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XMog1, a nuclear Ran-binding protein in Xenopus, is a functional homologue of Schizosaccharomyces pombe Mog1p that co-operates with RanBP1 to control generation of Ran-GTP

Francisco J. Nicolás^1,*, William J. Moore¹, Chuanmao Zhang^1,2 and Paul R. Clarke^1,

¹ Biomedical Research Centre, University of Dundee, Level 5, Ninewells Hospital and Medical School, Dundee, DD1 9SY, UK
² Department of Cell Biology and Genetics, College of Life Sciences, Peking University, Beijing 100871, China
^* Present address: ICRF Laboratories, 44 Lincoln's Inn Fields, London, WC2A 3PX, UK
Author for correspondence (e-mail: p.clarke{at}icrf.icnet.uk )

Accepted May 18, 2001

Ran is a multifunctional small GTPase of the Ras superfamily that plays roles in nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. By screening a Xenopus oocyte cDNA library for Ran-GTP-binding proteins using the two-hybrid system of co-expression in yeast, we identified XMog1, a 20.4 kDa polypeptide related to Mog1p in Saccharomyces cerevisiae and similar gene products in Schizosaccharomyces pombe, Arabidopsis and mammals. We show that cDNAs encoding XMog1 and S. cerevisiae Mog1p rescue the growth defect of S. pombe cells lacking mog1, demonstrating conservation of their functions. In Xenopus somatic cells and transfected mammalian cells, XMog1 is localised to the nucleus. XMog1 alone does not stimulate Ran GTPase activity or nucleotide exchange, but causes nucleotide release from Ran-GTP and forms a complex with nucleotide-free Ran. However, in combination with Ran-binding protein 1 (RanBP1), XMog1 promotes the release of GDP and the selective binding of GTP to Ran. XMog1 and RanBP1 also promote selective GTP loading onto Ran catalysed by the nuclear guanine nucleotide exchange factor, RCC1. We propose that Mog1-related proteins, together with RanBP1, facilitate the generation of Ran-GTP from Ran-GDP in the nucleus.

Key words: Ran, RCC1, Nucleocytoplasmic transport, Xenopus egg extracts

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