Structural insights into the early steps of receptor-transducer signal transfer in archaeal phototaxis

Electron paramagnetic resonance-based inter-residue distance measurements b etween site-directed spin-labelled sites of sensory rhodopsin II (NpSRII) a nd its transducer NpHtrII from Natronobacterium pharaonis revealed a 2:2 co mplex with 2-fold symmetry. The core of the complex is formed by the four t ransmembrane helices of a transducer dimer. Upon light excitation, the prev iously reported flap-like movement of helix F of NpSRII induces a conformat ional change in the transmembrane domain of the transducer. The inter-resid ue distance changes determined provide strong evidence for a rotary motion of the second transmembrane helix of the transducer. This helix rotation be comes uncoupled from changes in the receptor during the last step of the ph otocycle.