PROJECTION STRUCTURE OF FROG RHODOPSIN IN 2 CRYSTAL FORMS

Rhodopsin is the G protein-coupled receptor that upon light activation triggers the visual transduction cascade. Rod cell outer segment disc membranes were isolated from dark-adapted frog retinas and were extracted with Tween detergents to obtain two-dimensional rhodopsin crystals for electron crystallography. When Tween 80 was used, tubular structures with a p2 lattice (a = 32 Angstrom, b = 83 Angstrom, gamma = 91 degrees) were formed, The use of a Tween 80/Tween 20 mixture favored the formation of larger p22(1)2(1) lattices (a = 40 Angstrom, b = 146 Angstrom, gamma = 90 degrees), Micrographs from frozen hydrated frog rhodopsin crystals were processed, and projection structures to 7-Angstrom resolution for the p22(1)2(1) form and to 6-Angstrom resolution for the p2 form Here calculated, The maps of frog rhodopsin in both crystal forms are very similar to the 9-Angstrom map obtained previously for bovine rhodopsin and show that the arrangement of the helices is the same. In a tentative topographic model, helices 4, 6, and 7 are nearly perpendicular to the plane of the membrane, In the higher-resolution projection maps of frog rhodopsin, helix 5 looks more tilted than it appeared previously, The quality of the two frog rhodopsin crystals suggests that they would be suitable to obtain a three-dimensional structure in which all helices would be resolved.