X-ray diffraction studies of retinal rods. I. Structure of the disc membrane, effect of illumination

The structure of the retinal rod disc membrane and its modifications upon bleaching have been studied by X-ray diffraction. Three types of preparations are used: functioning isolated from retina, isolated rods from frog retina, oriented by a magnetic field, and stacked discs from cattle retina. X-rays are detected by a position-sensitive linear counter. Diffraction spectra are obtained in 10-100 s. The electron density profile favors models where the rhodopsin molecule spans the whole thickness of the membrane. Upon bleaching, a small increase of electron density appears instantly at the cytoplasmic edge of the membrane. In the intact retina this structural change is accompanied by disorder and slow swelling reactions which are not observed in the isolated rod outer segment. The diffraction signal arising from the protein distribution in the plane of the membrane has been reinvestigated carefully. Patterns identical to those of Blasie (Blaise (1969) J. Mol. Biol. 39, 407 and Blaise (1972) Biophys. J. 12, 191) can be obtained but these are shown to be dominated by artefacts. The actual signal is a single broad band around (55 A)-1, upon which bleaching has a negligible effect. No measurable displacement of rhodopsin in the thickness of the membrane occurs upon bleaching. Temperature effects on the protein distribution are found to be large only for disc membranes from cattle retina. In this material from a warm-blooded animal those effects are correlated with the occurrence, upon lowering the temperature, of a partial phase transition of the paraffin chains of the lipids. The position and the slope of the transition are not sensitive to bleaching.