A (1----4)-alpha-D-glucan-protein involved in liver glycogen biosynthesis

A macromolecular (1----4)-alpha-D-[14C]glucan-protein complex was synthesized with a rat liver preparation and uridine diphosphate D-[14C]glucose. The size of the complex is contributed by both the protein and the (1----4)-alpha-D-glucosyl-oligomer components. Iodoacetamide treatment did not change the migration properties on Bio-Gel A-50m. Therefore, disulfide bonds linking glucan-protein subunits seem not to be involved. The [14C]glucan-protein, precipitated by diluted trichloroacetic acid, was digested by alpha-amylase, phosphorylase a, and proteases. The extent of proteolysis was greater for a complex having fewer D-glucose units incorporated. After proteolytic digestion of that complex, the labeled fragments behaved on electrophoresis, and ion-exchange and gel chromatography as [14C]glucosylated peptides. These findings support previous conclusions that the primer for liver glycogen synthesis is a protein on which glycogen is built up by covalent attachment.