Orientation of the Escherichia coli outer membrane protein OmpX in phospholipid bilayer membranes determined by solid-state NMR

Abstract

The solid-state NMR orientation-dependent frequencies measured for membrane proteins in macroscopically oriented lipid bilayers provide precise orientation restraints for structure determination in membranes. Here we show that this information can also be used to supplement crystallographic structural data to establish the orientation of a membrane protein in the membrane. This is achieved by incorporating a few orientation restraints, measured for the Escherichia coli outer membrane protein OmpX in magnetically oriented lipid bilayers (bicelles), in a simulated annealing calculation with the coordinates of the OmpX crystal structure. The ¹H−¹⁵N dipolar couplings measured for the seven Phe residues of OmpX in oriented bilayers can be assigned by back-calculation of the NMR spectrum from the crystal structure and are sufficient to establish the three-dimensional orientation of the protein in the membrane, while the ¹⁵N chemical shifts provide a measure of cross-validation for the analysis. In C14 lipid bilayers, OmpX adopts a transmembrane orientation with a 7° tilt of its β-barrel axis relative to the membrane normal, matching the hydrophobic thickness of the barrel with that of the membrane.