Required for normal glycogen accumulation. The alpha 1-6 branches of glycogen play an important role in increasing the solubility of the molecule. 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. Monomer. Binds its carbohydrate substrate close to the active site, but also via regions close to the N-terminus; this may result in increased affinity and therefore increased catalytic efficiency. Defects in GBE1 are the cause of glycogen storage disease IV (GSD-IV). Belongs to the glycosyl hydrolase 13 family. GlgB subfamily.