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1 2 Cooperative and non-cooperative conformational changes of F-actin induced 3 by cofilin 4 Tomoki Aihara Q1 , Toshiro Oda 5 Structural Analysis Research Team, RIKEN SPring-8 Center, Kouto 1-1, Sayo, Hyogo 461-5146, Japan 6 7 9 article info 10 Article history: 11 Received 27 March 2013 12 Available online xxxx 13 Keywords: 14 Cofilin 15 Actin 16 ESR 17 MTSL 18 MSL 19 Cooperativity 20 21 abstract 22 Cofilin is an actin-binding protein that promotes F-actin depolymerization. It is well-known that cofilin- 23 coated F-actin is more twisted than naked F-actin, and that the protomer is more tilted. However, the 24 means by which the local changes induced by the binding of individual cofilin proteins proceed to the 25 global conformational changes of the whole F-actin molecule remain unknown. Here we investigated 26 the cofilin-induced changes in several parts of F-actin, through site-directed spin-label electron paramag- 27 netic resonance spectroscopy analyses of recombinant actins containing single reactive cysteines. We 28 found that the global, cooperative conformational changes induced by cofilin-binding, which were 29 detected by the spin-label attached to the Cys374 residue, occurred without the detachment of the 30 D-loop in subdomain 2 from the neighboring protomer. The two processes of local and global changes 31 do not necessarily proceed in sequence. 32 Ó 2013 Published by Elsevier Inc. 33 34 35 36 1. Introduction 37 Actin filament (F-actin) dynamics are controlled by various ac- 38 tin-binding proteins, and are essential for cellular movement and 39 functions. Cofilin is an actin-binding protein that binds to F-actin 40 in the dephosphorylated state [1]. The bound cofilin severs and 41 depolymerizes F-actin in a pH-dependent manner [2–5]. An elec- 42 tron microscopic study [6,7] revealed that cofilin binds two adja- 43 cent protomers on a single strand of F-actin. The structures of 44 cofilin-bound F-actin and naked F-actin are different: the crossover 45 repeat is shorter and each protomer on F-actin is more tilted than 46 those of naked F-actin. Consequently, cofilin-binding weakens the 47 lateral interactions of F-actin [8] and increases the bending flexibil- 48 ity [9]. These properties are associated with changes in the interac- 49 tions among F-actin protomers. However, it remains enigmatic 50 how the local changes induced by the binding of cofilin proceed 51 to the global conformational changes of the F-actin. Cofilin report- 52 edly binds cooperatively to F-actin [7,10]. One interpretation for 53 the cooperative binding is that cofilin binding influences not only 54 the protomer bound to cofilin but also the conformations of neigh- 55 boring protomers, thus enhancing the affinity of the neighboring 56 protomers to cofilin [2]. Hence, the cofilin-induced conformational 57 change is considered to propagate through the interface between 58 the protomers in F-actin. 59 Here we investigated the cofilin-induced changes on the local 60 interfaces between protomers in F-actin, through site-directed 61 spin-label electron paramagnetic resonance (SDSL-EPR) spectros- 62 copy analyses using recombinant actins containing single reactive 63 cysteines. In this study, we prepared human cardiac wild-type 64 a-actin (C374) and its point mutated actins: V43C, Y91C and 65 S323C (see Fig. S1). Val43 is located on the DNase-I binding loop 66 (D-loop) in subdomain 2 of actin, which interacts with the hydro- 67 phobic groove in subdomain 1 of the neighboring protomer in F-ac- 68 tin [11]. Tyr91 is in close proximity to the cofilin binding site on 69 F-actin, in the cofilin-saturated F-actin model [12]. Ser323 resides 70 on a small loop in subdomain 3, which interacts with subdomain 4 71 and is far from the cofilin-binding site. Cys374 (intrinsic cysteine) 72 is located in subdomain 1 near the binding site of the neighboring 73 D-loop. SDSL-EPR is a useful method to characterize the secondary, 74 tertiary and quaternary structures of proteins and to detect the 75 conformational changes induced by ligands or other proteins 76 [13–15]. By monitoring the mobility of the attached spin-label, 77 the steric restrictions imposed by the local environment of the 78 label or the motion of a polypeptide segment can be investigated 79 [16]. The SDSL-EPR probes are relatively small nitroxide spin-la- 80 bels, and thus they only minimally influence the labeled proteins. 81 Our results suggested that the detachment of the D-loop, in- 82 duced by cofilin-binding to the neighboring protomer, is not neces- 0006-291X/$ - see front matter Ó 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.bbrc.2013.04.076 Abbreviations: SDSL-EPR, site-directed spin-label electron paramagnetic reso- nance; G-actin, globular actin; F-actin, fibrous actin; V43C, expressed actin in which Val 43 and Cys374 are replaced by Cys and Ala, respectively; Y91C, expressed actin in which Tyr91 and Cys374 are replaced by Cys and Ala, respectively; S323C, expressed actin in which Ser323, and Cys374 are replaced by Cys and Ala, respectively. Corresponding author. Fax: +81 791 58 1879. E-mail address: [email protected] (T. Oda). Biochemical and Biophysical Research Communications xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Biochemical and Biophysical Research Communications journal homepage: www.elsevier.com/locate/ybbrc YBBRC 30299 No. of Pages 5, Model 5G 11 May 2013 Please cite this article in press as: T. Aihara, T. Oda, Cooperative and non-cooperative conformational changes of F-actin induced by cofilin, Biochem. Bio- phys. Res. Commun. (2013), http://dx.doi.org/10.1016/j.bbrc.2013.04.076

