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 Glycan biosynthesis; glycogen biosynthesis. Homodimer, tightly complexed to glycogen synthase. Additional isoforms seem to exist. Detected in liver (at protein level) (PubMed:9857012). Expressed preferentially in liver, heart, and pancreas (PubMed:9346895). 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-228. Belongs to the glycosyltransferase 8 family. Glycogenin subfamily.