Open AccessRemoval of transducer HtrI allows electrogenic proton translocation by sensory rhodopsin I.
Author(s) -
Roberto A. Bogomolni,
Walther Stoeckenius,
István Szundi,
Eduardo Perozo,
Karl D. Olson,
John L. Spudich
Publication year - 1994
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.91.21.10188
Subject(s) - bacteriorhodopsin , halorhodopsin , rhodopsin , phototaxis , biophysics , ion pump , chemistry , halobacterium salinarum , halobacteriaceae , proton pump , transmembrane protein , membrane , deprotonation , transduction (biophysics) , halobacterium , ion transporter , proton , ion channel , kinetics , ion , biochemistry , biology , retinal , receptor , atpase , botany , physics , organic chemistry , quantum mechanics , enzyme
Sensory rhodopsin I (sR-I) is a phototaxis receptor in halobacteria, which is closely related to the light-driven proton pump bacteriorhodopsin and the chloride pump halorhodopsin found in the same organisms. The three pigments undergo similar cyclic photoreactions, in spite of their different functions. In intact cells or isolated membranes sR-I is complexed with protein HtrI, the next link in the signal transduction chain, and does not function as an electrogenic ion pump. However, illumination of sR-I in membranes lacking HtrI causes pH changes in the medium, and its photoreaction kinetics become pH-dependent. We show here that in closed vesicles, near neutral pH it functions as an electrogenic 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 occurs because the 587-nm form of HtrI-free sR-I exists in a pH-dependent equilibrium with a 550-nm absorbing species generated through deprotonation of one group with a pKa of 7.2, which we have tentatively identified as Asp-76. We interpret the results in terms of a general model for ion translocation by the bacterial rhodopsins.