Acts as a glycogen-targeting subunit for PP1 and regulates its activity. Activates glycogen synthase, reduces glycogen phosphorylase activity and limits glycogen breakdown. Dramatically increases basal and insulin-stimulated glycogen synthesis upon overexpression in a variety of cell types. Interacts with PPP1CC catalytic subunit of PP1 and associates with glycogen. Forms complexes with glycogen phosphorylase, glycogen synthase and phosphorylase kinase which is necessary for its regulation of PP1 activity. Also interacts with EPM2A/laforin. 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. Ubiquitinated by NHLRC1/malin in a EPM2A/laforin-dependent manner.