On the Topography of Photoreceptor Membranes
Rhodopsin, the principal photoreceptor membrane protein (85%) cannot be spectrally degraded by proteases at any enzyme concentration (6 hrs incubation at 30°) as long as the membrane structure is intact.
If, however, the arrangement of the membrane is disturbed by detergents, the typical rhodopsin absorption maximum at 500 nm is rapidly destroyed by proteases. Thus, either rhodopsin becomes accessible to the enzymes after interruption of lipid-protein interactions or the detergent unfolds the rhodopsin molecule itself.
In order to distinguish between these two possibilities, photoreceptor membranes were pretreated with phospholipase C, which destroys the membrane bilayer arrangement by removing the polar head groups from the phospholipids. Again, rhodopsin is no longer spectrally resistant against proteolytic attack, which suggests that, indeed, the lipid bilayer protects rhodopsin.
Another photoreceptor membrane protein, retinol oxidoreductase, is very sensitive to proteases, also in the intact membrane.
Peptide analysis by SDS-gel electrophoresis shows a progressive proteolytic breakdown of all membrane proteins in detergent. However, in the intact membrane, rhodopsin is apparently only susceptible to a limited degree, which does not interfere with its absorption spectrum, while the other photoreceptor membrane proteins are largely degraded.