RanBP2 associates with Ubc9p and a modified form of RanGAP1

Ran is a small GTPase required for nuclear transport in eukaryotic cells [Gorlich, D. & Mattaj, I. W. (1996) Science 271, 1513-1518]. Mutants in Ran also show defects in mRNA processing, cell cycle regulation, and other aspects of nuclear function [Rush, M. G., Drivas, G. & D'Eustachio, P. (1996) BioEssays 18, 103-112; Sazer, S. (1996) Trends Cell Biol. 6, 81-85]. In an effort to understand the role of Ran in these diverse processes, we previously characterized 10 Ran interacting proteins (Rips) from Xenopus egg extracts. In this report, we present further characterization of a complex containing three of these Rips: p340(RanBP2), p88, and p18. We have cloned the Xenopus homologue of RanGAP1, and we show here that p88 is a modified form of this protein. In RanGAP assays, the p340(RanBP2)-p88-p18 complex contains GTPase-activating protein activity, indicating that RanGAP1 is not inactivated by modification. Rather, modification of RanGAP1 appears to be linked to its association with p340(RanBP2) because we did not observe unmodified RanGAP1 in p340(RanBP2) immunoprecipitates. We have also characterized p18, and we found that it is the Xenopus homologue of Ubc9p, an E2 ubiquitin-conjugating enzyme that is required for cell cycle regulation [Seufert, W., Futcher, B. & Jentsch, S. (1995) Nature (London) 373, 78-81]. Using antibodies directed against Xenopus Ubc9p, we have confirmed that Ubc9p associates with p340(RanBP2) in Xenopus extracts. These results suggest Ubc9p's role in cell cycle regulation may involve either modification of nuclear transport substrates or the nuclear transport machinery.