Characterization of PKC2, a gene encoding a second protein kinase C isotype of Saccharomyces cerevisiae

Protein kinase C (PKC) has attracted considerable attention over the past decade, primarily because of its presumed role in cellular growth control and tumourigenesis. Mammalian cells express at least 10 different isozymes of PKC; it is this complexity that has made elucidating the precise functions of PKC: so difficult. The identification of PKC homologues in organisms such as Drosophila, Xenopus, Dictyostelium, Aplysia and Caenorhabditis indicates that the enzyme is evolutionarily conserved, and this has stimulated our search for counterparts in the yeast Saccharomyces cerevisiae, in which powerful genetic analyses can be used. To date, only one PKC homologue, PKC1, has been identified in yeast and no biochemical activity has been definitively ascribed to the encoded protein. This, and the inability to identify other PKC homologues in yeast by DNA hybridization, has led to doubts about the existence of PKC isozymes in yeast. We have taken the approach of screening yeast expression libraries with anti-PKC antibodies in an attempt to identify further homologues.

We have identified a novel PKC isozyme, Pkc2p, encoded by the gene PKC2. We report here the sequence of PKC2 and a comparison showing its similarity to other PKCs. Phylogenetic analysis suggests that all known PKC genes, including PKC2, originated from a common ancestor. Disruption of the PKC2 protein-coding region, deleting the entire catalytic domain of the encoded enzyme, is not lethal to yeast growing on rich media. However, the pkc2 mutant, unlike wild-type strains, fails to grow on minimal media containing limited concentrations of amino acids. This implicates Pkc2p in the response of yeast cells to amino-acid starvation.

We have shown that yeast cells do express more than one PKC isozyme, by identifying and characterizing a novel PKC gene PKC2, the product of which may be involved in the cellular response to amino-acid starvation.