Glycogen synthase isolated from rabbit skeletal muscle as a D-form (synthase D2) is activated by a rat liver cytosolic protein differing from any of the known protein phosphatases (D2 activase). Although reversible by phosphorylation by cyclic AMP-dependent protein kinase, the activation is a result of limited proteolysis by D2 activase, which has been identified as the Ca2+-activated protease.