MOLECULAR MECHANISM OF PHOTOSIGNALING BY ARCHAEAL SENSORY RHODOPSINS

Two sensory rhodopsins (SRI and SRII) mediate color-sensitive phototax is responses in halobacteria. These seven-helix receptor proteins, str ucturally and functionally similar to animal visual pigments, couple r etinal photoisomerization to receptor activation and are complexed wit h membrane-embedded transducer proteins (HtrI and HtrII) that modulate a cytoplasmic phosphorylation cascade controlling the flagellar motor . The Htr proteins resemble the chemotaxis transducers from Escherichi a coli. The SR-Htr signaling complexes allow studies of the biophysica l chemistry of signal generation and relay, from the photobiophysics o f initial excitation of the receptors to the final output at the level of the flagellar motor switch, revealing fundamental principles of se nsory transduction and more broadly the nature of dynamic interactions between membrane proteins. We review here recent advances that have l ed to new insights into the molecular mechanism of signaling by these membrane complexes.