Evidence for the archaebacterial-type conformation about the bond between the β-ionone ring and the polyene chain of the chromophore retinal in chlamyrhodopsin

Abstract

Previous studies using retinal analogs (Wada, A., Sakai, M., Kinumi, T., Tsujimoto, K., Yamauchi, M. and Ito, M. (1994) J. Org. Chem. 59, 6992–6997; Wada, A., Sakai, M., Imamoto, Y., Shichida., Y., Yoshizawa, T. and Ito, M. (1993) Chem. Pharm. Bull. 41, 793–795) revealed that both retinochrome and visual pigments share a common chromophoric conformation in which the ring region of retinal is twisted ca. 50° with respect to the plane of the polyene chain, suggesting a highly conserved 6-s-cis conformation throughout rhodopsin-like pigments in animals. By contrast, 6-s-trans conformation was commonly observed or suggested in archaebacterial rhodopsins examined thus far. Here we have reconstituted in vivo both the photoreceptor for photobehavioral responses of the unicellular alga Chlamydomonas reinhardtii and the second archaeal sensory photoreceptor phoborhodopsin from a series of retinal analogs. Results exclusively point to the conclusion that in both photoreceptors retinal has the coplanar 6-s-trans conformation, though recent molecular cloning studies revealed no homology between Chlamydomonas photoreceptor (chlamyrhodopsin) and archaeal rhodopsins