Expression of mammalian PKB partially complements deletion of the yeast protein kinase Sch9

摘要

The SCH9 protein kinase gene from Saccharomyces cerevisiae is known as a close homologue and a multicopy suppressor of mutants defective in the cAMP-dependent protein kinase genes TPKJ-3 [1], We show here that Sch9 is structurally and functionally more related to mammalian protein kinase B (PKB) than to the yeast Tpk's.Our earlier results [2] indicated that Sch9 is indispensable for the induction of ribosomal protein gene expression, the repression of STRE-controlled genes, the activation of trehalase and the resumption of growth, that occur after the re-addition of a nitrogen source to nitrogen-depleted cells. These phenomena did not occur or occurred in a deregulated way in an sch9A strain. We show that expression of mammalian PKB in such a strain partially overcomes this signalling defect.Sch9A strains expressing mammalian PKB showed a normal colony morphology and normal growth, in contrast to the small colony and slow growth phenotype of an sch9A strain. Nitrogen source-induced repression of the STRE-controlled genes SSA3, CTT1, and HSP12 was also largely restored by expression of PKB, whereas induction of the ribosomal protein gene RPL25 was not restored. Nitrogen source-induced activation of trehalase, which was absent in sch9A mutants, occurred to a variable extent and with kinetics different from the wild type in sch9A strains expressing mammalian PKB.Taken together, our results indicate that Sch9 might be the functional homologue of mammalian PKB in yeast and therefore regulated by similar mechanisms as those. Hence they suggest a nutrient-induced phosphoinositide-dependent pathway for the control of Sch9 and its targets in yeast.