Phosphorylated seryl and threonyl, but not tyrosyl, residues are efficient specificity determinants for GSK‐3β and Shaggy

摘要

>Glycogen synthase kinase-3 is involved in diverse functions including insulin signalling and development. In a number of substrates, phosphorylation by glycogen synthase kinase-3 is known to require prior phosphorylation at a Ser in the +4 position relative to its own phosphorylation site. Here we have used synthetic peptides derived from a putative glycogen synthase kinase-3 site in the Drosophila translation initiation factor eIF2Bϵ to investigate the efficacy of residues other than Ser(P) as priming residues for glycogen synthase kinase-3β and its Drosophila homologue Shaggy. glycogen synthase kinase-3β phosphorylated peptides with Ser(P) and Thr(P) in the priming position, but peptides with Tyr(P), Thr, Glu or Asp were not phosphorylated. The V _max for the Thr(P) peptide was three times higher than that of the Ser(P) peptide. These data suggest that glycogen synthase kinase-3 is unique among phosphate-directed kinases. The priming site specificity of Shaggy is similar to that of mammalian glycogen synthase kinase-3β. This unpredicted efficacy of Thr(P) in the priming position suggests that there may be other unidentified substrates for these kinases.