Nuclear pores

• each nuclear pore complex has ~30 nucleoporins (proteins) which have an eightfold rotational symmetry
• nuclear envelope of typical mammalian cell is 3-4000NPCs.

  each NPC can transport up to 1000 macromolecules per second & in both directions simultaneously
• internal diameters of each NPC is ~ 40nm, to accomodate ribosomal subunits
• pore filled with unstructured protein contains numerous repeats of phenylalanin—glycine (FG) motifs.

  weak affinity for each other creates a gel-like mesh inside the NPC
• Mesh created from the weak affinity acts as a sieve, as it restricts the diffusion of large macromolecules but allows smaller molecules to pass through.

  • small molecules (<5000 daltons) have rapid diffusion so freely permeable
  • many cell proteins (40,000 daltons which are ~ 5nm diameter) are too large to diffuse passively via NPCs as nuclear compartment and cytosol can maintain different protein compositions
  • Mature cytosolic ribosomes (~30nm diameter) cannot diffuse through so protein synthesis is confined to cytosol
• Small proteins continually shuttle back and forth between the nucleus and cytosol as they:

  • need import/export signals
  • other proteins contain both nuclear localisation signals and nuclear export signals
  • relative rates of import & export determine the steady-state localisation of such shuttling proteins
  • changing rate of import & export (or both) changes the location of a protein