Orientation of rhodopsin alpha-helices in in retinal rod outer segment membranes studied by infrared linear dichroism.

Abstract

Frog retinal rod outer segments, oriented by a magnetic field, contained rhodopsin .alpha.-helical segments preferentially aligned perpendicular to the plane of the disc membrane, by the technique of IR linear dichroism. IR absorption parallel and perpendicular to the rod axes by peptide C.dbd.O groups, whose absorption band contains .alpha.-helical and random coil components at slightly different frequencies, showed positive dichroism centered on the .alpha.-helix frequency. The .alpha.-helical portion of the protein apparently has an average orientation in the transmembrane direction. IR spectra of rods in 2H2O Ringer''s solution exhibit 2 distinct peptide amino group absorption bands: the unexchanged N-H band, which is dichroic, and the proton-exchanged N-2H band, which is nondichroic. This implies that the oriented part of the protein is in the lipid bilayer, supporting a model for rhodopsin with a hydrophobic core containing partially oriented .alpha.-helices and hydrophilic ends consisting of unoriented polypeptide.