The isolated catalytic hairpin of the Ras-specific guanine nucleotide exchange factor Cdc25Mm retains nucleotide dissociation activity but has impaired nucleotide exchange activity

Cdc25(Mm) is a mammalian Ras-specific guanine nucleotide exchange factor (GEF). By homology modeling we show that it shares with Sos-GEF the structure of the putative catalytic HI hairpin where the dominant negative T1184E mutation is located. Similarly to Cdc25(MnT1184E), the isolated wild-type and mutant hairpins retain the ability to displace Ras-bound nucleotide, originate a stable Ras/GEF complex and downregulate the Ras pathway in vivo. These results indicate that nucleotide re-entry and Ras/GEF dissociation - final steps in the GEF catalytic cycle - require GEF regions different from the HI hairpin. GEF down-sizing could lead to development of novel Ras inhibitors. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.