Transbilayer Distribution of Lipids in Microbial Membranes

Abstract

Publisher Summary Asymmetry of membrane lipids means that lipids in the outer leaflet are different from those in the inner leaflet of the bilayer. The chapter outlines the evidence for lipid asymmetry in microbial membranes and discusses mechanisms for the maintenance of this asymmetry. Techniques based on degradation of phospholipids by lipolytic enzymes, nonenzymatic chemical modification of membrane phospholipids, and immunochemical techniques provide information on the transbilayer distribution of membrane lipids. The basic assumption for these procedures is that the enzyme, chemical reagent, or antibody has access in intact cells only to lipids localized in the outer leaflet of the lipid bilayer. The chapter describes the transbilayer distribution of outer membrane lipids and of cytoplasmic membrane lipids. Gram-negative bacteria are bounded by two membranes, the cytoplasmic membrane and the outer membrane. The cytoplasmic membrane is responsible for the major metabolic functions, including respiratory pathways, biosynthetic activities, and transport. However, the major function of the outer membrane is to serve as a barrier protecting the underlying peptidoglycan and cytoplasmic membrane by controlling access of nutrients and agents such as detergents, enzymes, and antibodies. Although their phospholipid content is similar, only the outer membrane contains lipopolysaccharides. Bidirectional translocation of phospholipids between the cytoplasmic and outer membranes of Gram-negative bacteria occurs at a rapid rate. Translocation of phosphatidylethanolamine is not coupled with membrane protein synthesis and is not driven by ATP or by newly synthesized phospholipids. Asymmetric distribution of lipids may be maintained by the lack of spontaneous transbilayer movement of the lipid molecule.