The structure of bacterial outer membrane proteins

Abstract

Integral membrane proteins come in two types, α-helical and β-barrel proteins. In both types, all hydrogen bonding donors and acceptors of the polypeptide backbone are completely compensated and buried while nonpolar side chains point to the membrane. The α-helical type is more abundant and occurs in cytoplasmic (or inner) membranes, whereas the β-barrels are known from outer membranes of bacteria. The β-barrel construction is described by the number of strands and the shear number, which is a measure for the inclination angle of the β-strands against the barrel axis. The common right-handed β-twist requires shear numbers slightly larger than the number of strands. Membrane protein β-barrels contain between 8 and 22 β-strands and have a simple topology that is probably enforced by the folding process. The smallest barrels form inverse micelles and work as enzymes or they bind to other macromolecules. The medium-range barrels form more or less specific pores for nutrient uptake, whereas the largest barrels occur in active Fe2+ transporters. The β-barrels are suitable objects for channel engineering, because the structures are simple and because many of these proteins can be produced into inclusion bodies and recovered therefrom in the exact native conformation. © 2002 Elsevier Science B.V. All rights reserved.