PDK1 Homologs Activate the Pkc1–Mitogen-Activated Protein Kinase Pathway in Yeast

PDK1 (phosphoinositide-dependent kinase 1) is a mammalian growth factor-regulated serine/threonine kinase. Using a genetic selection based on a mutant form of the yeast MAP kinase kinase Ste7, we isolated a gene,PKH2 , encoding a structurally and functionally conserved yeast homolog of PDK1. Yeast cells lacking bothPKH2 andPKH1 , encoding another PDK1 homolog, were nonviable, indicating that Pkh1 and Pkh2 share an essential function. A temperature-sensitive mutant,pkh1D398G pkh2, was phenotypically similar to mutants defective in the Pkc1–mitogen-activated protein kinase (MAPK) pathway. Genetic epistasis analyses, the phosphorylation of Pkc1 by Pkh2 in vitro, and reduced Pkc1 activity in thepkh1D398G pkh2mutant indicate that Pkh functions upstream of Pkc1. The Pkh2 phosphorylation site in Pkc1 (Thr-983) is part of a conserved PDK1 target motif and essential for Pkc1 function. Thus, the yeast PDK1 homologs activate Pkc1 and the Pkc1-effector MAPK pathway.