Infrared dichroism of twisted beta-sheet barrels. The structure of E-coli outer membrane proteins

The infrared dichroic ratios of the amide bands from oriented beta-barrels yield an experimental value for the mean orientation, beta, of the beta-str ands, relative to the barrel axis. For a barrel of n strands, this then giv es shear number, S, that characterizes the stagger of the beta-sheet. Combi ning values of beta and n specifies the barrel geometry by using the optimi zed model of Murzin, Lesk & Chothia for regular barrels. Application to pub lished infrared data on the Escherichia coli outer membrane protein, OmpA y ields S = 9 - 10 (n = 8), a barrel radius of 0.81(+/-0.01) nm, and an inter nal free volume of 0.031 nm(3) per residue, where the average twist of the beta-sheets is theta x 28 degrees, and their coiling angle is epsilon appro ximate to 1 degrees. Hydrophobic matching of the 2.6 nm transmembrane stret ch partly determines the shear number of the OmpA beta-barrel. (C) 2000 Aca demic Press.