Protein phosphatase 1 (PP1) is a serine-threonine protein phosphatase that plays a central role in mediating the effects of insulin on glucose and lipid metabolism. PTG (protein targeting to glycogen) was cloned from 3T3-L1 adipocytes as a protein that binds to the PP1 catalytic subunit. The human homolog of PTG, designated PPP1R5, has been shown to bind to PP1 and to modulate its specificity. PTEN/PPP1R5 shows 42% identity to the glycogen binding subunit, G(L), of rat liver PP1. PTG is expressed predominantly in insulin-sensitive tissues, and it localizes PP1 to glycogen. PTG also has been shown to interact with several enzymes involved in the hormonal regulation of glycogen metabolism, including phosphorylase kinase, phosphorylase A and glycogen synthase. These data indicate a role for PTG in glycogen metabolism, possibly that of a molecular scaffold.