Implication of Pkc1p protein kinase C in sustaining Cln2p level and polarized bud growth in response to calcium signaling in Saccharomyces cerevisiae

Protein kinase C, a highly conserved signaling molecule among eukaryotes, has been implicated in the regulation of cellular processes such as cell proliferation and polarized growth. In Saccharomyces cerevisiae, the unique protein kinase C Pkc1p is thought to have multiple functions, including the activation of the Mpk1p (Slt2p) MAP kinase pathway, which is essential for cell wall construction and bud emergence. However, little is known about the other functions of Pkc1p. In the course of screening for the mutants that suppress the Ca²⁺-sensitivity phenotype of the Ca²⁺-sensitive strain zds, we isolated a novel mutant allele (scz6/pkc1-834) of PKC1. Unlike the previously characterized PKC1 allele stt1-1, heat-shock-induced Mpk1p activation and cell-wall integrity were not impaired in the pkc1-834 mutant. By contrast, the mutant was defective in the maintenance of Ca²⁺-induced F-actin polarization in a manner independent of Mpk1p activation. This phenotype was caused by a decreased expression level of the G₁ cyclin Cln2p. The Rho1 small G protein molecular switch was suggested to be involved in the novel Pkc1p function. The Pkc1p novel function was required for posttranscriptional upregulation of Cln2p and appeared to be important for the coordinated regulation of polar bud growth and the cell cycle.