Seems to act as a glycogen-targeting subunit for PP1. PP1 is essential for cell division, and participates in the regulation of glycogen metabolism, muscle contractility and protein synthesis. Plays an important role in glycogen synthesis but is not essential for insulin activation of glycogen synthase (By similarity). Interacts with PPP1CC catalytic subunit of PP1, and associates with glycogen. Skeletal muscle and heart. The CBM21 domain is known to be involved in the localization to glycogen and is characteristic of some regulatory subunit of phosphatase complexes. Phosphorylation at Ser-46 by ISPK stimulates the dephosphorylation of glycogen synthase and phosphorylase kinase. Disease susceptibility is associated with variants affecting the gene represented in this entry. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay.