Nuclear protein import: Ran-GTP dissociates the karyopherin αβ heterodimer by displacing a from an overlapping binding site on β

The α subunit of the karyopherin heterodimer functions in recognition of the protein import substrate and the β subunit serves to dock the trimeric complex to one of many sites on nuclear pore complex fibers. The small GTPase Ran and the Ran interactive protein, p10, function in the release of the docked complex. Repeated cycles of docking and release are thought to concentrate the transport substrate for subsequent diffusion into the nucleus. Ran-GTP dissociates the karyopherin heterodimer and forms a stoichiometric complex with Ran-GTP. Here we report the mapping of karyopherin β's binding sites both for Ran-GTP and for karyopherin α. We discovered that karyopherin β's binding site for Ran-GTP shows a striking sequence similarity to the cytoplasmic Ran-GTP binding protein, RanBP1. Moreover, we found that Ran-GTP and karyopherin α bind to overlapping sites on karyopherin β. Having a higher affinity to the overlapping site, Ran-GTP displaces karyopherin α and binds to karyopherin β. Competition for overlapping binding sites may be the mechanism by which GTP bound forms of other small GTPases function in corresponding dissociation-association reactions. We also mapped Ran's binding site for karyopherin β to a cluster of basic residues analogous to those previously shown to constitute karyopherin α's binding site to karyopherin β.