The nature of the primer for glycogen synthesis in muscle

We and others have reported that glycogenin, the covalently bound protein found in muscle glycogen, also exists in muscle in a glycogen-free form (M_r, 38 000-39 000) that is autocatalytic, undergoes self-glucosylation and acts as a primer for glycogen synthesis. We now report that this entity is not present in a fresh muscle extract. Instead it exists within a pro form of much higher molecular mass which breaks down spontaneously to the M_r, 38 000-39 000 form. Such breakdown is accelerated by the addition of α-amylase and is prevented by protease inhibitors. Multiple intermediates of the breakdown process have been detected, each capable of undergoing glucosylation.