Structural and Functional Asymmetry of Bacterial Membranes

Abstract

Plasma membranes of both gram-positive and gram-negative bacteria perform a variety of functions including transport of ions and metabolites, secretion of proteins, energization, biosynthetic stages in cell wall peptidoglycan, lipopolysaccharide, and capsular polysaccharide formation, and the biosynthesis of membrane lipids. All of these functions are organized in a single membrane system, in contrast to the functional compartmentalization characteristic of the membranous organelles (e.g., mitochondria, endoplasmic reticulum, Golgi, and lysosomes) of eukaryotic cells (Stanier, 1970; Carlile, 1980). In addition to plasma membranes, gram-negative bacteria possess a distinctive outer membrane (Inouye, 1979), and certain groups of bacteria possess specialized intracellular membranes such as chromatophores of photosynthetic bacteria, cytomembranes of nitrifying organisms, and spore membranes of sporulating bacteria. The only other membrane structure seen in many bacteria, especially in gram-positive organisms, is the mesosome. The origins and functions of mesosomes have been widely reviewed and there is still much speculation and some evidence as to their role and significance in the bacterial cell and cell cycle (Ghosh, 1974; Salton and Owen, 1976; Higgins et al?,1981). Central to an understanding of the structure-function relationships of these various bacterial membrane systems are (1) resolution of the complexity of the components in plasma membranes, outer membranes of gram-negative bacteria, and specialized membranes such as chromatophores, and (2) establishment of the molecular architecture and sidedness of the membranes and derived vesicles. Much progress has been made in recent years by resolving the variety of polypeptides in membrane structures by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (SDS-PAGE) and by the application of two-dimensional (“crossed”) immunoelectrophoresis (CIE) in the identification of membrane antigens and/or enzymes under essentially nondenaturing solubilization conditions with nonionic detergents such as Triton X-100(Owen and Salton, 1975a; Owen and Smyth, 1977; Smyth et al., 1978).