Structure and Assembly of beta-Barrel Membrane Proteins

Abstract

Integral membrane proteins fall into two major structural classes; they consist of individual or bundled TM1 β-helices, or they form monomeric, dimeric, or trimeric TM β-barrels. These folds are dictated by the necessity to form oriented hydrogenbonded secondary structures in the highly ordered apolar environment of the lipid bilayer. No other structural motif has yet been confirmed for membrane proteins at high resolution, although the helical bundle structure of the nicotinic acetylcholine receptor may be surrounded by a concentric sheath of TM β-sheets (1). Apart from this possible exception, all membrane proteins of plasma and endoplasmic reticulum-derived membranes are β-helical, whereas the proteins of the outer membranes of Gram-negative bacteria and likely a fair number of proteins of the OMs of mitochondria and chloroplasts are of the β-barrel type. These two structural motifs are also recapitulated in various membrane-inserted toxins, which can be considered facultative integral membrane proteins. This review focuses on the structure and assembly of β-barrel membrane proteins. Because high resolution structures are known only for bacterial OM proteins and because our knowledge on the folding and assembly of these proteins is much more advanced than that of mitochondrial or chloroplast OM proteins, we will restrict our comments to the prokaryotic systems.