INVOLVEMENT OF DISTINCT G-PROTEINS, GPA2 AND RAS, IN GLUCOSE-INDUCED AND INTRACELLULAR ACIDIFICATION-INDUCED CAMP SIGNALING IN THE YEAST SACCHAROMYCES-CEREVISIAE

Adenylate cyclase activity in Saccharomyces cerevisiae is dependent on Ras proteins. Both addition of glucose to glucose-deprived (derepress ed) cells and intracellular acidification trigger an increase in the c AMP level in vivo, We show that intracellular acidification, but not g lucose, causes an increase in the GTP/GDP ratio on the Ras proteins in dependent of Cdc25 and Sdc25. Deletion of the GTPase-activating protei ns Ira1 and Ira2, or expression of the RAS2(val19) allele, causes an e nhanced GTP/GDP basal ratio and abolishes the intracellular acidificat ion-induced increase. In the iral Delta ira2 Delta strain, intracellul ar acidification still triggers a cAMP increase. Glucose also did not cause an increase in.the GTP/GDP ratio in a strain with reduced feedba ck inhibition of cAMP synthesis. Further investigation indicated that feedback inhibition by cAPK on cAMP synthesis acts independently of ch anges in the GTP/GDP ratio on Ras. Stimulation by glucose was dependen t on the G alpha-protein Gpa2, whose deletion confers the typical phen otype associated with a reduced cAMP level: higher heat resistance, a higher level of trehalose and glycogen and elevated expression of STRE -controlled genes. However, the typical fluctuation in these character istics during diauxic growth on glucose was still present. Overexpress ion of Ras2(val19) inhibited both the acidification- and glucose-induc ed cAMP increase even in a protein kinase A-attenuated strain. Our res ults suggest that intracellular acidification stimulates cAMP synthesi s in vivo at least through activation of the Ras proteins, while gluco se acts through the Gpa2 protein. Interaction of Ras2(val19),vith aden ylate cyclase apparently prevents its activation by both agonists.