Transfers the glycosyl residue from UDPG to the non-reducing end of alpha-1,4-glucan. In larval skeletal muscle, isoform B is required for the formation of autophagosomes during starvation and during cloroquine-induced vacuolar myopathy. [(1->4)-alpha-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + H(+) + UDP Glycan biosynthesis; glycogen biosynthesis. Isoform B interacts with Atg8a upon starvation. Isoform B colocalizes with Atg8a at autophagosomes upon starvation in larval skeletal muscle. In third instar larvae, isoform B is highly expressed in skeletal muscle but not detected in fat body. Belongs to the glycosyltransferase 3 family.