Phosphorylation of the S. cerevisiae Cdc25 in response to glucose results in its dissociation from Ras.

In the yeast Sacchromyces cerevisiae, addition of glucose to starved cells triggers a transient rise in the intracellular level of cyclic AMP that induces a protein phosphorylation cascade. The glucose signal is processed by the Cdc25/Ras/adenylyl cyclase pathway, where the role of Cdc25 is to catalyse the GDP-GTP exchange on Ras. The molecular mechanisms involved in the regulation of the activity of Cdc25 are unknown. We report here the use of highly selective anti-Cdc25 antibodies to demonstrate that Cdc25 is a phospho protein and that in response to glucose it is hyperphosphorylated, within seconds, by the cyclic AMP-dependent protein kinase. It is also demonstrated that, concomitantly with hyperphosphorylation, Cdc25 partially relocalizes to the cytoplasm, reducing its accessibility to membrane-bound Ras. These results are of general significance because of the highly conserved sequence of Ras-guanyl nucleotide exchange factors from yeasts to mammals.