Acts as a glycogen-targeting subunit for phosphatase PP1. Facilitates interaction of the PP1 with enzymes of the glycogen metabolism and regulates its activity. Suppresses the rate at which PP1 dephosphorylates (inactivates) glycogen phosphorylase and enhances the rate at which it activates glycogen synthase and therefore limits glycogen breakdown. Its activity is inhibited by PYGL, resulting in inhibition of the glycogen synthase and glycogen phosphorylase phosphatase activities of PP1. Dramatically increases basal and insulin-stimulated glycogen synthesis upon overexpression in hepatocytes (By similarity). Interacts with glycogen, PPP1CC catalytic subunit of PP1 and PYGL. Associates with glycogen particles. Forms complexes with debranching enzyme, glycogen phosphorylase, glycogen synthase and phosphorylase kinase which is necessary for its regulation of PP1 activity (By similarity). Highly expressed in liver (at protein level). Expressed predominantly in liver. Expressed moderately in heart. Expressed weakly in prostate, stomach, thyroid, lung, kidney, spleen and skeletal muscle. Expressed in lung at 12.5 and 16.5 dpc and declines thereafter. Expressed in epithelial cells of the bronchus and smooth muscle of the pulmonary artery at 13.5 and 16.5 dpc. Up-regulated by TITF1. The N-terminal region is required for binding to PP1, the central region is required for binding to glycogen and the C-terminal region is required for binding to PYGL.