PDK1 homologs activate the Pkc1-mitogen-activated protein kinase pathway in yeast

PDK1 (phosphoinositide-dependent kinase 1) is a mammalian growth factor-reg ulated serine/threonine kinase. Using a genetic selection based on a mutant form of the yeast MAP kinase kinase Ste7, we isolated a gene, PKH2, encodi ng a structurally and functionally conserved yeast homolog of PDK1. Yeast c ells lacking both PKH2 and PKH1, encoding another PDK1 homolog, were nonvia ble, indicating that Pkh1 and Pkh2 share an essential function. A temperatu re-sensitive mutant, pkh1(D398G) pkh2, was phenotypically similar to mutant s defective in the Pkc1-mitogen-activated protein kinase (MAPK) pathway. Ge netic epistasis analyses, the phosphorylation of Pkc1 by Pkh2 in vitro, and reduced Pkc1 activity in the pkh1(D398G) pkh2 mutant indicate that Pkh fun ctions upstream of Pkc1. The Pkh2 phosphorylation site in Pkc1 (Thr-983) is part of a conserved PDK1 target motif and essential for Pkc1 function. Thu s, the yeast PDK1 homologs activate Pkc1 and the Pkc1-effector MAPK pathway .