Essential Mycoplasma Glycolipid Synthase Adheres to the Cell Membrane by Means of an Amphipathic Helix.

Because of the lack of cell wall, Micoplasma species require a fine control of membrane fluidity and integrity. mg517 is an essential gene of Mycoplasma genitalium responsible for the biosynthesis of membrane glycoglycerolipids. It encodes for a unique glycosyltransferase (MG517) with processive activity, transferring activated glycosyl donors to either nude diacylglycerol or already glycosylated diacylglycerol. This dual activity, asserted to different enzymes in other species, is sensitive to and regulated by the presence of anionic lipid vesicles in vitro. We present here a computational model of the C-terminus domain of MG517 that complements a previous structural model of the N-terminus domain. By means of sequence analysis, molecular dynamics and metadynamics simulations, we have identified a short α-helix at the apical C-terminus of MG517 with clear amphipathic character. Binding to a membrane model is thermodynamically favored which suggests that this structural element guides the adhesion of MG517 to the cell membrane. We have experimentally verified that truncation of part of this helix causes a substantial reduction of glycoglycerolipids synthesis. The model proposes that MG517 recognizes and binds the diacylglycerol substrate embedded in the membrane by means of this α-helix at the C-terminus together with a previously identified binding pocket at the N-terminus.