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The Snf1 kinase and its mammalian orthologue, the AMP-activated protein kinase (AMPK), function as heterotrimers composed of a catalytic α-subunit and two non-catalytic subunits, β and γ. The β-subunit is thought to hold the complex together and control subcellular localization whereas the γ-subunit plays a regulatory role by binding to and blocking the function of an auto-inhibitory domain (AID) present in the α-subunit. In addition, catalytic activity requires phosphorylation by a distinct upstream kinase. In yeast, any one of three Snf1-activating kinases, Sak1, Tos3, or Elm1, can fulfill this role. We have previously shown that Sak1 is the only Snf1-activating kinase that forms a stable complex with Snf1. Here we show that the formation of the Sak1·Snf1 complex requires the β- and γ-subunits in vivo. However, formation of the Sak1·Snf1 complex is not necessary for glucose-regulated phosphorylation of the Snf1 …