Spectral diversity of vertebrate retinal photoisomerase RGRs

Vision begins with the photoisomerization of the chromophore 11-cis-retinal (11cR) to its all-trans form in visual rhodopsins. To regenerate visual rhodopsins, all-trans-retinal (atR) is re-isomerized to 11cR through a process known as the visual cycle in the eye. A key component of the vertebrate visual cycle is retinal G-protein-coupled receptor (RGR), which is a rhodopsin acting as a retinal photoisomerase that regenerates 11cR from atR by light. Although the spectral properties of visual rhodopsins have been extensively characterized, those of RGRs among different animals remain poorly understood. Here, we conducted a phylogenetically broad survey and found that RGRs are generally blue-absorbing rhodopsins, with absorption maximum wavelengths (λ_max) typically ranging from 470 to 495 nm. Strikingly, we identified an exceptional green-absorbing variant, zebrafish RGRb, which exhibits a substantial redshift with a λ_max of 528 nm. Mutational analysis revealed two residues, Ile98 and Ile181, as critical determinants of this redshifted phenotype. These findings uncover unexpected diversity in the spectral properties of RGRs and provide initial insights into their color-tuning mechanisms. In addition, comparison of bamboo shark and whale shark RGRs revealed no correlation between spectral properties and habitat depth, in contrast to the clear depth-related trends observed in shark visual rhodopsins. Our results establish an initial comprehensive framework for understanding the spectral diversity of vertebrate retinal photoisomerases and their adaptation to diverse light environments.