Synthesis of the nonbilayer-prone ␣-monoglucosyldiacylglycerol (MGlcDAG) is crucial for bilayer packing properties and the lipid surface charge density in the membrane of Acholeplasma laidlawii. The gene for the responsible, membrane-bound glucosyltransferase (alMGS) (EC 2.4.1.157) was sequenced and functionally cloned in Escherichia coli, yielding MGlcDAG in the recombinants. Similar amino acid sequences were encoded in the genomes of several Gram-positive bacteria (especially pathogens), thermophiles, archaea, and a few eukaryotes. All of these contained the typical EX 7 E catalytic motif of the CAZy family 4 of ␣-glycosyltransferases. The synthesis of MGlcDAG by a close sequence analog from Streptococcus pneumoniae (spMGS) was verified by polymerase chain reaction cloning, corroborating a connection between sequence and functional similarity for these proteins. However, alMGS and spMGS varied in dependence on anionic phospholipid activators phosphatidylglycerol and cardiolipin, suggesting certain regulatory differences. Fold predictions strongly indicated a similarity for alMGS (and spMGS) with the two-domain structure of the E. coli MurG cell envelope glycosyltransferase and several amphipathic membrane-binding segments in various proteins. On the basis of this structure, the alMGS sequence charge distribution, and anionic phospholipid dependence, a model for the bilayer surface binding and activity is proposed for this regulatory enzyme.Lipids are the local environment for most integral and peripheral membrane proteins, which often depend on the lipids for optimal function. The large diversity of lipids and the differences in composition and properties between membranes have made it difficult to find out common features of bilayer organization and how lipids and proteins are cooperating in local processes. Lipid-synthesizing pathways have been mapped for the most common types of lipids, and several of the corresponding enzymes catalyzing these reactions have been characterized. However, when it comes to the connection between regulation of bilayer properties and enzyme structure, very little is known (1). So far, only a few lipid-synthesizing enzymes have been crystallized. Which structural properties are involved in the catalytic mechanism of these lipid enzymes, and how are the membrane properties sensed (1)?In the well characterized plasma membrane of Acholeplasma laidlawii, the lipid composition is regulated in a manner to maintain (i) lipid phase equilibria, close to a potential bilayer to nonbilayer transition, (ii) a nearly constant radius of spontaneous curvature, and (iii) a certain anionic surface charge density of the lipid bilayer. The synthesis of the major nonbilayer-prone lipid in this membrane, monoglucosyldiacylglycerol (MGlcDAG) 1 (Scheme 1, step I), plays an important role to fulfill the two first points above but also the third, since it is strongly regulated by negatively charged lipids (e.g. the major in vivo lipid phosphatidylglycerol (PG)) (2). MGlcDAG is consecutively processed into...