Self-glucosylates, via an inter-subunit mechanism, to form an oligosaccharide primer that serves as substrate for glycogen synthase. L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = alpha-D-glucosyl-L-tyrosyl-[glycogenin] + H(+) + UDP [1,4-alpha-D-glucosyl](n)-L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = [1,4-alpha-D-glucosyl](n+1)-L-tyrosyl-[glycogenin] + H(+) + UDP Divalent metal ions. Required for self-glucosylation. Manganese is the most effective. Glycan biosynthesis; glycogen biosynthesis. Homodimer. Interacts (via C-terminus) with glycogen synthase GYS1 (By similarity). Interacts (via C-terminus) with GYS2; required for GYS2-mediated glycogen synthesis (PubMed:24982189). Skeletal muscle, heart, to a lesser extent in kidney, lung and brain. Self-glycosylated by the transfer of glucose residues from UDP-glucose to itself, forming an alpha-1,4-glycan of around 10 residues attached to Tyr-195. Phosphorylated. Belongs to the glycosyltransferase 8 family. Glycogenin subfamily.