Biosynthesis and Assembly of the Outer Membrane Proteins of Escherichia coli

Abstract

The envelope of Escherichia coli consists of two distinct membranes: the outer membrane and the inner or cytoplasmic membrane.1,2 The cell wall or the peptidoglycan layer is located between the two membranes.2 Both the outer and the cytoplasmic membranes show typical features in electron micrographs of a unit membrane having a thickness of about 75 A and consisting of phospholipids and proteins3 (Fig. 1). The cytoplasmic membrane is known to have many important functions such as energy metabolism, active transport, and synthesis of lipids, peptidoglycan, and lipopolysaccharide. The functions of the outer membrane are still somewhat obscure. So far only one enzyme, a phospholipase, has been suggested to exist in the outer membrane.4 One possible function of the outer membrane is thought to be as a barrier against penetration of certain substances such as antibiotics and macromolecules (see review by Nikaido5). However, many other substances which are required for cell growth should freely pass through the outer membrane, since the active transport systems for these substances are located exclusively in the cytoplasmic membrane.6,7 This suggests the existence of passive diffusion pores in the outer membrane. The lipopolysaccharide, which exists exclusively in the outer membrane, has been shown not to provide hydrophilic diffusion pores.8