A Polarized Viewpoint?

Abstract

Invertebrates, like vertebrates, use the light-activated heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) rhodopsin as a photoreceptor in visual signal transduction. Unlike vertebrate rhodopsin, which signals through transducin, invertebrate rhodopsin signals through G q -type G proteins to stimulate an inositol 1,4,5-trisphosphate-based signaling pathway (see Schertler). Murakami and Kouyama determined the 2.5 Å resolution crystal structure of squid rhodopsin, shedding new light on mechanisms of signaling involving this GPCR family. Crystallization of truncated squid rhodopsin (lacking a C-terminal extension that is not involved in signal transduction) revealed two cytoplasmic helices as well as the seven transmembrane helices characteristic of GPCRs. Transmembrane helices V and VI projected 25 Å into the cytoplasm to form, together with cytoplasmic helices VIII and IX, a structure that is not present in bovine rhodopsin, which the authors propose is involved in interactions with G q . Rhodopsin consists of the protein opsin and the covalently bound chromatophore retinal; the amino acid residues in contact with retinal in squid rhodopsin differ from those in bovine rhodopsin, and retinal assumes a less distorted configuration in the former. Intriguingly, the arrangement of chromatophores in crystallized squid rhodopsin may provide insight into the ability of invertebrates to detect the polarization plane of light. M. Murakami, T. Kouyama, Crystal structure of squid rhodopsin. Nature 453 , 363-367 (2008). [PubMed] G. F. X. Schertler, The rhodopsin story continued. Nature 453 , 292-293 (2008). [PubMed]