The integral membrane proteins with β-barrel structures are known from outer membranes of bacteria, mitochondria, and chloroplasts. The β-barrel is characterized by the number of antiparallel β-strands and by the shear number, which is a measure for the inclination angle of the β-strands against the barrel axis. The outer membrane proteins (OMPs) of bacteria form transmembrane β-barrels with even numbers of β-strands ranging from 8 to 22 with shear numbers from 8-24 . The strands are tilted by 36° to 44° relative to the barrel axis [1, 2]. Examples are OmpA [3, 4], OmpX [5-7], NspA , and PagP [9, 10] (8 β-strands); OmpT  (10 β-strands); NalP  and OmPlA  (12 β-strands); FadL  (14 β-strands); Omp32 , matrix porin , OmpF , and PhoE  (16 β-strands); maltoporin (LamB)  and sucrose porin (ScrY)  (18 β-strands); FepA , BtuB [22, 23], and FhuA [24, 25] (22 β-strands). Monomers (OmpA, FhuA, OmpG ), dimers (OmPlA) and trimers (OmpF, PhoE) are known. β-barrel membrane proteins serve a wide range of different functions. They can be non-specific diffusion pores (OmpA, OmpC, OmpF), specific pores (LamB, ScrY), active transporters (FhuA, FepA, BtuB), enzymes such as proteases (OmpT), lipases (OmPlA), acyltransferases (PagP), or, like TolC, involved in solute efflux . Some examples of β-barrel membrane proteins are shown above.
OmpA is a small ion channel , OmpT is a protease, NalP is an autotransporter, FadL is a long chain fatty acid transporter, PhoE is a diffusion pore, ScrY is a sucrose specific porin, OmPlA is a phospholipase. FhuA and BtuB are active transporters for ferrichrom iron and vitamin B12 uptake, respectively. OMPs of mitochondria are predicted to form similar TM β-barrels. Examples are the VDAC channels, out of which more than a dozen have been sequenced . The membrane protein structures shown above were generated with MolMol .
Recently developed screening algorithms for the genomic identification of β-barrel membrane proteins indicate that there are many still not characterized outer membrane proteins for example in the genomes of E. coli and P. aeruginosa [31, 32]. Soluble bacterial toxins that can insert into membranes, such as α-hemolysine from Staphylococcus aureus , perfringolysine O from Clostridium perfringens [34, 35] also form β-barrels, but these are oligomeric.
Kleinschmidt, J.H. 2005, Folding and Stability of monomeric β-barrel Membrane proteins In: Protein-Lipid Interactions, Ed. Tamm, L.K. John Wiley & Sons
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