Cyclic AMP-dependent protein kinase regulates pseudohyphal differentiationin Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the yeast Saccharomyce s cerevisiae differentiate to a filamentous growth form known as pseudohyph al differentiation. Filamentous growth is regulated by elements of the pher omone mitogen-activated protein (MAP) kinase cascade and a second signaling cascade involving the receptor Gpr1, the G alpha protein Gpa2, Ras2, and c yclic AMP (cAMP). We show here that the Gpr1-Gpa2-cAMP pathway signals via the cAMP-dependent protein kinase, protein kinase A (PKA), to regulate pseu dohyphal differentiation. Activation of PKA by mutation of the regulatory s ubunit Bcy1 enhances filamentous growth. Mutation and overexpression of the PKA catalytic subunits reveal that the Tpk2 catalytic subunit activates fi lamentous growth, whereas the Tpk1 and Tpk3 catalytic subunits inhibit fila mentous growth. The PKA pathway regulates unipolar budding and agar invasio n, whereas the MAP kinase cascade regulates cell elongation and invasion. E pistasis analysis supports a model in which PKA functions downstream of the Gpr1 receptor and the Gpa2 and Ras2 G proteins. Activation of filamentous growth by PKA does not require the transcription factors Ste12 and Tec1 of the MAP kinase cascade, Phd1, or the PKA targets Msn2 and Msn4. PKA signals pseudohyphal growth, in part, by regulating Flo8-dependent expression of t he cell surface flocculin Flo11. In summary, the cAMP-dependent protein kin ase plays an intimate positive and negative role in regulating filamentous growth, and these findings may provide insight into the roles of PKA in mat ing, morphogenesis, and virulence in other yeasts and pathogenic fungi.