Transient dissociation of the transducer protein from anabaena sensory rhodopsin concomitant with formation of the M state produced upon photoactivation.

Anabaena sensory rhodopsin (ASR), a microbial rhodopsin in the cyanobacterium sp. PCC7120, has been suggested to regulate cell processes in a light-quality-dependent manner (color-discrimination) through interaction with a water-soluble transducer protein (Tr). However, light-dependent ASR-Tr interaction changes have yet to be demonstrated. We applied the transient grating (TG) method to investigate protein-protein interaction between ASR with Tr. The molecular diffusion component of the TG signal upon photostimulation of ASR(AT) (ASR with an all-trans retinylidene chromophore) revealed that Tr dissociates from ASR upon formation of the M-intermediate and rebinds to ASR during the decay of M; that is, light induces transient dissociation of ASR and Tr during the photocycle. Further correlating the dissociation of the ASR-Tr pair with the M-intermediate, no transient dissociation was observed after the photoexcitation of the blue-shifted ASR(13C) (ASR with 13-cis, 15-syn chromophore), which does not produce M. This distinction between ASR(AT) and ASR(13C), the two isomeric forms in a color-sensitive equilibrium in ASR, provides a potential mechanism for color-sensitive signaling by ASR.