X-ray analysis of retinal photoreceptor structure

Abstract

X-ray diffraction intensities from frog and cattle retinal rod segments have been interpreted in terms of an electron density distribution along the rod axis. Phases were uniquely determined using direct methods of structure analysis. The correction factor was derived in explicit form and its value was obtained experimentally. The average electron density of the disc membrane was determined as a result of X-ray experiments using glycerol solutions. An absolute electron density scale for the disc membrane was thus obtained. The derived electron density levels provide a reality condition for the correctness of the disc membrane profile. It has been demonstrated that the electron density levels of the asymmetrical profile are realistic whereas those of the symmetrical profile are unrealistic. Therefore, the symmetrical profile is incorrect. Fourier syntheses of the disc structure at a resolution of 14 Å verify that the asymmetric disc membrane profile of Gras and Worthington (1969) is fundamentally correct. The present X-ray analysis confirms that the rhodopsin molecules are located on the inside of the disc. Fourier syntheses of untreated and glycerol-treated retinas and Fourier syntheses of bleached and unbleached retinas are shown and discussed.