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Page 1: Cooperative and non-cooperative conformational changes of F-actin induced by cofilin

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Biochemical and Biophysical Research Communications xxx (2013) xxx–xxx

YBBRC 30299 No. of Pages 5, Model 5G

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Contents lists available at SciVerse ScienceDirect

Biochemical and Biophysical Research Communications

journal homepage: www.elsevier .com/locate /ybbrc

Cooperative and non-cooperative conformational changes of F-actin inducedby cofilin

Tomoki Aihara, Toshiro Oda ⇑Structural Analysis Research Team, RIKEN SPring-8 Center, Kouto 1-1, Sayo, Hyogo 461-5146, Japan

a r t i c l e i n f o

2223242526272829303132

Article history:Received 27 March 2013Available online xxxx

Keywords:CofilinActinESRMTSLMSLCooperativity

0006-291X/$ - see front matter � 2013 Published byhttp://dx.doi.org/10.1016/j.bbrc.2013.04.076

Abbreviations: SDSL-EPR, site-directed spin-labelnance; G-actin, globular actin; F-actin, fibrous actin; VVal 43 and Cys374 are replaced by Cys and Ala, respecin which Tyr91 and Cys374 are replaced by Cys aexpressed actin in which Ser323, and Cys374 arerespectively.⇑ Corresponding author. Fax: +81 791 58 1879.

E-mail address: [email protected] (T. Oda).

Please cite this article in press as: T. Aihara, T. Ophys. Res. Commun. (2013), http://dx.doi.org/10

a b s t r a c t

Cofilin is an actin-binding protein that promotes F-actin depolymerization. It is well-known that cofilin-coated F-actin is more twisted than naked F-actin, and that the protomer is more tilted. However, themeans by which the local changes induced by the binding of individual cofilin proteins proceed to theglobal conformational changes of the whole F-actin molecule remain unknown. Here we investigatedthe cofilin-induced changes in several parts of F-actin, through site-directed spin-label electron paramag-netic resonance spectroscopy analyses of recombinant actins containing single reactive cysteines. Wefound that the global, cooperative conformational changes induced by cofilin-binding, which weredetected by the spin-label attached to the Cys374 residue, occurred without the detachment of theD-loop in subdomain 2 from the neighboring protomer. The two processes of local and global changesdo not necessarily proceed in sequence.

� 2013 Published by Elsevier Inc.

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1. Introduction

Actin filament (F-actin) dynamics are controlled by various ac-tin-binding proteins, and are essential for cellular movement andfunctions. Cofilin is an actin-binding protein that binds to F-actinin the dephosphorylated state [1]. The bound cofilin severs anddepolymerizes F-actin in a pH-dependent manner [2–5]. An elec-tron microscopic study [6,7] revealed that cofilin binds two adja-cent protomers on a single strand of F-actin. The structures ofcofilin-bound F-actin and naked F-actin are different: the crossoverrepeat is shorter and each protomer on F-actin is more tilted thanthose of naked F-actin. Consequently, cofilin-binding weakens thelateral interactions of F-actin [8] and increases the bending flexibil-ity [9]. These properties are associated with changes in the interac-tions among F-actin protomers. However, it remains enigmatichow the local changes induced by the binding of cofilin proceedto the global conformational changes of the F-actin. Cofilin report-edly binds cooperatively to F-actin [7,10]. One interpretation forthe cooperative binding is that cofilin binding influences not onlythe protomer bound to cofilin but also the conformations of neigh-

