Time-resolved X-ray solution scattering observations of light-induced structural changes in sensory rhodopsin II

Unicellular organisms respond to a changing environment through physical and chemical sensory cascades. Sensory rhodopsin II (SRII) is a blue-light receptor, which initiates a negative phototaxis response in the host archaea. Here, we apply time-resolved X-ray solution scattering (TR-XSS) to characterize the mechanism of signal transduction by SRII and explore how light-induced structural changes are modified when SRII is in complex with its transducer protein (HtrII). TR-XSS difference data are modeled as arising from an outward movement of helices E and F in combination with modest changes associated with helices C and the extracellular regions of helices D and E. The magnitude of the displacement of helices E and F is similar irrespective of whether or not HtrII is present. In combination with structural predictions of the full SRII:HtrII complex, TR-XSS provides insight into how conformational changes may be communicated from SRII to the signaling domain of HtrII.