THE YAK1 PROTEIN-KINASE OF SACCHAROMYCES-CEREVISIAE MODERATES THERMOTOLERANCE AND INHIBITS GROWTH BY AN SCH9 PROTEIN KINASE-INDEPENDENT MECHANISM

The growth defect associated with the loss of yeast A kinase activity can be alleviated by the overexpression or deletion of two other kinas es, Sch9 and Yak1, respectively. Using tests of epistasis, we have sho wn that Sch9 and Yak1 define separate signaling pathways and must, the refore, suppress the A kinase defect by different mechanisms. Neverthe less, the Yak1 kinase appears to regulate cellular processes that are under A kinase control. For example, acquisition of heat resistance is correlated with Yak1 kinase activity, such that YAK1-overexpressing c ells are over 200-fold more resistant than isogenic yak1 strains. Thes e results, for the first time, associate a phenotype, other than suppr ession of the A kinase growth defect, with the loss of Yak1 activity a nd argue a broader role for the Yak1 kinase in cell growth.