Two-dimensional crystallization of bovine rhodopsin

Abstract

Bovine rhodopsin has been clustered into two-dimensional crystals in highly purified native rod disk membranes and studied with negative staining and transmission electron microscopy. The lattice is P2 1 with dimensions of 8.3 × 7.9 nm and interaxis angles of 86 ± 3 degrees. 110 images of ordered areas were digitized and aligned with computer-correlation methods to calculate an average image with diffraction to the fourth order. The images were computer-filtered and reconstructed to approx. 2 nm resolution. When crystals appeared they covered 20–40% of the surface of the preparation and, since rhodopsin is at least 95% of the protein, there is no doubt that the crystals were due to rhodopsin. There appear to be two rhodopsin dimers per unit cell. Each rhodopsin molecules takes up about 7.5 nm 2 of membrane area and is estimated to be associated with about 12 lipids on each side of the membrane. The membrane area found for bovine rhodopsin supports the rhodopsin origin of rarely seen but more highly ordered two-dimensional crystals found in detergent-treated frog rod membranes (Corles, J.M., McCaslin, D.R. and Soctt, B.L. (1982) Proc. Natl. Acad. Sci. USA 79, 1116–1120). Furthermore, the rhodopsin membrane area is close to that of bacteriorhodopsin and is consistent with a seven transmembrane helix structure proposed for rhodopsin (for references see Dratz E.A. and Hargrave, D.A. (1983) Trends Biochem. Sci. 8, 128–131). Crystallization was accomplished by loweing the pH to 5.5 near the isoelectric point of rhodopsin, raising the salt concentration of 2 M (NH 4) 2SO 4, adding 5% glucose and 0.02% Hibitane (Ayerst), a cationic amphipathic antiseptic that favored crystal growth.