Incorporation of Outer Membrane Protein OmpG in Lipid Membranes : Protein-lipid Interactions and β-Barrel Orientation

Abstract

OmpG is an intermediate size, monomeric, outer membrane protein from Escherichia coli, with nβ ) 14 β-strands. It has a large pore that is amenable to modification by protein engineering. The stoichiometry (Nb ) 20) and selectivity (Kr ) 0.7-1.2) of lipid-protein interaction with OmpG incorporated in dimyristoyl phosphatidylcholine bilayer membranes was determined with various 14-position spinlabeled lipids by using EPR spectroscopy. The limited selectivity for different lipid species is consistent with the disposition of charged residues in the protein. The conformation and orientation (β-strand tilt and β-barrel order parameters) of OmpG in disaturated phosphatidylcholines of odd and even chain lengths from C(12:0) to C(17:0) was determined from polarized infrared spectroscopy of the amide I and amide II bands. A discontinuity in the protein orientation (deduced from the β-barrel order parameters) is observed at the point of hydrophobic matching of the protein with lipid chain length. Compared with smaller (OmpA; nβ ) 8) and larger (FhuA; nβ ) 22) monomeric E. coli outer membrane proteins, the stoichiometry of motionally restricted lipids increases linearly with the number of β-strands, the tilt (β ∼ 44°) of the β-strands is comparable for the three proteins, and the order parameter of the β-barrel increases regularly with nβ. These systematic features of the integration of monomeric β-barrel proteins in lipid membranes could be useful for characterizing outer membrane proteins of unknown structure.