin Biles and Connolly (2004). PCR products were digested with bothHindIII and BamHI, ligated into pKNT25–ftsZ (cut with the sameenzymes) and transformed into BTH101-containing pUT18–mreBD285A. Candidates that consistently gave blue cultures on NAplates containing X-gal (40 mg/ml) were selected and both plasmidsre-isolated. Re-isolated pKNT25–ftsZ-mutant plasmids were co-transformed into BTH101 strains with either pUT18–ftsZ,pUT18C–mreB or pUT18C–mreBD285A to confirm the ‘re-binding’phenotype and check protein variants could still bind FtsZ(Supplementary Figure S5A) before being sequenced. Primer se-quences are given as Supplementary Data.

Other methodsPurification of native FtsZ protein was carried out exactly asdescribed in Galli and Gerdes (2012). Affinity purification of His-tagged MreB was carried out using a HisTrap HP column (GEHealthcare). MreB depletion microscopy was carried out asdescribed in Kruse et al (2005). Cell-wall HADA staining wascarried out as described in Kuru et al (2012). In all cases, the fullprotocols are given as Supplementary Data.

Supplementary dataSupplementary data are available at The EMBO Journal Online(http://www.embojournal.org).

Acknowledgements

We would like to thank Waldemar Vollmer and Romain Mercier forcritical reading of the manuscript; Bernard Connolly for thegenerous gift of Pfu (exo-) D473G error-prone polymerase; ErkinKuru, Yves Brun and Michael VanNieuwenhze for the gift of theHADA fluorescent aas; and Elisa Galli for purified FtsZ protein. Wewould also like to thank and acknowledge Chen Chen and QingWang who conducted essential preliminary work on this project.CC generated the mreB point mutant library and conductedpreliminary work on mreB mutant-cell division phenotypes. QWengineered the pQW59 vector and the functional Ypet–linker–MreBfusion, which was further modified by AKF and used in this study.

Author Contributions: AKF carried out the majority of experi-ments, designed the experimental programme and co-authored themanuscript. KG designed parts of the experimental programme andco-authored the manuscript. This work was supported by theBBSRC and European Commission DIVINOCELL program.

Conflict of interest

The authors declare that they have no conflict of interest.



http://www.embojournal.org

ReferencesAaron M, Charbon G, Lam H, Schwarz H, Vollmer W,

Jacobs-Wagner C (2007) The tubulin homologue FtsZ contributesto cell elongation by guiding cell wall precursor synthesis inCaulobacter crescentus. Mol Microbiol 64: 938–952

Aarsman MEG, Piette A, Fraipont C, Vinkenvleugel TMF,Nguyen-Disteche M, Den Blaauwen T (2005) Maturation of theEscherichia coli divisome occurs in two steps. Mol Microbiol 55:1631–1645

Addinall SG, Cao C, Lutkenhaus J (1997) FtsN, a late recruitto the septum in Escherichia coli. Mol Microbiol 25: 303–309

Banzhaf M, Van den Berg van Saparoea B, Terrak M, Fraipont C,Egan A, Philippe J, Zapun A, Breukink E, Nguyen-Disteche M,Den Blaauwen T, Vollmer W (2012) Cooperativity of peptidogly-can synthases active in bacterial cell elongation. Mol Microbiol85: 179–194

Basu R, Chang F (2007) Shaping the actin cytoskeleton usingmicrotubule tips. Curr Opin Cell Biol 19: 88–94

Bendezu FO, De Boer PAJ (2008) Conditional lethality, divisiondefects, membrane involution, and endocytosis in mre and mrdshape mutants of Escherichia coli. J Bacteriol 190: 1792–1811

Bendezu FO, Hale CA, Bernhardt TG, De Boer PAJ (2009) RodZ(YfgA) is required for proper assembly of the MreB actincytoskeleton and cell shape in E. coli. EMBO J 28: 193–204

Bertsche U, Kast T, Wolf B, Fraipont C, Aarsman MEG, KannenbergK, Von Rechenberg M, Nguyen-Disteche M, Den Blaauwen T,Holtje J-V, Vollmer W (2006) Interaction between two murein(peptidoglycan) synthases, PBP3 and PBP1B, in Escherichia coli.Mol Microbiol 61: 675–690

