Orientation of β-barrel proteins OmpA and FhuA in lipid membranes. Chain length dependence from infrared dichroism

Abstract

The outer-membrane proteins OmpA and FhuA of Escherichia coli are monomeric β-barrels of widely differing size. Polarized attenuated total reflection infrared spectroscopy has been used to determine the orientation of the β-barrels in phosphatidylcholine host matrices of different lipid chain lengths. The linear dichroism of the amide I band from OmpA and FhuA in hydrated membranes generally increases with increasing chain length from diC(12:0) to diC(17:0) phosphatidylcholine, in both the fluid and gel phases. Measurements of the amide I and amide II dichroism from dry samples are used to deduce the strand tilt (β = 46° for OmpA and β= 44.5° for FhuA). These values are then used to deduce the order parameters, 〈P2(cos α)〉, of the β-barrels from the amide I dichroic ratios of the hydrated membranes. The orientational ordering of the β-barrels and their assembly in the membrane are discussed in terms of hydrophobic matching with the lipid chains. © 2005 American Chemical Society.