A common phosphate binding site explains the unique substrate specificity of GSK3 and its inactivation by phosphorylation

The inhibition of GSK3 is required for the stimulation of glycogen and prot ein synthesis by insulin and the specification of cell fate during developm ent. Here, we demonstrate that the insulin-induced inhibition of GSK3 and i ts unique substrate specificity are explained by the existence of a phospha te binding site in which Arg-96 is critical. Thus, mutation of Arg-96 aboli shes the phosphorylation of "primed" glycogen synthase as well as inhibitio n by PKB-mediated phosphorylation of Ser-9. Hence, the phosphorylated N ter minus acts as a pseudosubstrate, occupying the same phosphate binding site used by primed substrates. Significantly, this mutation does not affect pho sphorylation of "nonprimed" substrates in the Wnt-signaling pathway (Axin a nd beta -catenin), suggesting new approaches to design more selective GSK3 inhibitors for the treatment of diabetes.