ABSTRACTThe regulatory-targeting subunit (RGL, also called GM) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/RGLhas been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of RGL. To investigate the function of the phosphatase, RGLknockout mice were generated. Animals lacking RGLshow no obvious defects. The RGLprotein is absent from the skeletal and cardiac muscle of null mutants and present at ∼50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by ∼50% in the RGL-deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant RGLmice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by ∼90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and RGLnull mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that RGLis genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/RGLis not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.