RA-GEF, a novel Rap1A guanine nucleotide exchange factor containing a Ras/Rap1A-associating domain, is conserved between nematode and humans

A yeast two-hybrid screening for Ras-binding proteins in nematode Caenorhab ditis elegans has identified a guanine nucleotide exchange factor (GEF) con taining a Ras/Rap1A-associating (RA) domain, termed Ce-RA-GEF. Both Ce-RA-G EF and its human counterpart Hs-RA-GEF possessed a PSD-95/DlgA/ZO-1 (PDZ) d omain and a Ras exchanger motif (REM) domain in addition to the RA and GEF domains. They also contained a region homologous to a cyclic nucleotide mon ophosphate-binding domain, which turned out to be incapable of binding cAMP or cGMP. Although the REM and GEF domains are conserved with other GEFs ac ting on Ras family small GTP-binding proteins, the RA and PDZ domains are u nseen in any of them. Hs-RA-GEF exhibited not only a GTP-dependent binding activity to Rap1A at its RA domain but also an activity to stimulate GDP/GT P exchange of Rap1A both in vitro and in, vivo at the segment containing it s REM and GEF domains. However, it did not exhibit any binding or GEF activ ity toward Ras. On the other hand, Ce-RA-GEF associated with and stimulated GDP/GTP exchange of both Ras and Rap1A These results indicate that Ce-RA-G EF and Hs-RA-G;EF define a novel class of Rap1A GEF molecules, which are co nserved through evolution.