Bi E, Lutkenhaus J (1991) FtsZ ring structure associated withdivision in Escherichia coli. Nature 354: 161–164

Biles BD, Connolly BA (2004) Low-fidelity Pyrococcus furiosusDNA polymerase mutants useful in error-prone PCR. NucleicAcids Res 32: e176

Bork P, Sander C, Valencia A (1992) An ATPase domain common toprokaryotic cell cycle proteins, sugar kinases, actin, andhsp70 heat shock proteins. PNAS 89: 7290–7294

Butland G, Manuel J, Alvarez P, Li J, Yang W, Yang X, Canadien V,Starostine A, Richards D, Beattie B, Krogan N, Davey M,Parkinson J, Greenblatt J, Emili A (2005) Interaction networkcontaining conserved and essential protein complexes inEscherichia coli. Nature 433: 531–537

Corbin BD, Geissler B, Sadasivam M, Margolin W (2004)Z-Ring-independent interaction between a subdomain of FtsAand late septation proteins as revealed by a polar recruitmentassay. J Bacteriol 186: 7736–7744

Den Blaauwen T, Aarsman MEG, Vischer NOE, Nanninga N (2003)Penicillin-binding protein PBP2 of Escherichia coli localizespreferentially in the lateral wall and at mid-cell in comparisonwith the old cell pole. Mol Microbiol 47: 539–547

De Boer PAJ (2010) Advances in understanding E. coli cell fission.Curr Opin Microbiol 13: 730–737

De Boer P, Crossley R, Rothfield L (1992) The essential bacterialcell-division protein FtsZ is a GTPase. Nature 359: 254–256

De Pedro MA, Quintela JC, Holtje JV, Schwarz H (1997) Mureinsegregation in Escherichia coli. J Bacteriol 179: 2823–2834

Divakaruni AV, Ogorzalek Loo RR, Xie Y, Loo JA, Gober JW (2005)The cell-shape protein MreC interacts with extracytoplasmicproteins including cell wall assembly complexes in Caulobactercrescentus. PNAS 102: 18602–18607

Domınguez-Escobar J, Chastanet A, Crevenna AH, Fromion V,Wedlich-Soldner R, Carballido-Lopez R (2011) Processivemovement of MreB-associated cell wall biosynthetic complexesin bacteria. Science 333: 225–228

Erickson HP (2007) Evolution of the cytoskeleton. BioEssays 29:668–677

Fenton AK, Lambert C, Wagstaff PC, Sockett RE (2010) Manipulatingeach MreB of Bdellovibrio bacteriovorus gives diversemorphological and predatory phenotypes. J Bacteriol 192:1299–1311

Figge RM, Divakaruni AV, Gober JW (2004) MreB, the cellshape-determining bacterial actin homologue, co-ordinates cellwall morphogenesis in Caulobacter crescentus. Mol Microbiol 51:1321–1332

Fletcher DA, Mullins RD (2010) Cell mechanics and the cyto-skeleton. Nature 463: 485–492

Galli E, Gerdes K (2012) FtsZ-ZapA-ZapB interactome of Escherichiacoli. J Bacteriol 194: 292–302

Garner EC, Bernard R, Wang W, Zhuang X, Rudner DZ, Mitchison T(2011) Coupled, circumferential motions of the cell wall synthesismachinery and MreB filaments in B. subtilis. Science 333:222–225

Garrido T, Scnchez M, Palacios P, Aldea M, Vicente M (1993)Transcription of ftsZ oscillates during the cell cycle ofEsherichia coli. EMBO J 12: 3957–3965

Gerding Ma, Liu B, Bendezu FO, Hale CA, Bernhardt TG,De Boer PAJ (2009) Self-enhanced accumulation of FtsN atdivision sites and roles for other proteins with a SPOR domain(DamX, DedD, and RlpA) in Escherichia coli cell constriction.J Bacteriol 191: 7383–7401

