Proton-Based Structural Analysis of a Heptahelical Transmembrane Protein in Lipid Bilayers

The structures and properties of membrane proteins in lipid bilayers are expected to closely resemble those in native cell membrane environments, although they have been difficult to elucidate. By performing solid-state NMR measurements at very fast (100 kHz) magic-angle spinning rates and at high (23.5 T) magnetic field, severe sensitivity and resolution challenges are overcome, enabling the atomic-level characterization of membrane proteins in lipid environments. This is demonstrated by extensive 114 based resonance assignments of the fully protonated heptahelical membrane protein proteorhodopsin, and the efficient identification of numerous H-1-H-1 dipolar interactions, which provide distance constraints, inter-residue proximities, relative orientations of secondary structural elements, and protein cofactor interactions in the hydrophobic transmembrane regions. These results establish a general approach for high-resolution structural studies of membrane proteins in lipid environments via solid-state NMR.