Transducer binding establishes localized interactions to tune sensory rhodopsin II

In haloarchaea, sensory rhodopsin 11 (SRII) mediates a photophobic response to avoid photo-oxidative damage in bright light. Upon light activation the receptor undergoes a conformational change that activates a tightly bound transducer molecule (Htrll), which in turn by a chain of homologous reactions transmits the signal to the chernotactic eubacterial two-component system. Here, using single-molecule force spectroscopy, we localize and quantify changes to the intramolecular interactions within SRIl of Natronomonas pharaonis (NpSRII) upon NpHtrII binding. Transducer binding affected the interactions at transmembrane ot helices IF and G of NpSRII to which the transducer was in contact. Remarkably, the interactions were distributed asymmetrically and significantly stabilized a helix G entirely but oc helix F only at its extracellulartip. These findings provide unique insights into molecular mechanisms that "prime" the complex for signaling, and guide the receptor toward transmitting light-activated structural changes to its cognate transducer.