A new look at the biogenesis of glycogen.

The discovery of glycogenin as a self-glucosylating protein that primes glycogen synthesis has significantly increased our understanding of the structure and metabolism of this storage polysaccharide. The amount of glycogenin will influence how much glycogen the cell can store. Therefore, the production of active glycogenin primer in the cell has the potential to be the overall rate-limiting process in glycogen formation, capable of overriding the better understood hormonally controlled mechanisms of protein phosphorylation/dephosphorylation that regulate the activities of glycogen synthase and phosphorylase. There are indications that a similar covalent modification control is also being exerted on glycogenin. Glycogenin has the ability to glucosylate molecules other than itself and to hydrolyze UDPglucose. These are independent of self-glucosylation, so that glycogenin, even when it has completed its priming role and become part of the glycogen molecule, retains its catalytic potential. Another new comp...