Rhodopsin's secondary structure revisited: assignment of structural elements.

FT-IR spectroscopy has been applied to study the secondary structure of rhodopsin in dehydrated films of bovine rod photoreceptor membranes. Curve fitting analysis of the amide I band around 1658 cm-1 compares well with data obtained from samples in the hydrated state. Repeating this analysis on samples, treated with proteinase K or thermolysin, secondary structural elements at the cytoplasmic side of the photoreceptor membrane can be located. We present evidence for the location of a beta-sheet and a beta-turn near the lipid anchor in the C-terminal region of the protein, and for an alpha-helical structure in the third cytoplasmic loop.