Unique Retinal Binding Pocket of Primate Blue-Sensitive Visual Pigment.

The visual pigments of humans contain 11-cis retinal as the chromophore of light perception, and its photoisomerization to the all-trans form initiates visual excitation in our eyes. It is well-known that three isomeric states of retinal (11-cis, all-trans, and 9-cis) are in photoequilibrium at very low temperatures such as 77 K. Here we report the lack of formation of the 9-cis form in monkey blue (MB) at 77 K, as revealed by light-induced difference Fourier transform infrared spectroscopy. This indicates that the chromophore binding pocket of MB does not accommodate the 9-cis form, even though it accommodates the all-trans form by twisting the chromophore. Mutation of the blue-specific tyrosine at position 265 to tryptophan, which is highly conserved in other animal rhodopsins, led to formation of the 9-cis form in MB, suggesting that Y265 is one of the determinants of the unique photochemistry in blue pigments. We also found that 9-cis retinal does not bind to MB opsin, implying that the chromophore binding pocket does not accommodate the 9-cis form at physiological temperature. The unique property of MB is discussed on the basis of the results presented here.