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

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.