Snf1 Promotes Phosphorylation of the α Subunit of Eukaryotic Translation Initiation Factor 2 by Activating Gcn2 and Inhibiting Phosphatases Glc7 and Sit4

Snf1 is the ortholog of mammalian AMP-activated kinase and is responsible for activation of glucose-repressed genes at low glucose levels in budding yeast. We show that Snf1 promotes the formation of phosphorylated alpha subunit of eukaryotic translation initiation factor 2 (eIF2 alpha-P), a regulator of general and gene-specific translation, by stimulating the function of eIF2 alpha kinase Gcn2 during histidine starvation of glucose-grown cells. Thus, eliminating Snf1 or mutating its activation loop lowers Gcn2 kinase activity, reducing the autophosphorylation of Thr-882 in the Gcn2 activation loop, and decreases eIF2 alpha-P levels in starved cells. Consistently, eliminating Reg1, a negative regulator of Snf1, provokes Snf1-dependent hyperphosphorylation of both Thr-882 and eIF2 alpha. Interestingly, Snf1 also promotes eIF2 alpha phosphorylation in the nonpreferred carbon source galactose, but this occurs by inhibition of protein phosphatase 1 alpha (PP1 alpha; Glc7) and the PP2A-like enzyme Sit4, rather than activation of Gcn2. Both Glc7 and Sit4 physically interact with eIF2 alpha in cell extracts, supporting their direct roles as eIF2 alpha phosphatases. Our results show that Snf1 modulates the level of eIF2 alpha phosphorylation by different mechanisms, depending on the kind of nutrient deprivation existing in cells.