Hu P, Janga SC, Babu M, Dıaz-Mejıa JJ, Butland G, Yang W,Pogoutse O, Guo X, Phanse S, Wong P, Chandran S,Christopoulos C, Nazarians-Armavil A, Nasseri NK, Musso G,Ali M, Nazemof N, Eroukova V, Golshani A, Paccanaro A et al.(2009) Global functional atlas of Escherichia coli encompassingpreviously uncharacterized proteins. PLoS Biol 7: e96

Ishikawa S, Kawai Y, Hiramatsu K, Kuwano M, Ogasawara N (2006)A new FtsZ-interacting protein, YlmF, complements the activityof FtsA during progression of cell division in Bacillus subtilis.Mol Microbiol 60: 1364–1380

Jones LJ, Carballido-Lopez R, Errington J (2001) Control of cellshape in bacteria: helical, actin-like filaments in Bacillus subtilis.Cell 104: 913–922

Karczmarek A, Martınez-Arteaga R, RM-A Baselga, Alexeeva S,Hansen FG, Vicente M, Nanninga N, Den Blaauwen T (2007)DNA and origin region segregation are not affected by thetransition from rod to sphere after inhibition of Escherichia coliMreB by A22. Mol Microbiol 65: 51–63

Karimova G, Dautin N, Ladant D (2005) Interaction network amongEscherichia coli membrane proteins involved in cell divisionas revealed by bacterial two-hybrid analysis. J Bacteriol 187:2233–2243

Kruse T, Bork-Jensen J, Gerdes K (2005) The morphogeneticMreBCD proteins of Escherichia coli form an essentialmembrane-bound complex. Mol Microbiol 55: 78–89

Kruse T, M�ller-Jensen J, L�bner-Olesen A, Gerdes K (2003)Dysfunctional MreB inhibits chromosome segregation inEscherichia coli. EMBO J 22: 5283–5292

Kuru E, Hughes HV, Brown PJ, Hall E, Tekkam S, Cava F, De PedroMA, Brun YV, Vannieuwenhze MS (2012) In situ probing of newlysynthesized peptidoglycan in live bacteria with fluorescentD-amino acids. Angewandte Chemie 51: 12519–12523

Lutkenhaus J (2009) FtsN–trigger for septation. J Bacteriol 191:7381–7382

Lutkenhaus JF, Wolf-Watz H, Donachie WD (1980) ftsA-envA regionof the and identification of a new fts organization of genesin the ftsA-envA region of the Escherichia coli genetic mapand identification of a new fts locus (ftsZ). J Bacteriol 142:615–620

Ma X, Margolin W (1999) Genetic and functional analyses of theconserved C-Terminal core domain of Escherichia coli FtsZ.J Bacteriol 181: 7531–7544

Matsui T, Yamane J, Mogi N, Yamaguchi H, Takemoto H, Yao M,Tanaka I (2012) Structural reorganization of the bacterialcell-division protein FtsZ from Staphylococcus aureus.Acta Crystallogr D Biol Crystallogr 68: 1175–1188

Michie KA, Lowe J (2006) Dynamic filaments of the bacterialcytoskeleton. Ann Rev Biochem 75: 467–492

Miller JH (1972) Experiments in Molecular Genetics. Cold SpringHarbor: Cold Spring Harbor Press

Mohammadi T, Karczmarek A, Crouvoisier M, Bouhss A,Mengin-Lecreulx D, Den Blaauwen T (2007) The essentialpeptidoglycan glycosyltransferase MurG forms a complexwith proteins involved in lateral envelope growth as well aswith proteins involved in cell division in Escherichia coli.Mol Microbiol 65: 1106–1121

Muller P, Ewers C, Bertsche U, Anstett M, Kallis T, Breukink E,Fraipont C, Terrak M, Nguyen-Disteche M, Vollmer W (2007) Theessential cell division protein FtsN interacts with the murein(peptidoglycan) synthase PBP1B in Escherichia coli. J Biol Chem282: 36394–36402



Oliva MA, Cordell SC, Lowe J (2004) Structural insights intoFtsZ protofilament formation. Nat Struct Mol Biol 11: 1243–1250

