The specificity of interaction of archaeal transducers with their cognate sensory rhodopsins is determined by their transmembrane helices.

Chimeras of the Halobacterium salinarum transducers HtrI and HtrII were constructed to study the structural determinants for their specific interaction with the phototaxis receptors sensory rhodopsins I and II (SRI and SRII), respectively. Interaction of receptors and transducers was assessed by two criteria: phototaxis responses by the cells and transducer-modulation of receptor photochemical reaction kinetics in membranes. Coexpression of HtrI with SRII or HtrII with SRI did not result in interaction by either criterion. Each receptor was coexpressed with chimeric transducers in which various domains of the two transducers were interchanged. The results show that the presence of the two transmembrane helices of HtrI in a chimera is necessary and sufficient for functional transducer complexation with SRI, i.e., for wild-type SRI photoreactions and attractant and 2-photon repellent phototaxis responses. Additionally, a previously demonstrated chaperone-like facilitation of SRI folding or stability by HtrI was shown to depend only on the two transmembrane helices of HtrI in chimeric transducers. Similarly, the two transmembrane helices of HtrII specify interaction with the repellent receptor SRII according to motility analysis and laser-flash spectroscopy. The results support a model in which the membrane domains of the receptor/transducer complexes, consisting of the seven helices of the receptor interacting with the four-helix bundle of the transducer dimer, produce SRI- and SRII-specific signals to the flagellar motor by means of interchangeable cytoplasmic domains.