A Model for the Structure of Nuclear Pores
The nuclear pore complex acts as a supramolecular sieve that spans both membranes of the nuclear envelope. The proteins that make up the pore are called nucleoporins. Parallel rings, exhibiting octagonal symmetry, are situated near the inner and outer membranes of the nuclear envelope (Web Figure 1.6.A). Various nucleoporin proteins form the other structures, such as the nuclear ring, cytoplasmic filaments, and the nuclear basket. Small metabolites diffuse freely through the NPC, but the flow of large proteins and RNA is selective. According to a recent model, selectivity is derived from a meshwork of unstructured nucleoporins within the aqueous pore. To penetrate this meshwork into or out of the nucleus, macromolecules must be associated with specific carrier proteins, such as importins. Ribosomes presumably exit the nucleus by physically moving through the meshwork.