Transducer binding establishes localized interactions to tune sensory rhodopsin II.

In haloarchaea, sensory rhodopsin II (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 (HtrII), which in turn by a chain of homologous reactions transmits the signal to the chemotactic eubacterial two-component system. Here, using single-molecule force spectroscopy, we localize and quantify changes to the intramolecular interactions within SRII of Natronomonas pharaonis (NpSRII) upon NpHtrII binding. Transducer binding affected the interactions at transmembrane alpha helices F and G of NpSRII to which the transducer was in contact. Remarkably, the interactions were distributed asymmetrically and significantly stabilized alpha helix G entirely but alpha helix F only at its extracellular tip. 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.