Interactions of the beta-ionone ring with the protein in the visual pigment rhodopsin control the activation mechanism. An FTIR and fluorescence study on artificial vertebrate rhodopsins.

Abstract

The photoreactions of rhodopsin regenerated with three 9-cis retinal analogs, modified at or in the vicinity of the beta-ionone ring (namely 5,6-epoxy, 7,8-diH, diethyl-acyclic) have been investigated by UV-vis and FTIR difference spectroscopy. In parallel, the ability to catalyze the GDP-->GTP exchange of G-protein (transducin) has been monitored by time-dependent fluorescence spectroscopy. The first photoproduct obtained with all three pigments at liquid nitrogen temperature is a blue-shifted intermediate (BSI), followed by a lumi-like intermediate at 170 K. For the 5,6-epoxy-ISO and 7,8-diH-ISO pigment we obtain two further intermediates similar to the META-I and META-II states of native RHO. For the diethyl-acyclic-ISO pigment only one further intermediate can be stabilized at 280 K. As compared to META-II the respective photoproduct exhibits striking differences. The latter two pigments have also been investigated in the solubilized lipid-free state (detergent: dodecyl maltoside) at 280 K. For the 5,6-epoxy-ISO pigment, the UV-vis, FTIR, and activation data agree with the formation of a META-II-like photoproduct (81% activation). Less META-II formation is observed for the 7,8-dihydro-ISO pigment in membranes (65% activation), but full formation in detergent (100% activation). Neither the membrane-bound nor the solubilized diethyl-acyclic-ISO pigment forms a META-II-like intermediate (18% and 0% activation, respectively). Therefore, we conclude that the substitution of the beta-ionone ring by two ethyl groups abolishes steric interactions with the protein, which are essential for META-II formation.(ABSTRACT TRUNCATED AT 250 WORDS)