Retinal Chromophore Configuration in the O Intermediate of Sensory Rhodopsin II from <i>Natronomonas pharaonis</i>

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-<i>cis</i> chromophore, and the other is the O intermediate that is believed to have the protonated all-<i>trans</i> chromophore. The chromophore in the O intermediate is also thought to have the atypical 15-<i>syn</i> (C═N <i>cis</i>) configuration about the Schiff base moiety. In this communication, we study SRII from <i>Natronomonas pharaonis</i> (<i>Np</i>SRII) using Raman spectroscopy and find that the retinal chromophore configuration in the O intermediate is the 13-<i>cis</i>, 15-<i>anti</i> (C═N <i>trans</i>), contrary to the conventional notion. This result points out the revision of the chromophore structural changes underlying the long-lived signaling state of SRII.