SummaryMembers of the SNF1-related protein kinase-1 (SnRK1) subfamily of 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 ®rst report showing that the plant SnRK1 kinases are regulated by AMP in a manner similar to their mammalian counterparts. The possible physiological signi®cance of these ®ndings is discussed.

DIF-1, a polyketide, has been primarily characterized as a transcriptional regulator, but proteome-wide identification of peptides that change phosphorylation level in DIF-treated cell reveals many novel targets. Thus a small-molecule regulator of gene transcription plays additional roles as a multifunctional regulator of protein modification.

DIF-1 (differentiation-inducing factor1) is a polyketide produced by Dictyostelium prespore cells which induces initially uncommitted cells to differentiate as prestalk cells. Exposure of cells to DIF-1 causes transitory hypo-phosphorylation of seven serine residues in YelA, a protein with a region of strong homology to the MIF4G domain of the eukaryotic initiation factor eIF4G. Based upon its domain architecture, which in one important aspect closely resembles that of Death-Associated Protein 5 (DAP5), we predict a role in stimulating internal ribosome entry-driven mRNA translation. The two paradigmatic DIF-1 inducible genes are ecmA (extracellular matrixA) and ecmB. In support of a YelA function in DIF-1 signaling, a YelA null strain showed greatly increased expression of ecmA and ecmB in response to DIF-1. Also, during normal development in the null strain, expression of the two genes is accelerated. This is particularly evident for ecmB, a marker of stalk tube and supporting structure differentiation. Mutants in DIF-1 bio-synthesis or signaling display a rudimentary or no basal disc and, conversely, YelA null mutants produce fruiting bodies with a highly enlarged basal disc that ectopically expresses a stalk tube-specific marker. Thus YelA acts as an antagonist of DIF-1 signaling, with a consequent effect on cell type proportioning and it is predicted to act as a translational regulator.