Glucosyltransferase involved in the biosynthesis of the non-bilayer-prone membrane lipid alpha-monoglucosyldiacylglycerol. This is a major component for maintaining a certain 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). 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 Activated by the negatively charged lipid phosphatidylglycerol (PG). Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.