SACCHAROMYCES-CEREVISIAE CDC25(1028-1589) IS A GUANINE-NUCLEOTIDE RELEASING-FACTOR FOR MAMMALIAN RAS PROTEINS AND IS ONCOGENIC IN NIH3T3 CELLS

The best characterized yeast guanine nucleotide releasing factor is CD C25, which acts on RAS and thereby stimulates cAMP production in Sacch aromyces cerevisiae. In order to determine if CDC25 could be a specifi c GDP-GTP releasing factor for the mammalian proteins Ha-ras, Ki-ras, and N-ras, its functions were studied both in vitro and in NIH3T3 cell s. The 561 amino acids composing the C-terminal domain of CDC25 (CDC25 C-domain) released guanine nucleotides (both GDP and GTP) from Ha-, K i-, and N-ras but not from Rap1A, Rab5, and Rab11. CDC25 acted on onco genically activated Ha-ras even if the last 23 amino acids (167-189) o f the Ras proteins were not present. CDC25 transformed NIH3T3 cells; i ts transforming capacity was enhanced by overexpression of wild-type H a-ras. CDC25 C-domain probably exerts its effects through the activati on of cellular Ras proteins. These data suggest that the CDC25 C-domai n can function as an upstream activator of Ras proteins in a heterolog ous system and therefore could be a useful tool to study the regulatio n of Ras activation by growth factor receptors.