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Background: The yeast SNF1 protein kinase and the
mammalian AMP-activated protein kinase are highly conserved
heterotrimeric complexes that are “metabolic master
switches” involved in the switch from fermentative/anaerobic to
oxidative metabolism. They are activated by cellular stresses that
deplete cellular ATP, and SNF1 is essential in the response to glucose
starvation. In both cases, activation requires phosphorylation at a
conserved threonine residue within the activation loop of the kinase
domain, but identifying the upstream kinase(s) responsible for this has
been a challenging, unsolved problem. Results: Using a library of strains that express 119 yeast protein kinases as GST fusions, we identified Elm1p as the sole kinase that could activate the kinase domain of AMP-activated protein kinase in vitro. Elm1p also activated the purified SNF1 complex, and this correlated with phosphorylation of Thr210 in the activation loop. Removal of the C-terminal domain increased the Elm1p kinase activity, indicating that it is auto-inhibitory. Expression of activated, truncated Elm1p from its own promoter gave a constitutive pseudohyphal growth phenotype that was rescued by deletion of SNF1, showing that Snf1p was acting downstream of Elm1p. Deletion of ELM1 does not give an snf− phenotype. However, Elm1p is closely related to Pak1p and Tos3p, and a pak1Δ tos3Δ elm1Δ triple mutant had an snf1− phenotype, i.e., it would not grow on raffinose and that express 119 yeast protein kinases as GST fusions, we identified Elm1p as the sole kinase that could activate the kinase domain of AMP-activated protein kinase in vitro. Elm1p also activated the purified SNF1 complex, and this correlated with phosphorylation of Thr210 in the activation loop. Removal of the C-terminal domain increased the Elm1p kinase activity, indicating that it is auto-inhibitory. Expression of activated, truncated Elm1p from its own promoter gave a constitutive pseudohyphal growth phenotype that was rescued by deletion of SNF1, showing that Snf1p was acting downstream of Elm1p. Deletion of ELM1 does not give an snf− phenotype. However, Elm1p is closely related to Pak1p and Tos3p, and a pak1Δ tos3Δ elm1Δ triple mutant had an snf1− phenotype, i.e., it would not grow on raffinose and did not display hyperphosphorylation of the SNF1 target, Mig1p, in response to glucose starvation. Conclusions: Elm1p, Pak1p, and Tos3p are upstream kinases for the SNF1 complex that have partially redundant functions.
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