X-Ray Diffraction Analysis of Three-Dimensional Crystals of Bovine Rhodopsin Obtained from Mixed Micelles

Abstract

Rhodopsin, a prototypic G protein-coupled receptor responsible for absorption of photons in retinal rod photoreceptor cells, was selectively extracted from bovine rod outer segment membranes, employing mixed micelles of nonyl β-$\text{d}$-glucoside and heptanetriol. Highly purified rhodopsin was crystallized from solutions containing varying amounts of detergent and amphiphile. The crystals contained ground state rhodopsin molecules as judged by their red color and the linear dichroism originating from the 11-cis-retinal chromophore. However, when exposed to visible light, even at 4°C, rhodopsin was bleached and the crystals decomposed. Reflections in the diffraction pattern were observed out to 3.5-Å resolution at 100 K for the most ordered crystals. Diffraction data have been processed to 3.85-Å resolution. The symmetry of the diffraction pattern and the systematic absences indicate that the crystals have tetragonal symmetry, space group P4₁22 or P4₃22, a = b = 96.51 Å, c = 148.55 Å. A value of 4.12 ų/Da for V_M was obtained for one monomer in the asymmetric unit (eight molecules per unit cell). Our study is the first characterization of a three-dimensional crystal of a G protein-coupled receptor and may be valuable for future structural studies on related receptors of this important superfamily.