The nuclear envelope contains an inner and outer membrane, separated by the perinuclear space. It regulates transport between the nucleus and cytoplasm. The nuclear lamina lies underneath the inner nuclear membrane and provides structural support. Nuclear pore complexes control molecular traffic through the envelope. Small molecules can passively diffuse, while larger cargoes require transport receptors and Ran GTPase. Importins ferry proteins into the nucleus via nuclear localization signals, while exportins use nuclear export signals and RanGTP to transport proteins and RNAs out of the nucleus. This ensures proper distribution and functions of biomolecules in the cell.

1. Cell BiologyCell Biology
S. Rahgozar,PhD
University of Isfahan
Faculty of Science
3. The nucleus
3.1. The nuclear envelope and nuclear traffic
1392-93

2.  Structure of the nuclear envelope
The space between the inner and outer nuclear membranes is directly
connected with the lumen of the endoplasmic reticulum.

3.  Nuclear membrane Compared to ER membrane is
 Similar in Function
having ribosomes bound to its surface)
 Different in composition (outer membrane contains proteins that bind to cytoskeleton
and
inner membrane carries specific proteins
(ie; those bound to nuclear lamina)

4. Nuclear membranes are permeable to small non polar molecules.
Small polar
molecules
Macromolecules
Ions
May pass
through
pores
 Nuclear lamina
Contains 60-80kd fibrous proteins called
lamins
Function: structural support of the nucleus

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

6. o Laminar modifications
Post translational addition of specific types of lipids (prenylation)

7. o Laminar interactions
 Histones H2A and H2B

8.  LBR (lamin B receptor) and Emerin
 Nuclear proteins that function in DNA synthesis, transcription or
chromatin

9.  Nuclear pore complex
 Diameter: 120 nm
 Molecular mass: 125,000 kd
 Composed of 30 proteins (nucleoporins) in
multiple copies
(~30×the size of a ribosome)Molecular traffic through nuclear
pore complexes
(20-40 kd)

10. Model of the nuclear pore complex
o A structure with 8fold symmetry organised around a large central channel

11. 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 in
replication
Involved in
chromatin
assembly
bipartite

12. Distribution of Ran/GTP across the nuclear envelope
Examples for Small GTP binding proteins: Ran, Rab, Rac, Rho, Cdc42, Arf,
Ran GAP: Ran
GTPase activating
protein
Ran GEF: Ran
guanine nucleotide
exchange factor

13. Protein import through the nuclear pore complex
NLS: nuclear localization
sequence
Transport
proceeds by
sequential
binding to
specific
nuclear pore
proteins
located
further and
further
toward the
nuclear side
of the pore
complex.
FG-
proteins
After release of importin Ran/GDP is transported back to the nucleus by NTF2 receptor

14. Nuclear export
NES: nuclear export
sequence
 exportins and importins are members of a
family named karyopherins
 Ran/GTP promotes the formation of stable
complexes between exportins and their cargo
proteins
Whereas
It dissociates the complexes between importins
and their cargos

16.  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

17.  Transport of RNAs
Transport of a ribonucleoprotein complex (RNP) of 35-40kb from the nuclear pore in the salivary gland
of 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 proteins
including NTF2)

18. o tRNAs and miRNAs are exported by exportin-t and exportin5
Functional
snRNP
snRNP: small nuclear
ribonucleoprotein
5’7mGcap: 5’ 7-
methylguanosine cap
o Small RNAs function within the nucleus as components of the RNA processing
machinery.