PTG, A PROTEIN PHOSPHATASE 1-BINDING PROTEIN WITH A ROLE IN GLYCOGEN-METABOLISM

Protein dephosphorylation by phosphatase PP1 plays a central role in m ediating the effects of insulin on glucose and lipid metabolism, A PP1 C-targeting protein expressed in 3T3-L1 adipocytes (called PTG, for pr otein targeting to glycogen) was cloned and characterized. PTG was exp ressed predominantly in insulin-sensitive tissues. In addition to bind ing and localizing PP1C to glycogen, PTG formed complexes with phospho rylase kinase, phosphorylase a, and glycogen synthase, the primary enz ymes involved in the hormonal regulation of glycogen metabolism. Overe xpression of PTG markedly increased basal and insulin-stimulated glyco gen synthesis in Chinese hamster ovary cells overexpressing the insuli n receptor, which do not express endogenous PTG. These results suggest that PTG is critical for glycogen metabolism, possibly functioning as a molecular scaffold.