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electron paramagnetic reso-43C, expressed actin in whichtively; Y91C, expressed actinnd Ala, respectively; S323C,

replaced by Cys and Ala,

da, Cooperative and non-coope.1016/j.bbrc.2013.04.076

boring protomers, thus enhancing the affinity of the neighboringprotomers to cofilin [2]. Hence, the cofilin-induced conformationalchange is considered to propagate through the interface betweenthe protomers in F-actin.

Here we investigated the cofilin-induced changes on the localinterfaces between protomers in F-actin, through site-directedspin-label electron paramagnetic resonance (SDSL-EPR) spectros-copy analyses using recombinant actins containing single reactivecysteines. In this study, we prepared human cardiac wild-typea-actin (C374) and its point mutated actins: V43C, Y91C andS323C (see Fig. S1). Val43 is located on the DNase-I binding loop(D-loop) in subdomain 2 of actin, which interacts with the hydro-phobic groove in subdomain 1 of the neighboring protomer in F-ac-tin [11]. Tyr91 is in close proximity to the cofilin binding site onF-actin, in the cofilin-saturated F-actin model [12]. Ser323 resideson a small loop in subdomain 3, which interacts with subdomain 4and is far from the cofilin-binding site. Cys374 (intrinsic cysteine)is located in subdomain 1 near the binding site of the neighboringD-loop. SDSL-EPR is a useful method to characterize the secondary,tertiary and quaternary structures of proteins and to detect theconformational changes induced by ligands or other proteins[13–15]. By monitoring the mobility of the attached spin-label,the steric restrictions imposed by the local environment of thelabel or the motion of a polypeptide segment can be investigated[16]. The SDSL-EPR probes are relatively small nitroxide spin-la-bels, and thus they only minimally influence the labeled proteins.

Our results suggested that the detachment of the D-loop, in-duced by cofilin-binding to the neighboring protomer, is not neces-

rative conformational changes of F-actin induced by cofilin, Biochem. Bio-

Page 2: Cooperative and non-cooperative conformational changes of F-actin induced by cofilin

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YBBRC 30299 No. of Pages 5, Model 5G

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sary for the global, cooperative conformational changes of F-actin,although restricted conformational changes of the D-loop occur.

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A: G-actin

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2. Materials and methods

2.1. Proteins

The His-tagged gelsolin 4–6 fragment was expressed in Escheri-chia coli BL21 cells and purified as described by Ohki [17]. ThepCold-1-His-tagged G4-7 expression vector was a kind gift fromDr. Ohki (Waseda Univ.). The His-tagged TEV protease was pre-pared from Rosetta(DE3)pLysS E. coli cells, transformed by Dr. B.Helena (kindly provided by Dr. Ihara, RIKEN) as described [18].Cofilin was expressed and prepared as described previously [19].The expression vector was a kind gift from Prof. Maeda at NagoyaUniversity.

