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Nuclear Import & Export Overview. Nuclear structure Nuclear pore Import/ Export Signals Receptors Ran (Directionality) FG Platform (Gating) Regulation. Nuclear Structure. Origin Relates to Topology Parts chromatin matrix nucleoli lamina (M-phase disassembly) pores

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nuclear import export overview
Nuclear Import & Export Overview

Nuclear structure

Nuclear pore

Import/ Export Signals


Ran (Directionality)

FG Platform (Gating)


nuclear structure
Nuclear Structure

Origin Relates to Topology





lamina (M-phase disassembly)


inner membrane

outer membrane

lumen (peri-nuclear space)

11 4 signal mediated transport through nuclear pore complexes npc
11.4 Signal-mediated transport through nuclear pore complexes (NPC)

The nuclear pore complex

Figure 11-28

nuclear cargo
Nuclear Cargo
  • Imported
  • Polymerases
  • Histones
  • Transcription factors
  • Ribosomal proteins
  • Exported
  • tRNAs
  • mRNPs
  • Ribosomal subunits
  • Transcription factors

106 ribos=>560K ribo proteins imported/min

14,000 ribo subs exported/min

3-4K pores/cell=> 150 ribo proteins/min/pore

Also 100 histones/min/pore etc.

nuclear pore
Nuclear Pore

125MD octagonal symmetry, 3-4K pores/cell

central plug

nuclear basket

cytoplasmic fibers

About 30 (?) proteins “Nucleoporins” (FG repeats)

Passive diffusion, limit 40-60KD

Channel 9nm x 15 nm long

Opens w/ signal: gating mech. unknown


size exlcusion limits
Size Exlcusion Limits


(up to 50MDa)




Not cleaved

Folded proteins transported

NLS (nuclear localization signal)

Defined by transfection/microinjection experiments

e.g. stretch of basic residues SV40 T=PKKKRKV

drives import of pyruvate kinase

NES (nuclear export signal)

Defined by heterokaryon expt

e.g. leucine rich


11.4 Heterokaryon assay demonstrating that human hnRNP A1 protein (red) can cycle in and out of the cytoplasm but human hnRNP protein C (grn) cannot

Figure 11-32


3 Types

Importin beta (classic)

NTF2 (for Ran import)

TAP/p15 (mRNA export)

Identified with

affinity chromatography

permeabilized cell assays

Importin beta (karyopherins)

Large family of related Import & Export Receptors

Bind signals (direct or indirect), porins & RanGTP

cargo binding controlled by Ran

destabilizes import receptors

stabilizes export receptors

permeabilized cells
Permeabilized Cells

Cytosol lost

NPCs intact


RCC1: stabilizes exchange intermediate

RanGAP1, RanBP1, RanBP2: >105 fold cooperative

RanBP1 binds Ran->dissoc. then RanGAP

RanGAP sumo/ RanBP2 complex on pore

NTF2: import receptor for RanGDP


Facilitated Diffusion

Transport cycles of exportins and importins use the chemical potential of a primary gradient, namely that

of RanGTP, to drive the directed transport of cargo molecules

On its own, translocation per se would be a fully reversible process that would only allow an equilibration of cargoes between nucleus and cytoplasm. Directional transport is achieved by Ran-regulated cargo loading and unloading of the transport receptors. This also constitutes the input of energy in the form of one molecule of GTP hydrolyzed per transport cycle.



Masks NES