Molecular Mechanisms of Nuclear Protein Transport

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Abstract
Transport of proteins into and out of the nucleus occurs through nuclear pore complexes (NPC). A heterodimeric protein complex, composed of karyopherin a and β (or importin α and β) functions to target proteins containing a nuclear localization sequence (NLS) to the NPCs. Two additional proteins, the GTPase Ran and p10, are required to translocate the docked NLS protein into the nucleus. The α subunit of karyopherin functions as the NLS receptor, whereas the β subunit mediates docking to nucleoporins that contain peptide repeats. During import the karyopherin heterodimer dissociates: karyopherin α and import substrates enter and accumulate in the nucleoplasm, whereas karyopherin β accumulates at the nuclear pore complex. Ran-GTP induces dissociation of karyopherin α from β by forming a complex with karyopherin β and promotes the release of both karyopherin subunits from a docking site. Protein transport across the NPC may occur via guided diffusion involving the karyopherin-mediated docking and undocking of import substrate to multiple binding sites that extend from the cytoplasmic to the nucleoplasmic ends of the NPC.

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