Phosphorylation of the RAS2 Gene Product by Protein Kinase A Inhibits the Activation of Yeast Adenylyl Cyclase

The RAS2 gene product of Saccharomyces cerevisiae expressed in Escherichia coli was phosphorylated by protein kinase A in vitro to approximately 0.5-0.7 mol of phosphate per mol of protein. Neither protein kinase C nor protein kinase P phosphorylated the RAS2 protein significantly. The RAS2 protein is known to activate, in the presence of either Mg2+ and GTP or Mn2+, a yeast membrane preparation with an overexpressed adenylyl cyclase and a deficiency in endogenous RAS1 and RAS2 proteins. When the RAS2 protein was phosphorylated by protein kinase A prior to exposure to the yeast membranes, its capacity to activate the adenylyl cyclase was diminished by 40-60%, while activation by Mn2+ remained unaffected. The phosphorylated protein retained, however, its ability to bind GTP. Incubation of protein kinase A with a specific protein kinase A inhibitor prior to phosphorylation prevented the inhibition. Furthermore, the hydrolysis of GTP was not required for the observed inhibition. These data suggest that phosphorylation of the RAS2 gene product by protein kinase A may function as one mechanism by which the intracellular level of cAMP in yeast is regulated.