Functional analyses of mammalian protein kinase C isozymes in budding yeast and mammalian fibroblasts

Background: The PKC1 gene of Saccharomyces cerevisiae encodes a homologue of mammalian protein kinase C (PKC) that is required for yeast cell growth. Pkc1 has been proposed to regulate a protein kinase cascade which includes the Bck1, Mkk1/Mkk2 and Mpk1 kinases. The functional relationship between Pkc1 and mammalian PKCs is unknown. Another signal transduction in Saccharomyces cerevisiae, the mating pheromone signalling pathway, is mediated by a heterotrimeric G protein, and causes cell cycle arrest in the G1 interval. It is not clear whether PKC is involved in this pathway. The effects of over-expression of PKCs in mammalian cells have been widely studied to analyse the function of PKCs in vivo. Results: We isolated a human cDNA which encodes a protein kinase C type η (PKC-η) by complementation of pkc1 mutations in Saccharomyces cerevisiae. The human PKC-η was able to complement the growth defect caused by the deletion of PKC1, whereas PKC-η was unable to suppress the defect caused by deletion of BCK1. We also isolated human cDNAs that can suppress the adaptation defect of sst2. One of them encodes a protein kinase C type δ (PKC-δ). Expression of this gene in yeast stimulated an adaptation to the pheromone response. Human PKC-δ suppressed the adaptation defect of a pheromone receptor mutation lacking its C-terminal domain, but not that of a G protein β-subunit mutation eliminating signal-induced phosphorylation, and not the lethality of the gpa1 null mutation. Moreover, overexpression of PKC-η in NIH3T3 cells induced anchorage-independent growth. Conclusions: PKC-η has a biological activity which is closely related to Pkc1, and PKC-η activates the Pkc1-mediated pathway through an activation of the Bck1 kinase that is a homologue of MAP kinase kinase kinase. PKC-η appears to play a critical role in growth control of yeast and mammalian cells. Suppression experiments with PKC-δ suggest that PKC-δ desensitizes the pathway by regulating an aspect of G protein function.