Regulation of spinach SNF1-related (SnRK1) kinases by protein kinases and phosphatases is associated with phosphorylation of the T loop and is regulated by 5′-AMP

Members of the SNF1-related protein kinase-l (SnRK1) subfamily nf protein kinases are higher plant homologues of mammalian AMP-activated and yeast SNF1 protein kinases. Based on analogies with the mammalian system, we surmised that the SnRK1 kinases would be regulated by phosphorylation on a threonine [equivalent to Thr175 in Arabidopsis thaliana SnRK1 (AKIN10)] within the 'T loop' between the conserved DFG and APE motifs. We have raised an antibody against a phosphopeptide based on this sequence, and used it to show that inactivation of two spinach SnRK1 kinases by protein phosphatases, and reactivation by a mammalian upstream protein kinase, is associated with changes in the phosphorylation state of this threonine. We also show that dephosphorylation of this threonine by protein phosphatases, and consequent inactivation, is inhibited by low concentrations of 5'-AMP, via binding to the substrate (i.e., the kinase). This is the first report showing that the plant SnRK1 kinases are regulated by AMP in a manner similar to their mammalian counterparts. The possible physiological significance of these findings is discussed.