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 ¹H-based resonance assignments of the fully protonated heptahelical membrane protein proteorhodopsin, and the efficient identification of numerous ¹H-¹H 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.