The gene encoding Drosophila 5C actin was cloned into thepFastBac-1 vector, between the BamHI and HindIII restriction sites.From the vector, the coding sequence of human cardiac muscle a-actin was produced through site-directed mutagenesis, using aKOD-Plus-Mutagenesis Kit (TOYOBO). The L21 sequence [20] andthe Strep-tag II and TEV protease recognition sites were introducedto the N-terminal end (Asp1) of the resulting wild-type actin. Forsite-directed spin labeling (except for the wild-type, Cys374),Cys374 was mutated to Ala374, and Val43, Tyr91 and Ser323 weremutated to Cys43, Cys91 and Cys323, respectively. These recombi-nant actins were expressed by the Bac-to-Bac Baculovirus Expres-sion System (Invitrogen) in Sf9 insect cells, cultured in a cellmaster bio-reactor (Waken) [21]. The lysate of 6 billion Sf9 cellsexpressing actin was gently mixed with an excess of His-taggedgelsolin 4–6 fragment (about 100 mg) in binding buffer (50 mMKCl, 10 mM Tris–HCl (pH 8.0), 5 mM CaCl2, 0.5 mM ATP and0.5 mM DTT, containing complete EDTA-free protease inhibitor(Roche)) at 4 �C overnight (total volume �300 ml), to form thecomplex between the monomeric actin and the gelsolin 4–6 frag-ment. After ultracentrifugation at 200,000g, the resulting gelsolin4–6 fragment – actin complexes were purified by using Ni-Sephar-ose 6 Fast Flow resin (GE Healthcare). The purified complex in-cluded both the Sf9-intrinsic actin and recombinant actin. Therecombinant actin bound to the gelsolin 4–6 fragment was purifiedby the use of Strep-Tactin Superflow resin (IBA), and then it wasdialyzed against binding buffer at 4 �C overnight. The dialyzedsolution was mixed with His-tagged TEV protease at a molar ratioof 1:1 for 3 h at room temperature, to remove the extra N-terminalactin sequence. After the mixture was passed through the Strep-Tactin resin, the recombinant actin was dissociated from the gelso-lin 4–6 fragment by chromatography on a Ni-Sepharose 6 FastFlow column in chelating buffer (50 mM KCl, 10 mM Tris-HCl(pH 8.0), 50 lM MgCl2, 0.5 lM EGTA and 0.5 mM ATP and0.5 mM DTT), according to the method of Ohki [17]. The G-actinconcentration was determined from the absorbance at 290 nm,using an extinction coefficient of E290 = 0.63 ml mg�1 cm�1. Theyield of wild-type actin was about 1–2 mg per 6 billion cells. Thepolymerization activity of the wild-type was confirmed to be sim-ilar to that of chicken skeletal muscle actin.

341533953375335533353315Magnetic Field [G]

Fig. 1. Spectra of MTSL-C374 in various states of actin. A and B are the spectra ofMTSL-C374 in the G- and F-actin states, respectively. AS, AF, BS, BF in the lower fieldsand AS

0 , BS0 in the higher fields are the positions of notable peaks. The separations

[AS–AS0] and [BS–BS

0] of the outer peaks (2Azz) are 63.2 G and 69.7 G, respectively.The spectra labeled C–F are arranged in the order of the cofilin/actin binding ratios(numbers on the right).

2.2. EPR measurements

The 4-maleimido-2,2,6,6-tetramethyl-1-piperdinyloxy (MSL)spin label was purchased from Sigma-Aldrich Chemicals. (1-Oxyl-2,2,5,5-tetramethyl-D3-pyrroline-3-methyl) Methanethiosulfonate(MTSL) was purchased from Toronto Research Chemicals.

Recombinant wild-type actin or mutants in MOPS G-buffer(10 mM MOPS (pH 7.0), 0.2 mM CaCl2 and 0.2 mM ATP) were re-acted with the spin-label reagent at a molar ratio of 1.5 at room

Please cite this article in press as: T. Aihara, T. Oda, Cooperative and non-coopephys. Res. Commun. (2013), http://dx.doi.org/10.1016/j.bbrc.2013.04.076

temperature in the dark. After an hour, the solution was passedthrough a Sephadex G-25 disposable column (PD G-25, GE) equil-ibrated with MOPS G-buffer, to remove the un-reacted reagent.EPR spectra for the spin-labeled actin solution (about 30 lM) weremeasured as the G-actin state sample. The spin-labeled actin waspolymerized by F-buffer (addition of final concentrations of100 mM KCl and 2 mM MgCl2). After ultracentrifugation of theF-actin solution, the pellet was re-suspended in 15 lL F-buffer.EPR spectra for the suspension were measured as the F-actin sam-ple. In addition, after the F-actin solutions were incubated withcofilin at various molar ratios for an hour, the cofilin-F-actin com-plexes were ultracentrifuged. Cofilin does not promote depolymer-ization of F-actin though it binds to F-actin at the condition ofneutral pH [19]. The pellets were re-suspended in 15 lM F-buffer,and EPR spectra for the cofilin-F-actin complex were measured.EPR spectra were collected using a Bruker ELEXSYS E580 spectrom-eter. Sample solutions (ca. 15 lL) were loaded into capillaries (in-side diameter, 1.0 mm) and inserted into the resonator, and first-derivative EPR spectra were obtained with the following instru-ment settings: microwave power 5 mW, modulation amplitude1G and sweep time 40 s. After the EPR measurement, the sampleswere recovered from the capillaries to estimate the cofilin/actinbinding ratios from the bands on SDS-PAGE gels stained by FastGreen. The fractions of actin molecules with cofilin-induced con-formational changes at the various molar ratios were estimatedby fitting, using the spectra of F-actin (panel B in Figs. 1–3) andcofilin-saturated F-actin (panel F in Figs. 1–3) and the Xeprsoftware.

