Howard Fried's Lab Web Page Nucleocytoplasmic Transport and RNA Protein Interactions Transport of RNA and protein molecules out of and into the nucleus is essential for gene expression and the proper functioning of a myriad of eukaryotic cellular activities. Nuclear transport is an energy-dependent process that occurs by way of the nuclear pore complex, a massive structure within the nuclear envelope that contains an aqueous, gated channel.
Transport is selective since only specific molecules enter or leave the nucleus.
Selectivity is achieved through interactions between a variety of specific molecular signals in the transported materials and an equivalent variety of soluble transport receptors that bind these signals and define different transport pathways. Nuclear import and export are also interdependent activities, since factors that move cargo into one compartment must be recycled back to the opposite compartment to begin the process anew. A GTPase called Ran maintains circularity and directionality by regulating the interactions between transport factors in the nucleus and cytoplasm. Interestingly, some transport pathways seem only to require Ran in a GTP-bound state while others also require GTP hydrolysis by Ran. The mechanistic basis for the difference in Ran function between pathways is presently unknown.