Tyrosine glycosylation is involved in muscle-glycogen synthesis

Rabbit-muscle glycogen contains a covalently bound protein having Mr 37,000 that the authors will henceforth refer to as glycogenin. It is completely insoluble in water at pH 5, and may be generated as a precipitate as a result of the combined action on glycogen of α-amylase and glucoamylase, or by treatment with anhydrous hydrogen fluoride. In the former case the protein still carries some of the glucose residues of glycogen (10-30 per mole of glycogenin). The linkage between glycogen and glycogenin has been identified as a novel glycosidic-amino acid bond. The authors demonstrated glucosylation with UDP[/sup 14/C]glucose by a muscle extract of two rabbit-muscle proteins contained in the same extract. The relation of these proteins to glycogenin, and whether the amino acid undergoing glucosylation is tyrosine, remains to be explored. The discovery of glycogenin is, the authors believe, an important clue to the mechanism of biogenesis of glycogen and may represent a previously unsuspected means of metabolic control of the glycogen content of the cell and the location of glycogen within the cell. The facts that the linkage between glycogen and glycogenin is via tyrosine, that insulin stimulates glycogen synthesis, and acts on its receptor by causing it to become an active tyrosine kinase, may be linked by a common thread