Photocyclic behavior of rhodopsin induced by an atypical isomerization mechanism

Abstract

Vertebrate rhodopsin (Rh) contains 11-<i>cis</i>-retinal as a chromophore to convert light energy into visual signals. On absorption of light, 11-<i>cis</i>-retinal is isomerized to all-<i>trans</i>-retinal, constituting a one-way reaction that activates transducin (G_t) followed by chromophore release. Here we report that bovine Rh, regenerated instead with a six-carbon-ring retinal chromophore featuring a C¹¹=C¹² double bond locked in its <i>cis</i> conformation (Rh6mr), employs an atypical isomerization mechanism by converting 11-<i>cis</i> to an 11,13-<i>dicis</i> configuration for prolonged G_t activation. Time-dependent UV-vis spectroscopy, HPLC, and molecular mechanics analyses revealed an atypical thermal reisomerization of the 11,13-<i>dicis</i> to the 11-<i>cis</i> configuration on a slow timescale, which enables Rh6mr to function in a photocyclic manner similar to that of microbial Rhs. With this photocyclic behavior, Rh6mr repeatedly recruits and activates G_t in response to light stimuli, making it an excellent candidate for optogenetic tools based on retinal analog-bound vertebrate Rhs. Overall, these comprehensive structure-function studies unveil a unique photocyclic mechanism of Rh activation by an 11-<i>cis</i>-to-11,13-<i>dicis</i> isomerization.