Cell BiologyCell Biology

S. Rahgozar,PhD

University of IsfahanFaculty of Science

3. The nucleus

3.1. The nuclear envelope and nuclear traffic

1392-93

Structure of the nuclear envelope

The space between the inner and outer nuclear membranes is directlyconnected with the lumen of the endoplasmic reticulum.

Nuclear membrane Compared to ER membrane is

Similar in Functionhaving ribosomes bound to its surface)

Different in composition (outer membrane contains proteins that bind to cytoskeletonand

inner membrane carries specific proteins(ie; those bound to nuclear lamina)

Nuclear membranes are permeable to small non polar molecules.

Small polarmolecules

Macromolecules

Ions

May passthroughpores

Nuclear lamina

Contains 60-80kd fibrous proteins calledlamins

Function: structural support of the nucleus

Model of lamin assembly

At least 7 distinct proteins of lamins are coded from 3 lamin genes of A, B and C

o Laminar modifications

Post translational addition of specific types of lipids (prenylation)

o Laminar interactions

Histones H2A and H2B

LBR (lamin B receptor) and Emerin

Nuclear proteins that function in DNA synthesis, transcription orchromatin

Nuclear pore complex

Diameter: 120 nm

Molecular mass: 125,000 kd

Composed of 30 proteins (nucleoporins) inmultiple copies

(~30×the size of a ribosome)Molecular traffic through nuclearpore complexes

(20-40 kd)

Model of the nuclear pore complex

o A structure with 8fold symmetry organised around a large central channel

Nuclear localization signals (1984, Smith, Cell. studying SV40 T Ag)

Selective transport of proteins

o Nuclear localization signals (specific aa sequences)

o Nuclear transport receptors (importins)

Involved inreplication

Involved inchromatinassembly

bipartite

Distribution of Ran/GTP across the nuclear envelope

Examples for Small GTP binding proteins: Ran, Rab, Rac, Rho, Cdc42, Arf,

Ran GAP: RanGTPase activatingprotein

Ran GEF: Ranguanine nucleotideexchange factor

Protein import through the nuclear pore complex

NLS: nuclear localizationsequence

Transportproceeds bysequentialbinding tospecificnuclear poreproteinslocatedfurther andfurthertoward thenuclear sideof the porecomplex.

FG-proteins

After release of importin Ran/GDP is transported back to the nucleus by NTF2 receptor

Nuclear export

NES: nuclear exportsequence

exportins and importins are members of afamily named karyopherins

Ran/GTP promotes the formation of stablecomplexes between exportins and their cargoproteins

Whereas

It dissociates the complexes between importinsand their cargos

Regulation of nuclear protein import

Inhibition by binding to a cytoplasmic inhibitory factor which masks its NLS

ubiquitin-mediated

Cell stimulation

Inhibition by direct phosphorylation of NLS

Transport of RNAs

Transport of a ribonucleoprotein complex (RNP) of 35-40kb from the nuclear pore in the salivary glandof an insect (A), and unfolding of the RNA during its translocation to the cytoplasm

N

C

Transport of tRNAs, rRNAs, miRNAs and snRNAs is through Ran/GTP

Transport of mRNAs in through mRNA exporter (a complex of two proteinsincluding NTF2)

o tRNAs and miRNAs are exported by exportin-t and exportin5

FunctionalsnRNP

snRNP: small nuclearribonucleoprotein

5’7mGcap: 5’ 7-methylguanosine cap

o Small RNAs function within the nucleus as components of the RNA processingmachinery.