Physical analysis of light-scattering changes in bovine photoreceptor membrane suspensions.

We have used electron microscopy and model calculations to analyze the physical basis of light-scattering signals from suspensions of photoreceptor membranes. These signals have previously been used to probe interactions between photoactivated rhodopsin (R*) and the peripheral membrane enzyme, GTP-binding protein (G) (Kühn et al., 1981, Proc. Natl. Acad. Sci. USA., 78:6873-6877). Although there is no unique physical interpretation of these signals, we have shown in this study that they were qualitatively unchanged when the rod outer segment fragments (containing stacked disks) were fragmented by sonication or osmotic shock to produce spherical disk membrane vesicles. An exact treatment of the scattering process for spherical vesicles enabled us to evaluate the effects of changing membrane thickness, refractive index, or vesicle diameter. We present a particular redistribution of mass upon R*-G interaction that fits the experimental data.