Structure and function of pore-forming beta-barrels from bacteria.

Crystallographic studies of the past ten years have revealed that many outer membrane proteins and bacterial toxins are constructed on the beta-barrel motif. Two structural classes can be identified. The first class, represented by the porins, includes monomeric or multimeric proteins where each beta-barrel is formed from a single polypeptide. The second class features proteins where the beta-barrel is itself a multimeric assembly, to which each subunit contributes a few beta-strands. In addition to structural investigations, much work has also been devoted to the functional aspects of these proteins, and to the relationships between structure and function. Here we present a review of the structural and the functional properties of some of the best-studied examples of these various classes of proteins, namely the general-diffusion, specific and ligand-gated porins, multidrug efflux proteins and the staphylococcal toxin alpha-hemolysin.