THE C9 METHYL-GROUP OF RETINAL INTERACTS WITH GLYCINE-121 IN RHODOPSIN

The visual pigment rhodopsin is a prototypical G protein-coupled recep tor. These receptors have seven trans-membrane helices and are activat ed by specific receptor-ligand interactions. Rhodopsin is unusual in t hat its retinal prosthetic group serves as an antagonist in the dark i n the 11-cis conformation but is rapidly converted to an agonist on ph otochemical cis to trans isomerization. Receptor-ligand interactions i n rhodopsin were studied in the light and dark by regenerating site-di rected opsin mutants with synthetic retinal analogues. A progressive d ecrease in light-dependent transducin activity was observed when a mut ant opsin with a replacement of Gly(121) was regenerated with 11-cis-r etinal analogues bearing progressively larger R groups (methyl, ethyl, propyl) at the C9 position of the polyene chain. A progressive decrea se in light activity was also observed as a function of increasing siz e of the residue at position 121 for both the 11-cis-9-ethyl- and the 11-cis-9-propylretinal pigments. In contrast, a striking increase of r eceptor activity in the dark-i.e., without chromophore isomerization-w as observed when the molecular volume at either position 121 of opsin or C9 of retinal was increased. The ability of bulky replacements at e ither position to hinder ligand incorporation and to activate rhodopsi n in the dark suggests a direct interaction between these two sites. A molecular model of the retinal-binding site of rhodopsin is proposed that illustrates the specific interaction between Gly(121) and the C9 Methyl group of 11-cis-retinal. Steric interactions in this region of rhodopsin are consistent with the proposal that movement of transmembr ane helices 3 and 6 is concomitant with receptor activation.