High Cationic Charge and Bilayer Interface-Binding Helices in a Regulatory Lipid Glycosyltransferase†,‡
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In the prokaryote Acholeplasma laidlawii, membrane bilayer properties are sensed and regulated by two interface glycosyltransferases (GTs), synthesizing major nonbilayer- (alMGS GT) and bilayer-prone glucolipids. These enzymes are of similar structure, as many soluble GTs, but are sensitive to lipid charge and curvature stress properties. Multivariate and bioinformatic sequence analyses show that such interface enzymes, in relation to soluble ones of similar fold, are characterized by high cationic charge, certain distances between small and cationic amino acids, and by amphipathic helices. Varying surface contents of Lys/Arg pairs and Trp indicate different membrane-binding subclasses. A predicted potential (cationic) binding helix from alMGS was structurally verified by solution NMR and CD. The helix conformation was induced by a zwitterionic as well as anionic lipid environment, and the peptide was confined to the bilayer interface. Bilayer affinity of the peptide, analyzed by surface plasmon resonance, was higher than that for soluble membrane-seeking proteins/peptides and rose with anionic lipid content. Interface intercalation was supported by phase equilibria in membrane lipid mixtures, analyzed by 31P NMR and DSC. An analogous, potentially binding helix has a similar location in the structurally determined Escherichia coli cell wall precursor GT MurG. These two helices have little sequence conservation in alMGS and MurG homologues but maintain their amphipathic character. The evolutionary modification of the alMGS binding helix and its location close to the acceptor substrate site imply a functional importance in enzyme catalysis, potentially providing a mechanism by which glycolipid synthesis will be sensitive to membrane surface charge and intrinsic curvature strain.
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