Potluri L-P, Kannan S, Young KD (2012) ZipA is required forFtsZ-dependent preseptal peptidoglycan synthesis prior toinvagination during cell division. J Bacteriol 194: 5334–5342

Salje J, Van den Ent F, De Boer P, Lowe J (2011) Direct membranebinding by bacterial actin MreB. Mol Cell 43: 478–487

Shiomi D, Sakai M, Niki H (2008) Determination of bacterial rodshape by a novel cytoskeletal membrane protein. EMBO J 27:3081–3091

Sliusarenko O, Heinritz J, Emonet T, Jacobs-Wagner C (2012) High-throughput, subpixel-precision analysis of bacterial morphogen-esis and intracellular spatio-temporal dynamics. Mol Microbiol80: 612–627

Swulius MT, Jensen GJ (2012) The helical MreB cytoskeleton inE. coli MC1000/pLE7 is an artifact of the N-terminal YFP tag.J Bacteriol 194: 6382–6386

Tan Q, Awano N, Inouye M (2011) YeeV is an Escherichia coli toxinthat inhibits cell division by targeting the cytoskeleton proteins,FtsZ and MreB. Mol Microbiol 79: 109–118

Typas A, Banzhaf M, Gross CA, Vollmer W (2012) From theregulation of peptidoglycan synthesis to bacterial growth andmorphology. Nat Rev Microbiol 10: 123–136

Van den Ent F, Amos LA, Lowe J (2001) Prokaryotic origin of theactin cytoskeleton. Nature 413: 39–44

Van den Ent F, Johnson CM, Persons L, De Boer P, Lowe J (2010)Bacterial actin MreB assembles in complex with cell shapeprotein RodZ. EMBO J 29: 1081–1090

Van der Ploeg R, Verheul J, Vischer NOE, Alexeeva S, HoogendoornE, Postma M, Banzhaf M, Vollmer W, Den Blaauwen T (2013)Colocalization and interaction between elongasome and divisomeduring a preparative cell division phase in Escherichia coli.Mol Microbiol 87: 1047–1087

Van Teeffelen S, Wang S, Furchtgott L, Huang KC, Wingreen NS,Shaevitz JW, Gitai Z (2011) The bacterial actin MreB rotates,and rotation depends on cell-wall assembly. PNAS 108:15822–15827

Varma A, De Pedro MA, Young KD (2007) FtsZ directs a secondmode of peptidoglycan synthesis in Escherichia coli. J Bacteriol189: 5692–5704

Varma A, Young KD (2009) In Escherichia coli, MreB and FtsZ directthe synthesis of lateral cell wall via independent pathways thatrequire PBP 2. J Bacteriol 191: 3526–3533

Vats P, Rothfield L (2007) Duplication and segregation of the actin(MreB) cytoskeleton during the prokaryotic cell cycle. PNAS 104:17795–17800

Vats P, Shih Y-L, Rothfield L (2009) Assembly of the MreB-associated cytoskeletal ring of Escherichia coli. Mol Microbiol72: 170–182

Vollmer W, Bertsche U (2008) Murein (peptidoglycan) structure,architecture and biosynthesis in Escherichia coli. Biochim BiophysActa 1778: 1714–1734

Wachi M, Doi M, Tamaki S, Park W, Nakajima-Iijima S, MatsuhashiM (1987) Mutant isolation and molecular cloning of mre genes,which determine cell shape, sensitivity to mecillinam, andamount of penicillin-binding proteins in Escherichia coli.J Bacteriol 169: 4935–4940

Wachi M, Matsuhashi M (1989) Negative control of cell division bymreB, a gene that functions in determining the rod shape ofEscherichia coli cells. J Bacteriol 171: 3123–3127

White CL, Kitich A, Gober JW (2010) Positioning cell wall syntheticcomplexes by the bacterial morphogenetic proteins MreB andMreD. Mol Microbiol 76: 616–633

Wientjes FB, Nanninga N (1989) Rate and topography ofpeptidoglycan synthesis during cell division in Escherichia coli:concept of a leading edge. J Bacteriol 171: 3412–3419



Documents

Direct interaction of FtsZ and MreB is required for septum synthesis and cell division in Escherichia coli