Biological Sciences: Lamellar Structure of Bleached and Unbleached Rod Photoreceptor Membranes

Abstract

WITHIN the past 3 yr, X-ray diffraction data from intact retinal rod outer segments have been obtained in several laboratories with sufficient resolution to permit analyses of the principal electron density fluctuations associated with the disk membranes^1–6. In 1969, Gras and Worthington¹ published diffraction patterns from fresh rod outer segments (Rana catesbeiana), showing the first 8 orders of a 314 Å repeat. They interpreted their data in terms of a linear strip density model, which featured an asymmetric membrane with the wider high density strip adjacent to the intradisk space. They considered it reasonable to assign the photopigment molecules to this wider strip. Later that year, Blaurock and Wilkins² published slightly higher resolution diffraction data from the rod outer segments of Rana temporaria, but handled their data somewhat differently. Their choice of an electron density profile was based on an initial interpretation of the Patterson map in terms of a lipid bilayer. Their phase choices were subsequently found to agree with swelling and shrinking studies using Ringer solutions of various tonicities. Their membrane profile appeared largely symmetric, in contrast to the linear strip model. The membrane profiles derived by both groups featured lipid bilayer regions, but the symmetry of the membrane, and hence the distribution of rhodopsin in the membrane, was in dispute.