Incorporation of outer membrane protein OmpG in lipid membranes:: Protein-lipid interactions and β-barrel orientation

OmpG is an intermediate size, monomeric, outer membrane protein from Escherichia coli, with n(beta) = 14 beta-strands. It has a large pore that is amenable to modification by protein engineering. The stoichiometry (N-b = 20) and selectivity (K-r = 0.7-1.2) of lipid-protein interaction with OmpG incorporated in dimyristoyl phosphatidylcholine bilayer membranes was determined with various 14-position spin-labeled 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 (beta-strand tilt and beta-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 beta-barrel order parameters) is observed at the point of hydrophobic matching of the protein with lipid chain length. Compared with smaller (OmpA; n(beta) = 8) and larger (FhuA; n(beta) = 22) monomeric E. coli outer membrane proteins, the stoichiometry of motionally restricted lipids increases linearly with the number of beta-strands, the tilt (beta similar to 44 degrees) of the beta-strands is comparable for the three proteins, and the order parameter of the beta-barrel increases regularly with n(beta). These systematic features of the integration of monomeric beta-barrel proteins in lipid membranes could be useful for characterizing outer membrane proteins of unknown structure.