Glycogen synthase kinase-3 from rabbit skeletal muscle. Separation from cyclic-AMP-dependent protein kinase and phosphorylase kinase.

Publisher Summary This chapter discusses glycogen synthase kinase-3 from rabbit skeletal muscle. Glycogen synthase kinase-3 is one of the five glycogen synthase kinases that are identified in skeletal muscle, and is of major importance in determining the kinetic properties of glycogen synthase in vivo. It catalyzes the phosphorylation of three serine residues on glycogen synthase, converting the enzyme from a form that is almost fully active in the absence of glucose-6P, to one that is largely dependent on this allosteric activator. Glycogen synthase kinase-3 also has a second activity that is not shared by any other protein kinase—namely, the ability to activate an enzyme termed the MgATP-dependent protein phosphatase. Glycogen synthase may also contain traces of a modified form of phosphorylase kinase that has lost its sensitivity to regulation by calcium ions, and is therefore, no longer inhibited by ethylene glycol tetraacetic acid (EGTA). This is largely removed by passing glycogen synthase through phosphocellulose.