Abstract
The α‐isoform of glycogen synthase kinase‐3 (GSK3α) was inactivated by 80% towards a synthetic peptide substrate upon incubation with Mg‐ATP and either MAP kinase‐activated protein (MAPKAP) kinase‐1 or p70 S6 kinase. Inactivation by either kinase resulted from the phosphorylation of Ser‐21 and was reversed by treatment with protein phosphatase 2A1. Phosphorylation also decreased GSK3α activity towards glycogen synthase, inhibitor‐2 and c‐jun. The specificity of GSK3a was similar to GSK3β, but with the synthetic peptide substrate heparin stimulated the dephosphorylated form of GSK3α (6‐fold) more than GSK3β(1.8‐fold). After phosphorylation, both isoforms were stimulated 15–20‐fold by heparin.