Removal of the transducer protein from sensory rhodopsin I exposes sites of proton release and uptake during the receptor photocycle

The phototaxis receptor sensory rhodopsin-I (SR-I) was genetically truncated in the COOH terminus which leads to overexpression in Halobacterium salinarium and was expressed in the presence and absence of its transducer, HtrI. Pyranine (8-hydroxyl-1,3,6-pyrene-trisulfonate) was used as a pH probe to show that proton release to the bulk phase results from the SR-I587 to S373 photoconversion, but only in the absence of transducer. The stoichiometry is 1 proton/S373 molecule formed. When SR-I is overexpressed in the presence of HtrI, the kinetics of the thermal return of S373 to SR-I587 is biphasic. A kinetic dissection indicates that overexpressed SR-I is present in two pools: one pool which generates an SR-I molecule possessing a normal (i.e., transducer-interacting) pH-independent rate of S373 decay, and a second pool which shows the pH-dependent kinetics of transducer-free S373 decay. The truncated SR-I receptor functions normally based on the following criteria: (i) Truncated SR-I restores phototaxis (attractant and repellent responses) when expressed in a strain lacking native SR-I, but containing HtrI. (ii) The absorption spectrum and the flash-induced absorption difference spectrum are indistinguishable from those of native SR-I. (iii) The rate of decay of S373 is pH-dependent in the absence of HtrI but not in the presence of HtrI. The data presented here indicate that a proton-conducting path exists between the protonated Schiff base nitrogen and the extramembranous environment in the transducer-free receptor, and transducer binding blocks this path.