3. Results and discussion

3.1. Selection of single cysteine sites on actin and spin-label reagents

The SDSL-EPR method is a tool that provides information aboutthe local environment around the spin-labeled amino acid in a pro-tein. Using the method, we measured the conformational changesin the interfaces between actin protomers, induced by cofilin-bind-

rative conformational changes of F-actin induced by cofilin, Biochem. Bio-

Page 3: Cooperative and non-cooperative conformational changes of F-actin induced by cofilin

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341533953375335533353315Magnetic Field [G]

A: G-actin

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Fig. 2. Spectra of MTSL-V43C in various states of actin. A and B are the spectra ofMTSL-V43C in the G- and F-actin states, respectively. AF, BS, BF in the lower fieldsand BS

0 in the higher fields are the positions of notable peaks. A has a componentrevealing single dynamic motion, as the inverse line-width of the central resonanceline (DH�1

0 ) = 0.60 G�1. The separation [BS–BS0] of the outer peaks is 69.0 G. The

spectra labeled C–F are arranged in the order of the cofilin/actin binding ratios(numbers on the right).

341533953375335533353315

Magnetic Field [G]

A: G-actin

B: F-actin

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Fig. 3. Spectra of MTSL-Y91C in various states of actin. A and B are the spectra of theG- and F-actin states, respectively. AS, AF, BS, BF in the lower fields and AS

0 , BS0 in the

higher fields are the positions of notable peaks. The separations [AS–AS0] and [BS–BS

0]of the outer peaks (2Azz) are 44.8 G and 48.2 G, and their DH�1

0 values are 0.43 G�1

and 0.29 G�1, respectively. The spectra labeled C–F are arranged in the order of thecofilin/actin binding ratios (numbers on the right). The DH�1

0 value of spectrum F is0.31 G�1. FS in the lower fields and FS

0 in the higher fields are the positions ofnotable peaks. The separation [FS–FS

0] of the peaks is 69.9 G.

T. Aihara, T. Oda / Biochemical and Biophysical Research Communications xxx (2013) xxx–xxx 3

YBBRC 30299 No. of Pages 5, Model 5G

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ing. For this purpose, we prepared wild type actin (C374) and threecysteine mutated actins (V43C, Y91C and S323C), and selected MSLand MTSL as the spin-labeling reagents. In this report, we refer tothe spin-labeled actin as MSL-G-actin or MTSL-C374 actin. MSLwas reproducibly attached to each G-actin with a labeling effi-

Please cite this article in press as: T. Aihara, T. Oda, Cooperative and non-coopephys. Res. Commun. (2013), http://dx.doi.org/10.1016/j.bbrc.2013.04.076

ciency higher than 0.85 mol/mol. Each spectrum of MSL-G-actinwas different from that of MSL-F-actin. In addition, the spectra ofMSL-C374, MSL-V43C and MSL-Y91C changed when a 3-fold molarratio of cofilin was mixed with F-actin (cofilin-saturated F-actinstate), but that of MSL-S323C was not (Fig. S2). Similarly, MTSLwas also attached to each G-actin (C374, V43C and Y91C) at an effi-ciency higher than 0.90 mol/mol, and the spectra of the MTSL-labeled actin were also clearly different in the various states. Thespectra of the MTSL-labeled actins provided better data to under-stand the changes in the transition from F-actin to the cofilin-saturated F-actin state, as compared to the MSL labeling (especiallyY91C), because of the clear peak separation and the invariant peakpositions (see below). Therefore, we used MTSL for the detailedanalyses, except for S323C.

