Structural analysis of photo-intermediates of squid rhodopsin

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Photochemical properties of invertebrate rhodopsin are different largely from vertebrate rhodopsin. Recent crystallographic studies of squid and bovine rhodopsins suggest profound differences in the structure of these photoreceptors. Here, we perform X-ray crystallographic studies of squid bathorhodopsin and an artificial 9-cis isomer isorhodopsin. Analyses of diffraction data of frozen crystals that were illuminated at these wavelengths provided the structural information of each isomeric state. The result showed that the formation of bathorhodopsin is accompanied by a large movement of the central moiety of the retinal polyene chain towards the cytoplasmic side with small movements of the s-ionone ring and the Schiff base ; i. e., the retinal takes on a right-handed screwed all-trans configuration. Conversely, the retinal in isorhodopsin takes on a planar 9-cis configuration ; i. e., as compared to the 11-cis retinal in the unphotolysed state, a limited configurational change is observed around the C9=C10 bond.