X-Ray Crystallographic Analysis of 9-cis-Rhodopsin, a Model Analogue Visual Pigment

Recent progress in high‐resolution structural study of rhodopsin has been enabled by a novel selective extraction procedure with rod photoreceptor cells. In this study, we applied the method for rapid and efficient preparation of a purified analogue pigment using bovine rod outer segment membranes with 9‐cis‐retinal. After complete bleaching of the membranes and subsequent regeneration with the exogenous retinal, 9‐cis‐rhodopsin is selectively extracted from the membranes using combination of zinc and heptylthioglucoside. The solubilized sample, even with a small amount of contaminating retinal oximes, is shown to be pure enough for three‐dimensional crystallization. The X‐ray diffraction from 9‐cis‐rhodopsin crystals was examined and the electron density map at 2.9 Å resolution in the chromophore region can be fitted well with the model of 9‐cis‐retinal Schiff base.