3.2. Mtsl-c374

Fig. 1 shows the spectra of MTSL-C374 in the various states of ac-tin. The spectra for G- and F-actin, shown in panels A and B, had twopeaks (AS, AF and BS, BF) in the lower field, indicating that MTSL-C374fluctuates at two kinds of rotational rates in the G- and F-actinstates. This was also reported previously [22,23], and the two peaksexhibit fast motion in the solvent and slow motion in the protein.The AS and BS peaks were assigned as slow motion, and the separa-tion (2Azz) expanded from 63.2 G of AS–AS

0 in the G-actin spectrum(Fig. 1A) to 69.7 G of BS–BS

0 in the F-actin spectrum (Fig. 1B). Thismeans that the slow motion observed in F-actin was more sup-pressed, probably by steric restriction. On the other hand, therewas no clear change in the mobility of the fast motion. As shownin the spectra of Fig. 1B–F, the peak height of BS normalized to BF de-creased with an increase in the cofilin-binding ratio to actin. Theseresults indicated that the fraction of the slow motion of MTSL-C374decreases upon cofilin-binding. In other words, MTSL-C374 prefersthe faster motion in cofilin-saturated F-actin. The change would beinduced indirectly by cofilin-binding, through a shift of the C-termi-nal region. According to Galkin’s model [12], C374 is far away fromthe cofilin-binding site, and cofilin interacts with residues of 143–147 and 343–346 in subdomain 1.

3.3. Mtsl-v43c

The spectrum of MTSL-V43C in G-actin, shown in Fig. 2A, has asingle peak in the lower field (AF), indicating that MTSL-V43C fluc-tuates at a single rate. This fluctuation was categorized into themotion of MTSL attached to a loop surface, from the inverse line-width of the central resonance line (DH�1

0 = 0.60 G�1) [15]. In theF-actin state (Fig. 2B), the single peak was divided into two peaks(BS, BF). The separation of BS–BS

0 was 69.0 G, suggesting that theslow motion of MTSL-V43C was also suppressed in the F-actinstate, to a similar degree as in MTSL-C374. The peak height of BS

in the MTSL-V43C spectra (Fig. 2B–F) decreased with cofilin-bind-ing. This dependency also was similar to that in the MTSL-C374spectra. In cofilin-saturated F-actin (Fig. 2F), the BS peak was small.In other words, MTSL-V43C in F-actin also became mobile by cofi-lin-binding. V43 probably forms a longitudinal interaction be-tween the protomers in F-actin, but cofilin-binding disrupts theinteraction. This interpretation is supported by previous reports.In our F-actin model [11], the tip (V43, V45) of the D-loop formsa longitudinal interaction with the hydrophobic groove of theneighboring protomer on a single strand of F-actin. In the previousEPR study [14], the V43 and V45 residues were more restrictedthan the others in the D-loop of F-actin. On the other hand, inGalkin’s model [12], cofilin disrupts this longitudinal interaction,since it pushes the D-loop away by binding to the root (K50) ofthe D-loop.

rative conformational changes of F-actin induced by cofilin, Biochem. Bio-

Page 4: Cooperative and non-cooperative conformational changes of F-actin induced by cofilin

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Fig. 4. Plots of the fractions of actin molecules that undergo conformationalchanges in the presence of cofilin, against the measured binding ratios of cofilin toactin. Circles, MTSL-C374; triangles, MTSL-V43C; squares, MTSL-Y91C. Each cofilin/actin binding ratio became saturated at about 1.2.

Table 1Environment around each residue.

Polymerization Cofilin-binding Cooperativity

V43 Tight Loose NoY91 Tight Tighter NoS323 Tight No change –C374 Tight Loose Yes

4 T. Aihara, T. Oda / Biochemical and Biophysical Research Communications xxx (2013) xxx–xxx

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3.4. Mtsl-y91c

Fig. 3 shows the spectra of MTSL-Y91C. Two peaks (AS, AS) arepresent in the lower field of the G-actin spectrum (Fig. 3A). Theseparation slightly expanded from AS–AS

0 (44.8 G) to BS–BS0

(48.4 G) in the spectrum of F-actin (Fig. 3B). Interestingly, uponcofilin addition to F-actin (Fig. 3C–F), BS split into two peaks (BS,FS). FS was the new peak, and the separation between FS–FS

0 was69.9 G. The peak height of FS increased, and the height of BS de-creased with increases in the cofilin-binding ratio to F-actin, indi-cating that the fraction with slower motion increased. Thedependency of Y91C was opposite to those of C374 and V43C.MTSL-Y91 would be directly restricted by cofilin binding. TheY91 side chain of actin resides close to V20 of cofilin, in the atomicmodel of fully cofilin-decorated F-actin [12]. Therefore, we con-cluded that the changes in the MTSL-Y91C spectra reflect the directbinding of cofilin to F-actin.

