Protein transport 6

Question 1

Transport across the nuclear envelope

Answer 1

• Nucleus = separated from rest of cell by nuclear envelope
• Envelope = made out of 2 membranes
• Large molecules use NPC (non-selective, large proteins, 1,000-10,000 NPC per nucleus)
• Nucleo-cytoplasmic transport system essential
• Active transport involves NTRs
• NTRs that facilitate transport of cargo = importing
• Importins interact w/ NLS in rich basic residue
• Exportins interact w/ NES rich in hydrophobic residues

Question 2

GTP hydrolysis in Ran

Answer 2

• Ran = 25kDa Rad-like GTP binding protein, cycles btw GDP/GTP
• Regulated by Ran-interacting factors e/g/ GAP, GEF
• Ran-GAP = cytoplasmic side, Ran-GDP high on cytoplasm
• Ran-GEF = found in nucleoplasm bound to histones, Ran-GTP high in nucleus
• Importins bind RanGTP
• Cargo bind importins in Ran-free state, cargo binds Ran-GTP
• GTP hydrolysis provides vectorality + E source for transport across membrane

Question 3

Fg-Nups

Answer 3

• NPC is composed of multiple copies of nups
• Nup w/ F+G = involved in selectivity
• Many transport receptors have hydrophobic bs on surface
• Systematic deletion = lethal
• Exchange of phenylalanine btw low bs is rapid

Question 4

NPC as entropic barrier

Answer 4

• Entropy of particle ↓ as FG restrict movement → activation barrier
• Can be overcome in particle has favoured;e enthalpies interaction w/ FG-filament through bs on importin
• Ea of barrier needs to be low
• 2 models
  1. Selective phase model (sieve formed in NPC btw FG repeats, size of mesh determines upper size limit of diffusion, binding transport receptors to FG repeats dissolves FG-FG mesh)
  2. Virtual gating (NPC = catalyst that ↓ Ea for translocation, both sides of NPC = crowded by FG-Nups forms ↑ selective barrier for large molecules, binding of transport receptors to FG repeat overcomes barrier