Glycogenin is a self-glucosylating protein involved in the initiation of glycogen synthesis. Rabbit skeletal muscle glycogenin, transiently expressed in COS cells, was found exclusively in the low speed supernatant fraction, with M(r) 37-38,000. The protein was capable of self-glucosylation and was, if suitably primed, an effective substrate for glycogen synthase. Rabbit muscle glycogen synthase was similarly expressed, to a level 7-10-fold over the endogenous activity. Most of the expressed protein was found in the low speed pellet fraction. However, when co-expressed with glycogenin, a significant increase in the proportion of glycogen synthase in the soluble fraction was observed. Therefore, glycogenin interacts with glycogen synthase in the cell and redistributes the synthase to the soluble fraction. Co-expression of an inactive form of glycogenin did not affect glycogen synthase localization. The expressed glycogenin could be detected as two distinguishable species, differing slightly in electrophoretic mobility, depending on the glucose concentration of the cell culture medium. At 25 mM glucose, a form of M(r) 38,000 was observed; however, upon transfer to 5 mM glucose, it converted to a species of slightly lower M(r). The M(r) of the 38,000-dalton species could be also be reduced by treatment of the cell extract with alpha-amylase. It was additionally found that the 38,000-dalton glycogenin was a much more effective glycogen synthase substrate than the lower M(r) species. These results, therefore, raise the possibility of a novel mechanism for the control of glycogen metabolism in which glucose levels would regulate the glucosylation state of glycogenin, which in turn would determine glycogenin's efficacy as a substrate for elongation by glycogen synthase.