Protein Kinase A Contributes to the Negative Control of Snf1 Protein Kinase in Saccharomyces cerevisiae

Snf1 protein kinase regulates responses to glucose limitation and other stresses. Snf1 activation requires phosphorylation of its T-loop threonine by partially redundant upstream kinases (Sak1, Tos3, and Elm1). Under favorable conditions, Snf1 is turned off by Reg1-Glc7 protein phosphatase. Thereg1 mutation causes increased Snf1 activation and slow growth. To identify new components of the Snf1 pathway, we searched for mutations that, likesnf1 , suppressreg1 for the slow-growth phenotype. In addition to mutations in genes encoding known pathway components (SNF1 ,SNF4 , andSAK1 ), we recovered “fast” mutations, designatedfst1 andfst2 . Unusual morphology of the mutants in the Σ1278b strains employed here helped us identifyfst1 andfst2 as mutations in the RasGAP genesIRA1 andIRA2 . Cells lacking Ira1, Ira2, or Bcy1, the negative regulatory subunit of cyclic AMP (cAMP)-dependent protein kinase A (PKA), exhibited reduced Snf1 pathway activation. Conversely, Snf1 activation was elevated in cells lacking the Gpr1 sugar receptor, which contributes to PKA signaling. We show that the Snf1-activating kinase Sak1 is phosphorylatedin vivo on a conserved serine (Ser1074) within an ideal PKA motif. However, this phosphorylation alone appears to play only a modest role in regulation, and Sak1 is not the only relevant target of the PKA pathway. Collectively, our results suggest that PKA, which integrates multiple regulatory inputs, could contribute to Snf1 regulation under various conditions via a complex mechanism. Our results also support the view that, like its mammalian counterpart, AMP-activated protein kinase (AMPK), yeast Snf1 participates in metabolic checkpoint control that coordinates growth with nutrient availability.