A PAIR OF FUNCTIONALLY REDUNDANT YEAST GENES (PPZ1 AND PPZ2) ENCODING TYPE 1-RELATED PROTEIN PHOSPHATASES FUNCTION WITHIN THE PKC1-MEDIATED PATHWAY

The PKC1 gene of Saccharomyces cerevisiae encodes a homolog of mammalian protein kinase C that is required for yeast cell growth. Loss of PKC1 function results in cell lysis due to an inability to remodel the cell wall properly during growth. The PKC1 gene has been proposed to regulate a bifurcated pathway, on one branch of which function four putative protein kinases that catalyze a linear cascade of protein phosphorylation culminating in the activation of the mitogen-activated protein kinase homolog, Mpk1p. Here we describe two genes whose overexpression suppress both an mpk1DELTA mutation and a mpkc1DELT1A mutation. One of these genes is identical to the previously identified PPZ2 gene. The PPZ2 gene is predicted to encode a type 1-related protein phosphatase and is functionally redundant with a closely related gene, designated PPZ1. Deletion of both PPZ1 and PPZ2 resulted in a temperature-dependent cell lysis defect similar to that observed for bck1DELTA, mkk1,2DELTA, or mpk1DELTA mutants. However, ppz1,2DELTA mpk1DELTA triple mutants displayed a cell lysis defect at all temperatures. The additivity of the ppz1,2DELTA defect with the mpk1DELTA defect, combined with the results of genetic epistasis experiments, suggested either that the PPZ1 - and PPZ2-encoded protein phosphatases function on a branch of the PKC1-mediated pathway different from that defined by the protein kinases or that they play an auxiliary role in the pathway. The other suppressor gene, designated BCK2 (for bypass of C kinase), is predicted to encode a 92-kDa protein that is rich in serine and threonine residues. Genetic interactions between BCK2 and other pathway components suggested that BCK2 functions on a common pathway branch with PPZ1 and PPZ2.