This is the oldest known and well-studied super-family of transmembrane beta-barrels. Porins are large enough to allow passive diffusion i.e., they act as channels that are specific to different types of molecules. They are found in the outer membranes of Gram-negative bacteria, mitochondria and chloroplasts. The amino acid composition of the transmembrane beta strands is unique since polar and non-polar residues alternate along them. This way, the non-polar residues face outwards in order to interact with the non-polar lipid bilayer, while the polar residues face the barrel interior in order to interact with the aqueous channel. Porins typically control the diffusion of small metabolites like sugars, ions, and amino acids. The archetypical general diffusion porins of Gram-negative Bacteria are composed of 16-stranded beta-barrels and are active only as homo-trimers (trimeric porins). However, other families have been found deviating from this motif such as the sugar porins which are trimeric porins composed of 18-stranded beta-barrels or the monomeric porins composed of 14-stranded beta-barrels.
Published in Chapter:
Bacterial ß-Barrel Outer Membrane Proteins: A Common Structural Theme Implicated in a Wide Variety of Functional Roles
Pantelis G. Bagos (University of Central Greece, and University of Athens, Greece) and Stavros J. Hamodrakas (University of Athens, Greece)
Copyright: © 2009
|Pages: 26
DOI: 10.4018/978-1-60566-076-9.ch010
Abstract
ß-barrel outer membrane proteins constitute the second and less well-studied class of transmembrane proteins. They are present exclusively in the outer membrane of Gram-negative bacteria and presumably in the outer membrane of mitochondria and chloroplasts. During the last few years, remarkable advances have been made towards an understanding of their functional and structural features. It is now wellknown that ß-barrels are performing a large variety of biologically important functions for the bacterial cell. Such functions include acting as specific or non-specific channels, receptors for various compounds, enzymes, translocation channels, structural proteins, and adhesion proteins. All these functional roles are of great importance for the survival of the bacterial cell under various environmental conditions or for the pathogenic properties expressed by these organisms. This chapter reviews the currently available literature regarding the structure and function of bacterial outer membrane proteins. We emphasize the functional diversity expressed by a common structural motif such as the ß-barrel, and we provide evidence from the current literature for dozens of newly discovered families of transmembrane ß-barrels.