Disassembly of RanGTP-karyopherin beta complex, an intermediate in nuclear protein import

We previously showed that RanGTP forms a 1:1 complex with karyopherin beta that renders RanGTP inaccessible to RanGAP (Floer, M., and Blobel, G. (1996) J. Biol. Chem. 271, 5313-5316) and karyopherin beta functionally inactive (Rexach, M., and Blobel, G. (1995) Cell 83, 683-692). Recycling of both factors for another round of function requires dissociation of the RanGTP-karyopherin beta complex. Here we show using BIAcoreTM, a solution binding assay, and GTP hydrolysis and exchange assays, with yeast proteins, that karyopherin beta and RanGTP are recycled efficiently in a reaction that involves karyopherin alpha, RanBP1, RanGAP, and the C terminus of the nucleoporin Nup1. We find that karyopherin alpha first releases RanGTP from karyopherin beta in a reaction that does not require GTP hydrolysis. The released RanGTP is then sequestered by RanBP1, and the newly formed karyopherin alphabeta binds to the C terminus of Nup1. Finally, RanGTP is converted to RanGDP via nucleotide hydrolysis when RanGAP is present. Conversion of RanGTP to RanGDP can also occur via nucleotide exchange in the presence of RanGEF, an excess of GDP, and if RanBP1 is absent. Additional nucleoporin domains that bind karyopherin alphabeta stimulate recycling of karyopherin beta and Ran in a manner similar to the C terminus of Nup1.