A cAMP-dependent protein kinase A pathway controls pseudohyphal differentiation in Saccharomyces cerevisiae

Pseudohyphal differentiation in diploid cells of Saccharomyces cerevisiae is a cellular response to nitrogen limitation and fermentable carbon source. This is an excellent system for investigating the mechanisms by which cells sense and respond to their environment. S. cerevisiae also serves as a model system to dissect signaling pathways controlling morphogensis and virulence in human and plant fungal pathogens.; Here we show that in S. cerevisiae, the G protein-coupled receptor Gpr1 and the G protein alpha subunit Gpa2 are required for cAMP production in response to glucose. Gpr1 is also essential for yeast pseudohyphal differentiation. Consistent with previous observations that Gpa2, CAMP, and the high affinity cAMP phosphodiesterase Pde2 control filamentous growth, the cAMP-dependent protein kinase A is required for pseudohyphal differentiation. Strikingly, the three PKA catalytic subunits (Tpk1, 2, 3), which are redundant for vegetative growth, have distinct functions in filamentous growth: Tpk2 plays a positive role and activates filamentation, whereas the more distantly related Tpk1 and Tpk3 subunits repress pseudohyphal growth. The PKA and MAP kinase pathways converge to stimulate expression of the cell wall flocculin Flo11, which is required for cell-cell adhesion and filamentous growth. Genetic and biochemical data indicate that the Tpk2 catalytic subunit of PKA phosphorylates the transcriptional activator Flo8 and promotes its binding to a 250 by region of FLO11 promoter. In addition, Tpk2 phosphorylates the transcriptional repressor Sfl1 and inactivates its binding to the same DNA fragment, possibly by preventing the formation of Sfl1-Sfl1 homodimers.; Previous models had suggested that the transcription factors Sok2 and Phd1 might function downstream of PKA to control pseudohyphal growth. Here we show that Sok2 functions in a separate pathway and represses expression of Phd1 and Ash1, proteins required for cell elongation and expression of the FLO11 gene.; Taken together, this study has elucidated a conserved CAMP pathway that governs gene expression and cellular differentiation in S. cerevisiae . It has answered a basic scientific question about how yeast cells sense an extracellular signal (glucose) and respond to this stimulus by controlling gene expression and cellular differentiation, Moreover, information presented in this study will contribute to our understanding of how cAMP governs morphogenesis and pathogenicity in fungi infecting both human and plant.