Immunoblotting as well as enzyme assays demonstrate the presence of the self-glucosylating protein, glycogenin, in the protein-glycogen complex, in the sarcoplasmic reticulum and in phosphorylase kinase. In all three compartments glycogenin occurs in different, albeit, defined glucosylated forms, which upon deglucosylation are converted into a 42 kDa form. We suggest that phosphorylase kinase might have a dual function in glycogen biogenesis: firstly, control of glycogen degradation in the protein-glycogen complex via phosphorylation of glycogen phosphorylase b; secondly, regulation of glycogen biosynthesis on the sarcoplasmic reticular membranes via phosphorylation and thereby inhibition of glycogen synthase.
