How Many Pathways Are Available for Nuclear Protein Import in Cells.

Abstract

In eukaryotic cells, functional macromolecules such as protein and RNA traverse the nuclear envelope, thus permitting communication between the nucleus and the cytoplasm. The molecular mechanism of the nucleocytoplasmic transport has been extensively studied and considerable knowledge concerning its fundamental machinery has been accumulated. That is, it has been shown that importins (karyopherins) function as a receptor/carrier molecule of nuclear localization signal (NLS)-containing karyophilic proteins and that a small GTPase Ran acts to assure the directionality of the transport through the nuclear pore complexes (NPCs). In addition, the subject of how many transport factors/pathways exist in cells to carry a variety of karyhophiles into the nucleus has been examined. In this article, we focus on the molecular divergence of the nucleocytoplasmic transport pathways and review some recent findings on the issue of multiplicity, which has been examined using intracellular signaling molecules such as STAT (signal transducer and activator of transcription) and Smad as transport substrates.