RANBP2 ASSOCIATES WITH UBC9P AND A MODIFIED FORM OF RANGAP1

Ran is a small GTPase required for nuclear transport in eukaryotic cel ls [Gorlich, D. & Mattaj, I. W. (1996) Science 271, 1513-1518]. Mutant s in Ran also show defects in mRNA processing, cell cycle regulation, and other aspects of nuclear function [Rush, M. G., Drivas, G. & D'Eus tachio, 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 d iverse processes, we previously characterized 10 Ran interacting prote ins (Rips) from Xenopus egg extracts. In this report, we present furth er 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 Ran GAP assays, the p340(RanBP2)-p88-p18 complex contains GTPase-activatin g protein activity, indicating that RanGAP1 is not inactivated by modi fication. Rather, modification of RanGAP1 appears to be linked to its association with p340(RanBP2) because we did not observe unmodified Ra nGAP1 in p340(RanBP2) immunoprecipitates. We have also characterized p 18, and we found that it is the Xenopus homologue of Ubc9p, an E2 ubiq uitin-conjugating enzyme that is required for cell cycle regulation [S eufert, W., Futcher, B. & Jentsch, S. (1995) Nature (London) 373, 78-8 1]. Using antibodies directed against Xenopus Ubc9p, we have confirmed that Ubc9p associates with p340(RanBP2) in Xenopus extracts. These re sults suggest Ubc9p's role in cell cycle regulation may involve either modification of nuclear transport substrates or the nuclear transport machinery.