Crystals of native and modified bovine rhodopsins and their heavy atom derivatives

Rhodopsin, the pigment protein responsible for dim-light vision, is a G protein-coupled receptor that converts light absorption into the activation of a G protein, transducin, to initiate the visual response. We have crystallised detergent-solubilised bovine rhodopsin in the native form and after chemical modifications as needles 10-40 mum in cross-section. The crystals belong to the trigonal space group P3(1), with two molecules of rhodopsin per asymmetric unit, related by a non-crystallographic 2-fold axis parallel with the crystallographic screw axis along c (needle axis). The unit cell dimensions are a = 103.8 Angstrom, c = 76.6 Angstrom for native rhodopsin, but vary over a wide range after heavy atom derivatisation, with a between 101.5 Angstrom and 113.9 Angstrom, and c between 76.6 Angstrom and 79.2 Angstrom. Rhodopsin molecules are packed with the bundle of transmembrane helices tilted from the c-axis by about 100degrees. The two molecules in the asymmetric unit form contacts along the entire length of their transmembrane helices 5 in an antiparallel orientation, and they are stacked along the needle axis according to the 3-fold screw symmetry. Hence hydrophobic contacts are prominent at protein interfaces both along and normal to the needle axis. The best crystals of native rhodopsin in this crystal form diffracted X-rays from a microfocused synchrotron source to 2.55 Angstrom maximum resolution. We describe steps taken to extend the diffraction limit from about 10 Angstrom to 2.6 Angstrom. (C) 2004 Elsevier Ltd. All rights reserved.