title: Crystal structure of squid rhodopsin.
abstract: Invertebrate phototransduction uses an inositol-1,4,5-trisphosphate signalling cascade in which photoactivated rhodopsin stimulates a G(q)-type G protein, that is, a class of G protein that stimulates membrane-bound phospholipase Cbeta. The same cascade is used by many G-protein-coupled receptors, indicating that invertebrate rhodopsin is a prototypical member. Here we report the crystal structure of squid (Todarodes pacificus) rhodopsin at 2.5 A resolution. Among seven transmembrane alpha-helices, helices V and VI extend into the cytoplasmic medium and, together with two cytoplasmic helices, they form a rigid protrusion from the membrane surface. This peculiar structure, which is not seen in bovine rhodopsin, seems to be crucial for the recognition of G(q)-type G proteins. The retinal Schiff base forms a hydrogen bond to Asn 87 or Tyr 111; it is far from the putative counterion Glu 180. In the crystal, a tight association is formed between the amino-terminal polypeptides of neighbouring monomers; this intermembrane dimerization may be responsible for the organization of hexagonally packed microvillar membranes in the photoreceptor rhabdom.
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Midori
authors.initials: M
authors.lastname: Murakami
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Kouyama
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D018360:Crystallography, X-Ray; D049832:Decapodiformes / Q000737:chemistry*; D019281:Dimerization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D012172:Retinaldehyde / Q000378:metabolism; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism; D012545:Schiff Bases; D014785:Vision, Ocular / Q000502:physiology; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: D012545:Schiff Bases; D014867:Water; D012243:Rhodopsin; D012172:Retinaldehyde