CHARACTERIZATION OF PKC2, A GENE ENCODING A 2ND PROTEIN-KINASE-C TYPEOF SACCHAROMYCES-CEREVISIAE

Background: Protein kinase C (PKC) has attracted considerable attentio n over the past decade, primarily because of its presumed role in cell ular growth control and tumourigenesis. Mammalian cells express at lea st 10 different isozymes of PKC; iris this complexity that has made el ucidating the precise functions of PKC so difficult. The identificatio n of PKC homologues in organisms such as Drosophila, Xenopus, Dictyost elium, Aplysia and Caenorhabditis indicates that the enzyme is evoluti onarily conserved, and this has stimulated our search for counterparts in the yeast Saccharomyces cerevisiae, in which powerful genetic anal yses can be used. To date, only one PKC homologue, PKC1, has been iden tified in yeast and no biochemical activity has been definitively ascr ibed 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 a ttempt to identify further homologues. Results: We have identified a n ovel 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 P KC2, originated from a common ancestor. Disruption of the PKC2 protein -coding region, deleting the entire catalytic domain of the encoded en zyme, is not lethal to yeast growing on rich media. However, the pkc2 mutant, unlike wild-type strains, fails to grow on minimal media conta ining limited concentrations of amino acids. This implicates Pkc2p in the response of yeast cells to amino-acid starvation. Conclusion: We h ave shown that yeast cells do express more than one PKC isozyme, by id entifying and characterizing a novel PKC gene PKC2, the product of whi ch may be involved in the cellular response to amino-acid starvation.