An all-trans-retinal-binding opsin peropsin as a potential dark-active and light-inactivated G protein-coupled receptor.

Peropsin or retinal pigment epithelium-derived rhodopsin homolog, found in many animals, belongs to the opsin family. Most opsins bind to 11-cis-retinal as a chromophore and act as light-activated G protein-coupled receptors. Some peropsins, however, bind all-trans-retinal and isomerise it into 11-cis form by light, and peropsin has been suggested to supply other visual opsins with 11-cis-retinal. Additionally, peropsin has some amino acid sequence motifs that are highly conserved among G protein-coupled opsins. Here, using chimeric mutant peropsins, we found that peropsin potentially generates an "active form" that drives G-protein signalling in the dark by binding to all-trans-retinal and that the active form photo-converts to an inactive form containing 11-cis-retinal. Comparative spectroscopic analysis demonstrated that spider peropsin exhibited catalytic efficiency for retinal photoisomerisation that was much lower than a retinal photoisomerase, squid retinochrome. The chimeric peropsins, constructed by replacing the third intracellular loop region with that of Gs- or Gi-coupled opsin, were active and drove Gs- or Gi-mediated signalling in the dark, respectively, and were inactivated upon illumination in mammalian cultured cells. These results suggest that peropsin acts as a dark-active, light-inactivated G protein-coupled receptor and is useful as a novel optogenetic tool.