Biogenesis of Outer Membrane Proteins
The cell envelope of Gram-negative bacteria consists of two membranes, the inner membrane, which is a phospholipid bilayer, and the outer membrane, which is an asymmetrical bilayer with phospholipids and lipopolysaccharides (LPS) in the inner and outer monolayers, respectively. The membranes are separated by the peptidoglycan-containing periplasm. Both membranes contain proteins. Whereas integral inner-membrane proteins span the membrane by hydrophobic α-helical segments, outer-membrane proteins (OMPs) present an entirely different structure, the β-barrel. These proteins consist mainly of antiparallel amphipathic β-strands, exposing their hydrophobic residues to the membrane and their hydrophilic residues to the interior of the barrel (Koebnik et al., 2000). Most integral OMPs are involved in transport processes, such as the import of nutrients or the export of proteins. Examples are the porins, which form hydrophilic pores in the outer membrane to allow for the passage of small hydrophilic molecules by passive diffusion, the receptors, which present a binding site for specific nutrients and mediate their uptake in an energy-consuming process, and the secretins, which form large oligomers and are involved in protein secretion (Bitter and Tommassen, 1999; Koebnik et al., 2000). Other OMPs, for example OmpA protein of Escherichia coli, have a structural role in maintaining the integrity of the outer membrane, or function in adhesion to or invasion of eukaryotic target cells, e.g. the Opa proteins of pathogenic Neisseria (Billker et al., 2000).
KeywordsOuter Membrane Signal Peptidase PPIase Activity Periplasmic Side Small Hydrophilic Molecule
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