Transfers a segment of a (1->4)-alpha-D-glucan chain to a primary hydroxy group in a similar glucan chain. Glycan biosynthesis; glycogen biosynthesis. Expressed during the transition between the late exponential and stationary growth phases, coincident with maximal glycogen accumulation. Present with 1230 molecules/cell in log phase SD medium. Belongs to the glycosyl hydrolase 13 family. GlgB subfamily.