Importin α:: a multipurpose nuclear-transport receptor

The importin alpha/beta heterodimer targets hundreds of proteins to the nuclear-pore complex (NPC) and facilitates their translocation across the nuclear envelope. Importin a binds to classical nuclear localization signal (cNLS)-containing proteins and links them to importin 0, the karyopherin that ferries the ternary complex through the INK. A second karyopherin, the exportin CAS, recycles importin a back to the cytoplasm. In this article, we discuss control mechanisms that importin alpha exerts over the assembly and disassembly of the ternary complex and we describe how new groups of importin a genes arose during the evolution of metazoan animals to function in development and differentiation. We also describe activities of importin a that seem to be distinct from its housekeeping functions in nuclear transport.