Processive glucosyltransferase involved in the biosynthesis of both the non-bilayer-prone alpha-monoglucosyldiacylglycerol and the bilayer-forming membrane lipid alpha-diglucosyldiacylglycerol. These are major components for maintaining the anionic lipid surface charge density, for balancing the bilayer to non-bilayer phase equilibria and for keeping a constant lipid bilayer spontaneous curvature (curvature packing stress). Catalyzes the transfer of a glucosyl residue from UDP-Glc to diacylglycerol (DAG) acceptor to form the corresponding alpha-glucosyl-DAG (1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol), which then acts as acceptor to give alpha-diglucosyl-DAG product (3-O-(alpha-D-glucopyranosyl-alpha-(1->2)-D-glucopyranosyl)-1,2-diacyl-sn-glycerol). It can only use UDP-Glc as sugar donor. a 1,2-diacyl-sn-glycerol + UDP-alpha-D-glucose = a 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol + H(+) + UDP a 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol + UDP-alpha-D-glucose = 1,2-diacyl-3-O-[alpha-D-glucosyl-(1-> 2)-alpha-D-glucosyl]-sn-glycerol + H(+) + UDP Activated by the negatively charged lipids phosphatidylglycerol (PG), cardiolipin (CL), nonbilayer-prone 1,3-DAG, 1,2-dioleoylphosphatidylglycerol (DOPG) and 1,2-dioleoylphosphatidylserine (DOPS). Inhibited by 1,2-diacyl-3-O-(alpha-D-galactopyranosyl)-sn-glycerol, 1,2-diacyl-3-O-[6-O-acyl(alpha-D-glucopyranosyl)]-sn-glycerol and 1,2-diacyl-3-O-[alpha-D-glucopyranosyl-(1->2)-O-(6-O-acyl-alpha-D-glucopyranosyl)]-sn-glycerol. Glycolipid metabolism; diglucosyl-diacylglycerol biosynthesis. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.