REMOVAL OF TRANSDUCER HTRI ALLOWS ELECTROGENIC PROTON TRANSLOCATION BY SENSORY RHODOPSIN-I

Sensory rhodopsin I (sR-T) is a phototaxis receptor in halobacteria, w hich is closely related to the light-driven proton pump bacteriorhodop sin and the chloride pump halorhodopsin found in the same organisms. T he three pigments undergo similar cyclic photoreactions, in spite of t heir different functions. In intact cells or isolated membranes sR-I i s complexed with protein HtrI, the next link in the signal transductio n chain, and does not function as an electrogenic ion pump. However, i llumination of sR-I in membranes lacking HtrI causes pH changes in the medium, and its photoreaction kinetics become pH-dependent. We show h ere that in closed vesicles, near neutral pH it functions as an electr ogenic proton pump capable of generating at least -80 mV transmembrane potential. The action spectrum shows a maximum 37 nm below the 587-nm absorption maximum of the native pigment. This apparent discrepancy o ccurs because the 587-nm form of HtrI-free sR-I exists in a pH-depende nt equilibrium with a 550-nm absorbing species generated through depro tonation of one group with a pK(a) of 7.2, which we have tentatively i dentified as Asp-76. We interpret the results in terms of a general mo del for ion translocation by the bacterial rhodopsins.