Retinal Chromophore Configuration in the O Intermediate of Sensory Rhodopsin II from Natronomonas pharaonis

Sensory rhodopsin II (SRII) is a prototype photosensor that binds the retinal Schiff base chromophore. Upon photoabsorption, SRII is transformed into the signaling state, where two long-lived photointermediates are known to contribute. One is the M intermediate containing the deprotonated 13-cis chromophore, and the other is the O intermediate that is believed to have the protonated all-trans chromophore. The chromophore in the O intermediate is also thought to have the atypical 15-syn (C═N cis) configuration about the Schiff base moiety. In this communication, we study SRII from Natronomonas pharaonis (NpSRII) using Raman spectroscopy and find that the retinal chromophore configuration in the O intermediate is the 13-cis, 15-anti (C═N trans), contrary to the conventional notion. This result points out the revision of the chromophore structural changes underlying the long-lived signaling state of SRII.