GUANINE-NUCLEOTIDE EXCHANGE FACTORS - A FAMILY OF POSITIVE REGULATORSOF RAS AND RELATED GTPASES

The past year has seen remarkable advances in our understanding of how members of the Ras superfamily of GTPases are activated. Not only hav e new guanine-nucleotide exchange factors (GEFs) that activate these G TPases been identified, but in some cases, the mechanisms by which GEF s are connected to events at the cell surface have also been described . We have also come to appreciate the possibilities that GEFs may be m ultifunctional and that they may contribute to tissue-specific regulat ion of their cognate GTPase.