The 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. All nuclear transport 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 molecular signals in the transported materials and an equivalent variety of soluble transport receptors that bind these signals. 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 interactions between transport factors in the nucleus and cytoplasm. In animal cells, SRP consists of a 300 nucleotide RNA and six SRP polypeptides. Figure courtesy of Dr. Christian Zwieb, Univ.