321

322

323

324

325

326

327

328

329330331

332333

334335336

337338

3.5. Transition from F-actin to cofilin-saturated F-actin states

To understand how the conformational change proceeds fromF-actin to cofilin-saturated F-actin, the fractions of actin moleculesthat underwent conformational changes in the presence of cofilinwere plotted against the measured binding ratios of cofilin to actin(Fig. 4). The fractions were estimated by using the spectra of nakedF-actin (panel B in Figs. 1–3) and cofilin-saturated F-actin (panel Fin Figs. 1–3). The estimation was rationalized by the result that thepeak positions of the EPR spectra of all recombinant actins wereinvariant upon the addition of cofilin, and thus the intermediatespecies can be expressed by a linear combination of the two spec-tra (Fig. S3).

The fractions of MTSL-V43C and of MTSL-Y91C linearly in-creased. The linear increase in the fraction of MTSL-Y91C is reason-able, because the spectral change of MTSL-Y91C would be directlyinduced by cofilin-binding. The spectra of MTSL-V43C in F-actinalso changed linearly, together with the cofilin/actin binding ratio.This suggested that cofilin binding disrupts the longitudinal inter-action, including V43, with the neighboring protomer in F-actin.

Please cite this article in press as: T. Aihara, T. Oda, Cooperative and non-coopephys. Res. Commun. (2013), http://dx.doi.org/10.1016/j.bbrc.2013.04.076

On the other hand, although the plots of MTSL-C374 almostoverlapped with those of MTSL-V43C and MTSL-Y91C up to a ratioof 0.5 mol/mol, the fractions of MTSL-C374 were larger than thoseexpected in the plots above 0.5 mol/mol. This transition was notobserved in the other sites. These latter plots of MTSL-C374 sug-gested that the conformational changes around the C374 residueare not directly coupled with cofilin-binding. In other words, cofi-lin-binding influences the conformational changes around theC374 residue not only in cofilin-bound but also in cofilin-unboundprotomers at ratios above 0.5 mol/mol. Conversely, the longitudi-nal interaction of V43 in the cofilin-unbound adjacent protomersof F-actin would persist. The linear relationship in MTSL-V43Cand the sigmoidal relationship in MTSL-C374 were also observedin MSL-labeled V43C and C374, respectively (data not shown).

The results of our EPR experiments are summarized in Table 1.MTSL-V43C and MTSL-Y91C would detect the local changes in-duced by cofilin-binding because those effects were not coopera-tive; we deduced that MTSL-V43C monitors the attachment anddetachment of the D-loop with the neighboring protomer, and thatMTSL-Y91C is directly included in the cofilin-binding process.MTL-S324C scarcely detected the cofilin-induced changes, and thusthe loop including S323, at the interface of the adjacent subdo-mains 3–4, might not be affected by cofilin-binding.

On the other hand, MTSL-C374 would detect the global changesinduced by cofilin-binding, because the effect is cooperative. Itshould be noted that the global change occurs without the detach-ment of the D-loop at a high cofilin/actin ratio, and the two pro-cesses do not necessarily proceed sequentially. However, theglobal change would also include the restricted change of theD-loop without the detachment, since fluorescently-labeled Q41Cexhibits cooperative dependency [24]. What does MTSL-C374 de-tect in F-actin? In the F-actin model [11,25], the region includingC374 is located in the outermost part of subdomain 1 and interactswith subdomain 4 of the protomer on the counter-strand in F-actin.In contrast, in the cofilin-saturated F-actin model [12], this interac-tion appears to be weaker. Hence, one probable interpretation isthat MTSL-C374 detects the change in the inter-strand, rather thanintra-strand, interaction. On basis of this interpretation, the cofilin-induced changes of MTSL-C374 in our results suggest that the inter-stand interactions in F-actin are cooperatively weakened by in-creased cofilin-binding. Finally, the SDSL-EPR analysis providedindependent conformational information about the interactions inF-actin.

Acknowledgments

We thank Dr. Arata (Osaka University) for critical reading of themanuscript. This work was supported by Japan Society for thePromotion of Science (JSPS) KAKENHI Grant 2350200 (TO).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.bbrc.2013.04.076.

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