A mouse CDC25-like product enhances the formation of the active GTP complex of human ras p21 and Saccharomyces cerevisiae RAS2 proteins

GDP-dissociation stimulators (GDSs) are the key element for the regeneration of the active state of ras proteins, but despite intensive investigations, little is so far known about their functional and structural properties, particularly in mammals. A growing number of genes from various organisms have been postulated to encode GDSs on the basis of sequence similarity with the Saccharomyces cerevisiae CDC25 gene, whose product acts as a GDS of RAS proteins. However, except for CDC25 and the related SDC25 C-domain, no biochemical evidence of ras GDS activity for these CDC25-like proteins has yet been available. We show that the product of a recently isolated mouse CDC25-like gene (CDC25Mm) can strongly enhance (more than 1000 times) the GDP release from both human c-Ha-ras p21 and yeast RAS2 in vitro. As a consequence, the CDC25Mm induces a rapid formation of the biologically active Ras.GTP complex. This GDS is much more active on the GDP than on the GTP complex and has a narrow substrate specificity, since it was found to be inactive on several ras-like proteins. The mouse GDS can efficiently substitute for yeast CDC25 in an in vitro adenylylcyclase assay on RAS2 cdc25 yeast membranes. Our results show that a cloned GDP to GTP exchange factor of mammalian ras belongs to the novel family of CDC25-like proteins.