Phosphorylated seryl and threonyl, but not tyrosyl, residues are efficientspecificity determinants for GSK-3 beta and Shaggy

Glycogen synthase kinase-3 is involved in diverse functions including insul in signalling and development. In a number of substrates, phosphorylation b y 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 h ave used synthetic peptides derived from a putative glycogen synthase kinas e-3 site in the Drosophila translation initiation factor eIF2B epsilon to i nvestigate the efficacy of residues other than Ser(P) as priming residues f or glycogen synthase kinase-3 beta and its Drosophila homologue Shaggy, gly cogen synthase kinase-3 beta phosphorylated peptides with Ser(P) and Thr(P) in the priming position, but peptides with Tyr(P), Thr, Glu or Asp were no t phosphorylated, The V-max for the Thr(P) peptide was three times higher t han that of the Ser(P) peptide. These data suggest that glycogen synthase k inase-3 is unique among phosphate-directed kinases, The priming site specif icity of Shaggy is similar to that of mammalian glycogen synthase kinase-3 beta, This unpredicted efficacy of Thr(P) in the priming position suggests that there may be other unidentified substrates for these kinases, (C) 1999 Federation of European Biochemical Societies.