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 (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). 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 glycogen phosphorylase, glycogen synthase and phosphorylase kinase.