THE C-TERMINAL PART OF THE CDC25 GENE-PRODUCT HAS RAS-NUCLEOTIDE EXCHANGE ACTIVITY WHEN PRESENT IN A CHIMERIC SDC25-CDC25 PROTEIN

The CDC25 gene from S. cerevisiae encodes an activator of Ras proteins . The C-terminal part of a structurally-related protein encoded by the SDC25 gene is characterised by a Ras-guanine nucleotide exchange acti vity in vitro whereas the C-terminal part of CDC25 gives no detectable exchange activity. A chimera between the 3' regions of these two gene s was constructed by homeologous recombination. This chimeric gene sup presses cdc25 mutations. When expressed in E. coli, the chimeric produ ct is detectable by antibodies directed against the carboxy-terminal C DC25 peptide and has an exchange-factor activity on the Ras2 protein. Therefore, the carboxy-terminal parts of both the CDC25 and the SDC25 gene products are structurally and functionally similar. The CDC25 par t of the chimeric protein contains an intrinsic guanine exchange facto r which does not require an additional cofactor.