Physical interaction between lipid A and phospholipids: a study with spin-labeled phospholipids.

When mixed bilayers containing spin-labeled phosphatidylethanolamine (or phosphatidylglycerol) and Escherichia coli B lipopolysaccharide were prepared, electron spin resonance signals indicated that the patches that were initially present and contained only phospholipids or only lipopolysaccharides were unusually stable and that little lateral diffusion of phospholipids into lipopolysaccharide domains, or vice versa, took place. These results explain how the outer layer of the outer membrane, which essentially contains only lipopolysaccharides in addition to proteins, can be generated and maintained stably in bacterial cells. Furthermore, the stability of pure lipopolysaccharide domains may have important implications in the mode of action of endotoxins in the body of the host. For example, lipopolysaccharide molecules may tend to form stable domains or patches spontaneously in the animal cell membrane, and special mechanisms (such as the binding to a special receptor) may be needed to disperse the lipopolysaccharide molecules within the host cell membranes.