Relationship between protein kinase C and derepression of different enzymes.

The PKC1 gene in the yeast Saccharomyces cerevisiae encodes for protein kinase C which is known to control a MAP kinase cascade consisting of different kinases: Bck1, Mkk1 and Mkk2, and Mpk1. This cascade affects the cell wall integrity but the phenotype of pkc1Delta mutants suggests additional targets that have not yet been identified [Heinisch et al., Mol. Microbiol. 32 (1999) 671-680]. The pkc1Delta mutant, as opposed to other mutants in the MAP kinase cascade, displays defects in the control of carbon metabolism. One of them occurs in the derepression of SUC2 gene after exhaustion of glucose from the medium, suggesting an involvement of Pkc1p in the derepression process that is not shared by the downstream MAP kinase cascade. In this work, we demonstrate that Pkc1p is required for the increase of the activity of enzymatic systems during the derepression process. We observed that Pkc1p is involved in the derepression of invertase and alcohol dehydrogenase activities. On the other hand, it seems not to be necessary for the derepression of the enzymes of the GAL system. Our results suggest that Pkc1p is acting through the main glucose repression pathway, since introduction of an additional mutation in the PKC1 gene in yeast strains already presenting mutations in the HXKII or MIG1 genes does not interfere with the typical derepressed phenotype observed in these single mutants. Moreover, our data indicate that Pkc1p participates in this process through the control of the cellular localization of the Mig1 transcriptional factor.

Evidence for involvement of Saccharomyces cerevisiae protein kinase C in glucose induction of HXT genes and derepression of SUC2.

The PKC1 gene in the yeast Saccharomyces cerevisiae encodes protein kinase C that is known to control a mitogen-activated protein (MAP) kinase cascade consisting of Bck1, Mkk1 and Mkk2, and Mpk1. This cascade affects the cell wall integrity but the phenotype of Pkc1 mutants suggests additional targets which have not yet been identified. We show that a pkc1Delta mutant, as opposed to mutants in the MAP kinase cascade, displays two major defects in the control of carbon metabolism. It shows a delay in the initiation of fermentation upon addition of glucose and a defect in derepression of SUC2 gene after exhaustion of glucose from the medium. After addition of glucose the production of both ethanol and glycerol started very slowly. The V(max) of glucose transport dropped considerably and Northern blot analysis showed that induction of the HXT1, HXT2 and HXT4 genes was strongly reduced. Growth of the pkc1Delta mutant was absent on glycerol and poor on galactose and raffinose. Oxygen uptake was barely present. Derepression of invertase activity and SUC2 transcription upon transfer of cells from glucose to raffinose was deficient in the pkc1Delta mutant as opposed to the wild-type. Our results suggest an involvement of Pkc1p in the control of carbon metabolism which is not shared by the downstream MAP kinase cascade.