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 and DAG is the preferred substrate. 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 lipids phosphatidylglycerol (PG), cardiolipin (CL), dodecylphosphate-rac-glycerol (PDG), 1,2-dioleoyl-phosphatidylglycerol (DOPG) and phosphatidylserine (PS). The enzyme binds the membrane surface via electrostatic association with anionic lipids and is accompanied by hydrophobic interactions and by a conformational change induced by nonbilayer-prone lipids. The enzyme is supposed to be located on the cytosolic side of the membrane. PG could also protect the enzyme from proteolytic digestion of the proteinase K, possibly by orienting a lysine-rich face toward the membrane. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.