Ran is a small GTPase that is required for protein import, mRNA export, and the maintenance of nuclear structures. To gain a better understanding of Ran's role in the nucleus, we have sought to use Xenopus egg extracts for the purification and characterization of proteins from egg extracts bound with a high affinity to a glutathione-S-transferase-Ran fusion protein (GST-Ran). We found that GST-Ran associates specifically with at least 10 extract proteins. We determined the identifies of six Ran-interacting proteins (Rips), and found that they include RanBP2/Nup358, Nup153, Importin beta, hsc70, RCC1, and RanBP1. On the basis of peptide sequence, a seventh Rip (p88) seems to be similar but not identical to Fug1/RanGAP1, the mammalian Ran-GTPase-activating protein. Gel filtration analysis of endogenous extract proteins suggests that Importin beta acts as a primary GTP-Ran effector. Both Ran and Importin beta are coimmunoprecipitated by anti-p340RanBP2 antibodies in the presence of nonhydrolyzable GTP analogues, suggesting that Ran-Importin beta complexes interact with p340RanBP2. Two other Rips, p18 and p88, are coprecipitated with p340RanBP2 in a nucleotide-independent manner. Analysis of the Ran-GTPase pathway in Xenopus extracts allows the examination of interactions between Ran-associated proteins under conditions that resemble in vivo conditions more closely than in assays with purified components, and it thereby allows additional insights into the molecular mechanism of nuclear transport.
Importin alpha can migrate into the nucleus in an importin beta- and Ran-independent manner
