Plays an important role in preventing glycogen hyperphosphorylation and the formation of insoluble aggregates, via its activity as glycogen phosphatase, and by promoting the ubiquitination of proteins involved in glycogen metabolism via its interaction with the E3 ubiquitin ligase NHLRC1/malin. Shows strong phosphatase activity towards complex carbohydrates in vitro, avoiding glycogen hyperphosphorylation which is associated with reduced branching and formation of insoluble aggregates (PubMed:16901901, PubMed:23922729, PubMed:26231210, PubMed:25538239, PubMed:25544560). Dephosphorylates phosphotyrosine and synthetic substrates, such as para-nitrophenylphosphate (pNPP), and has low activity with phosphoserine and phosphothreonine substrates (in vitro) (PubMed:11001928, PubMed:11220751, PubMed:11739371, PubMed:14532330, PubMed:16971387, PubMed:18617530, PubMed:22036712, PubMed:23922729, PubMed:14722920). Has been shown to dephosphorylate MAPT (By similarity). Forms a complex with NHLRC1/malin and HSP70, which suppresses the cellular toxicity of misfolded proteins by promoting their degradation through the ubiquitin-proteasome system (UPS). Acts as a scaffold protein to facilitate PPP1R3C/PTG ubiquitination by NHLRC1/malin (PubMed:23922729). Also promotes proteasome-independent protein degradation through the macroautophagy pathway (PubMed:20453062). Does not bind to glycogen (PubMed:18617530). Lacks phosphatase activity and might function as a dominant-negative regulator for the phosphatase activity of isoform 1 and isoform 7 (PubMed:18617530, PubMed:22036712). Has phosphatase activity (in vitro). H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Homodimer (PubMed:16971387, PubMed:18617530, PubMed:23922729, PubMed:25538239, PubMed:25544560). Interacts with itself (PubMed:14532330). Interacts with PPP1R3B, PPP1R3C, PPP1R3D, HIRIP5, and EPM2AIP1 (PubMed:12782127, PubMed:12915448, PubMed:14532330, PubMed:16901901, PubMed:18070875). Binds glycogen and Lafora bodies (PubMed:11739371, PubMed:14532330, PubMed:14706656, PubMed:15102711, PubMed:18617530, PubMed:22036712). Interacts with NHLRC1/malin (via the NHL repeats) (PubMed:15930137, PubMed:22036712, PubMed:23922729). Forms a complex with NHLRC1/malin and HSP70 (PubMed:19036738). Interacts with PPP1R3D; in the presence of NHLC1/malin the interaction leads to ubiquitination and autophagic degradation of PPP1R3D. Interacts (via the phosphatase domain) with MAPT/Tau; the interaction dephosphorylates MAPT (PubMed:19542233). Isoform 1 and isoform 2 interact to form a heterodimeric complex that lacks phosphatase activity (in vitro) (PubMed:18617530). Active phosphatase isoform 7 and isoform 1 interact with each other, but give rise to lower phosphatase activity than isoform 1 or isoform 7 by themselves (in vitro) (PubMed:22036712). Active phosphatase isoform 7 and inactive isoform 2 interact with each other, but give rise to lower phosphatase activity than isoform 7 by itself (in vitro) (PubMed:22036712). Interacts with PRDM8 (PubMed:22961547). Under glycogenolytic conditions localizes to the nucleus. Colocalizes with glycogen synthase in punctate structures in the cytoplasm (PubMed:11739371, PubMed:14532330). Primarily associated with polyribosomes at the rough endoplasmic reticulum, and also detected at the plasma membrane (PubMed:11001928, PubMed:11220751, PubMed:11883934, PubMed:18311786). Also found in the nucleus. Expressed in heart, skeletal muscle, kidney, pancreas and brain. Isoform 4 is also expressed in the placenta. The CBM20 domain mediates binding to cytoplasmic glycogen and to Lafora polyglucosan bodies. Polyubiquitinated by NHLRC1/malin. Phosphorylation on Ser-25 by AMPK affects the phosphatase activity of the enzyme and its ability to homodimerize and interact with NHLRC1, PPP1R3C or PRKAA2. The disease is caused by variants affecting the gene represented in this entry. Produced by alternative splicing. Produced by alternative splicing. May be due to an intron retention. Produced by alternative splicing. Produced by alternative initiation at Met-244 of isoform 1. Transcript amplified but protein not detected. Produced by alternative splicing. Active phosphatase. Produced by alternative splicing. Belongs to the protein-tyrosine phosphatase family. Probable cloning artifact.