Rod Visual Pigment Optimizes Active State to Achieve Efficient G Protein Activation as Compared with Cone Visual Pigments*

Background: The relationship between the properties of visual pigment and photoreceptor amplification efficiency is controversial. Results: Rhodopsin activates G protein more efficiently than cone pigments. Conclusion: Visual pigment properties are directly related to the photoreceptor sensitivity diversification. Significance: Amplification of cell responses is regulated by receptor molecules. Most vertebrate retinas contain two types of photoreceptor cells, rods and cones, which show different photoresponses to mediate scotopic and photopic vision, respectively. These cells contain different types of visual pigments, rhodopsin and cone visual pigments, respectively, but little is known about the molecular properties of cone visual pigments under physiological conditions, making it difficult to link the molecular properties of rhodopsin and cone visual pigments with the differences in photoresponse between rods and cones. Here we prepared bovine and mouse rhodopsin (bvRh and mRh) and chicken and mouse green-sensitive cone visual pigments (cG and mG) embedded in nanodiscs and applied time-resolved fluorescence spectroscopy to compare their G(t) activation efficiencies. Rhodopsin exhibited greater G(t) activation efficiencies than cone visual pigments. Especially, the G(t) activation efficiency of mRh was about 2.5-fold greater than that of mG at 37 degrees C, which is consistent with our previous electrophysiological data of knock-in mice. Although the active state (Meta-II) was in equilibrium with inactive states (Meta-I and Meta-III), quantitative determination of Meta-II in the equilibrium showed that the G(t) activation efficiency per Meta-II of bvRh was also greater than those of cG and mG. These results indicated that efficient G(t) activation by rhodopsin, resulting from an optimized active state of rhodopsin, is one of the causes of the high amplification efficiency of rods.