Alterations at Dispersed Sites Cause Phosphorylation and Activation of SNF1 Protein Kinase during Growth on High Glucose

The SNF1/AMP-activated protein kinases are central energy regulators in eukaryotes. SNF1 of Saccharomyces cerevisiae is inhibited during growth on high levels of glucose and is activated in response to glucose depletion and other stresses. Activation entails phosphorylation of Thr(210) on the activation loop of the catalytic subunit Snf1 by Snf1-activating kinases. We have used mutational analysis to identify Snf1 residues that are important for regulation. Alteration of Tyr(106) in the alpha C helix or Leu(198) adjacent to the Asp-Phe-Gly motif on the activation loop relieved glucose inhibition of phosphorylation, resulting in phosphorylation of Thr(210) during growth on high levels of glucose. Substitution of Arg for Gly(53), at the N terminus of the kinase domain, increased activation on both high and low glucose. Alteration of the ubiquitin-associated domain revealed a modest autoinhibitory effect. Previous studies identified alterations of the Gal83 (beta) and Snf4 (gamma) subunits that relieve glucose inhibition, and we have here identified a distinct set of Gal83 residues that are required. Together, these results indicate that alterations at dispersed sites within each subunit of SNF1 cause phosphorylation of the kinase during growth on high levels of glucose. These findings suggest that the conformation of the SNF1 complex is crucial to maintenance of the inactive state during growth on high glucose and that the default state for SNF1 is one in which Thr(210) is phosphorylated and the kinase is active.