ARRANGEMENT OF RHODOPSIN TRANSMEMBRANE ALPHA-HELICES

Rhodopsins(1), the photoreceptors in rod cells, are G-protein-coupled receptors with seven hydrophobic segments containing characteristic co nserved sequence patterns that define a large family(2,3). Members of the family are expected to share a conserved transmembrane structure. Direct evidence for the arrangement of seven alpha-helices was obtaine d from a 9 Angstrom projection map of bovine rhodopsin(4). Structural constraints inferred from a comparison of G-protein-coupled receptor s equences were used to assign the seven hydrophobic stretches in the se quence to features in the projection map(5), A low-resolution three-di mensional structure of bovine rhodopsin(6) and two projection structur es of frog rhodopsin(7) confirmed the position of the three least tilt ed helices, 4, 6 and 7, A more elongated peak of density for helix 5 i ndicated that it is tilted or bent(6,7), but helices 1, 2 and 3 were n ot resolved, Here we have used electron micrographs of frozen-hydrated two-dimensional frog rhodopsin crystals to determine the structure of frog rhodopsin, Seven rods of density in the map are used to estimate tilt angles for the seven helices. Density visible on the extracellul ar side of the membrane suggests a folded domain, Density extends from helix 6 on the intracellular side, and a short connection between hel ices 1 and 2, and possibly a part of the carboxy terminus, are visible .