AD: Nuclear Pore Complexes

It has several rings that slide relative to each other to open or shrink the pore size.

It was shown that in fact the pore is filled with NUPs (nucleoporins) with FG regions (phenylalanine, glycine).

Selective phase/hydrogel model: The FG-repeats form a three-dimensional sieve with defined mesh-size. The sieve can be traversed by compounds small enough to diffuse through the mesh and by importin β.

Virtual gate/polymer brush model: The pore is filled with FG repeats not necessarily forming a mesh, but compounds would cause a decrease in entropy by entering which is unfavorable.

Forest model: FG repeats and other compounds form a highly ordered mesh with defined channels, preventing uncontrolled diffusion. There are short FG repeats (“shrubs”) and larger ones (“trees”), leading to a path for active transport in the center and a path for passive pathway on the sides.

Reduction of dimensionality model: The channel walls are covered with a FG repeat bilayer, leaving a small tube for passive diffusion. Active transport is facilitated by two-dimensional random walk of karyopherins.

• N-terminal Ran-binding domain (CRIME)

• Hydrophobic pocket for interaction with FG repeats (AI)

• Cargo binding domain

• General structural motive: HEAT repeats (two antiparallel helices held together by hydrophobic interactions, forming a superhelix)

Ran-GTPases are small GTPases that function as protein switches.

GEFs: guanine nucleotide exchange factors

• trigger the release of GDP from Ran-GTPases

• probably displace the GDP

• GTP can bind afterwards because it has a very high concentration in the cell

• release of the cargo in nuclear transport

  • import: cargo is released when RanGTP binds

  • export: cargo is released when RanGTP is hydrolyzed

Necessary factors:

• RanBPs (GAP)

• RCC1 (GEF)

• responsible for import, too

• transports proteins with a NLS+PY motif

• spiral made out of HEAT repeats

two basic, K- and R-rich motives with a distance of 10-12 hydrophobic aa

• In the cytoplasm, importin α and β form a complex with a protein containing NLS.

• The complex travels through the NPC.

• RanGTP binds importin β, leading to its dissociation.

• Importin α also dissociates and is bound by its nuclear export factor CAS/RanGTP.

• Upon export, both RanGTPs (binding importin β and importin α) undergo GTP hydrolysis and dissociate.

—> To import one molecule, we need at least 2 GTP!

• have exportin and importin properties

• cargo binds to the inner surface of the receptor

• export cargo and Ran interact directly

• import cargo binding interferes with RanGTP binding

CRM1 is an extremely versatile exportin (100-1000 different cargos).

• no direct interaction between RanGTP and cargo

  • but: positive binding cooperativity – when one binds, the other one is more likely to bind as well

• When RanGTP binds, the acidic loop secures it and the cargo binding site (NES binding cleft) opens.

• The cargo binds and export takes place.

• RanBP1 binds to RanGTP and displaces the acidic loop.

• The RanBP1-RanGTP complex dissociates.

  • GTP hydrolysis leads to dissociation of the complex.

• The unloaded CRM1 is imported again.

OR: complex NES with folded domains

• RanGTP acts as “genome-positioning signal” (binds to genome during mitosis/miosis) and therefore causes GTP/GDP gradient in the cell

  • high GTP concentrations: This is where nucleus should be

• This principle is also used for spindle assembly, positioning etc.