ACTIVATING AMPHIPHILES CAUSE A CONFORMATIONAL CHANGE OF THE 1,2-DIACYLGLYCEROL 3-GLUCOSYLTRANSFERASE FROM ACHOLEPLASMA-LAIDLAWII MEMBRANES ACCORDING TO PROTEOLYTIC DIGESTION

1,2-Diacylglycerol 3-glucosyltransferase synthesizes the major nonbila yer-prone lipid monoglucosyldiacylglycerol (MGlcDAG) in the membrane o f Acholeplasma laidlawii, which is important for the spontaneous curva ture, and is a regulatory site for the lipid surface charge density, A potential connection between activity and a conformational change of this enzyme, governed by essential lipid activators, was studied with purified MGlcDAG synthase in different lipid aggregates, Critical frac tions of anionic phospholipids 1,2-dioleoyl-phosphatidylglycerol (DOPG ) and 1,2-dioleoyl-phosphatidylserine (DOPG) were essential for the re storation of enzyme activity, while the zwitterionic 1,2-dioleoyl-phos phatidylcholine (DOPC) and the uncharged di glucosyldiacylglycerol (DG lcDAG) were not, Proteolytic resistance had a very good correlation wi th the enzyme activity in various lipid CHAPS mixed micelles. Anionic lipids DOPG and DOPS could protect the exposed MGlcDAG:synthase from d igestion, whereas DOPC and DGlcDAG could not, Similar features were ob served in liposome bilayers, Likewise, the detergent dodecylphosphogly cerol (PGD), with a phosphatidylglycerol-Like headgroup, could also st imulate the MGlcDAG synthase activity efficiently with a concomitant p rotection toward proteolytic digestion, Neither proteolytic resistance nor restored enzyme activity was observed using soluble glycerol 3-ph osphate, It is concluded that in addition to critical amounts, both th e negatively charged headgroup and hydrophobic chains of the activator amphiphiles, but not a certain aggregate curvature, seem necessary fo r a proper conformation and the resulting active state of the MGlcDAG synthase.