Outer Membrane Proteins

Abstract

The membrane of Gram-negative bacteria consists of three layers: (i) the cytoplasmic or inner membrane, (ii) the peptidoglycan layer and (iii) the outer membrane (Figure 1). The cytoplasmic membrane consists of a phospholipid bilayer in which proteins are present which can play a role in solute transport, generation of energy and synthesis of cell envelope components. The peptidoglycan layer consists of building blocks composed of N-acetylglucosamine, N-acetylmuramic acid and a few amino acids (usually L-alanine, D-glutamic acid, meso-diaminopimelic acid and D-alanine). It determines the cell shape and prevents lysis of the cytoplasmic membrane, and therefore of the cell, under hypotonic conditions. The outer membrane of Enterobacteria-ceae protects the cell content against detergents, exogenous enzymes such as proteases and phospholipases and hydrophobic and large molecular weight antibiotics. This is the result of its specific structure. In the outer membrane the inner leaflet of the lipid bilayer consists of phospholipids whereas the outer leaflet consists of the fatty acids of the lipid A part of LPS (lipopolysaccharide). The phosphate and negatively charged KDO (keto-deoxyoctonate) residues of the inner core of the LPS molecule interact with divalent cations, thereby forming a virtually impermeable layer. Pore proteins form aqueous channels through the outer membrane which are permeable for hydrophilic molecules with a molecular mass of up to 600 to 700 daltons (assuming the molecules are more or less global). Such pores allow a fast permeation of nutrients, and reduce or prevent entry of hydrophobic and large molecular weight toxin and antibiotic molecules (reviewed in Lugtenberg and van Alphen, 1983).