Self-glucosylating initiator of glycogen synthesis. Catalyzes the formation of a short alpha (1,4)-glucosyl chain covalently attached to internal tyrosine residues. These chains act as primers for the elongation reaction catalyzed by glycogen synthase (By similarity). 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 Interacts with glycogen synthase GSY2. Belongs to the glycosyltransferase 8 family. Glycogenin subfamily.