1. Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
2. RIKEN Harima Institute/SPring-8, 1-1-1, Kouto, Sayo, Hyogo 679-5148, Japan

Correspondence to: Tsutomu Kouyama^1,2 Correspondence and requests for materials should be addressed to T.K. (Email: kouyama@bio.phys.nagoya-u.ac.jp).

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 C. 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 Å resolution. Among seven transmembrane -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.

1. Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
2. RIKEN Harima Institute/SPring-8, 1-1-1, Kouto, Sayo, Hyogo 679-5148, Japan

Correspondence to: Tsutomu Kouyama^1,2 Correspondence and requests for materials should be addressed to T.K. (Email: kouyama@bio.phys.nagoya-u.ac.jp).

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