GPA2P, A G-PROTEIN ALPHA-SUBUNIT, REGULATES GROWTH AND PSEUDOHYPHAL DEVELOPMENT IN SACCHAROMYCES-CEREVISIAE VIA A CAMP-DEPENDENT MECHANISM

The small GTP-binding protein Ras and heterotrimeric G-proteins are ke y regulators of growth and development in eukaryotic cells. In mammali an cells, Ras functions to regulate the mitogen-activated protein kina se pathway in response to growth factors, whereas many heterotrimeric GTP-binding protein alpha-subunits modulate cAMP levels through adenyl yl cyclase as a consequence of hormonal action. In contrast, in the ye ast Saccharomyces cerevisiae, it is the Ras1 and Ras2 proteins that re gulate adenylyl cyclase. Of the two yeast G-protein alpha-subunits (GP A1 and GPA2), only GPA1 has been well studied and shown to negatively regulate the mitogen-activated protein kinase pathway upon pheromone s timulation, In this report, we show that deletion of the GPA2 gene enc oding the other yeast G-protein alpha-subunit leads to a defect in pse udohyphal development, Also, the GPA2 gene is indispensable for normal growth in the absence of Ras2p. Both of these phenotypes can be rescu ed by deletion of the PDE2 gene product, which inactivates cAMP by cle avage, suggesting that these phenotypes can be attributed to low level s of intracellular cAMP. In support of this notion, addition of exogen ous cAMP to the growth media was also sufficient to rescue the phenoty pe of a GPA2 deletion strain, Taken together, our results directly dem onstrate that a G-protein alpha-subunit can regulate the growth and ps eudohyphal development of S. cerevisiae via a cAMP-dependent mechanism , Heterologous expression of mammalian G-protein alpha-subunits in the se yeast GPA2 deletion strains could provide a valuable tool for the m utational analysis of mammalian G-protein function in an in vivo null setting.