Porins from Mitochondrial and Bacterial Outer Membranes: Structural and Functional Aspects

Abstract

The cell envelope of Gram-negative bacteria consists of three different layers, the outer membrane, the murein and the inner membrane (Beveridge, 1981). The inner membrane represents a real diffusion barrier and contains, similar to the mitochondrial inner membrane, the respiration chain and a large number of transport systems. The outer membrane of Gram-negative bacteria plays an important role in the physiology of these organisms. All nutrients or antibiotics either hydrophilic or hydrophobic have to cross this permeability barrier which means that it has special sieving properties. The active components of molecular sieving of the outer membrane are due to presence of a few major proteins called “porins” (Nakae, 1976). The porins are organized as trimers of three identical subunits and form transmembrane channels that have more general properties and sort according to the molecular weight of the solutes (Benz, 1985; Nikaido & Vaara, 1985). On the other hand, also specific porins have been identified in the outer membrane of Gram-negative bacteria which contain binding sites for substrates (Benz, 1988). The porin trimers form basically one channel in the outer membrane with three openings faced to the cell surface and one outlet into the periplasmic space (Engel et al., 1985; Lepault et al., 1988).