Electrophysiological characterization of specific interactions between bacterial sensory rhodopsins and their transducers

The halobacterial phototaxis receptors sensory rhodopsin I and II (SRI, SRI I) enable the bacteria to seek optimal light conditions for ion pumping by bacteriorhodopsin and/or halorhodopsin. The incoming signal is transferred across the plasma membrane by means of receptor-specific transducer protein s that bind tightly to their corresponding photoreceptors. To investigate t he receptor/transducer interaction, advantage is taken of the observation t hat both SRI and SRII can function as proton pumps. SRI from Halobacterium salinarium, which triggers the positive phototaxis, the photophobic recepto r SRII from Natronobacterium pharaonis (pSRII), as well as the mutant pSRII -F86D were expressed in Xenopus oocytes, Voltage-clamp studies confirm that SRI and pSRII function as light-driven, outwardly directed proton pumps wi th a much stronger voltage dependence than the ion pumps bacteriorhodopsin and halorhodopsin, Coexpression of SRI and pSRII-F86D with their correspond ing transducers suppresses the proton transport, revealing a tight binding and specific interaction of the two proteins. These latter results may be e xploited to further analyze the binding interaction of the photoreceptors w ith their downstream effecters.