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J. Biol. Chem., Vol. 279, Issue 41, 42964-42969, October 8, 2004

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Five Residues in the HtrI Transducer Membrane-proximal Domain Close the Cytoplasmic Proton-conducting Channel of Sensory Rhodopsin I^*

Xinpu Chen and John L. Spudich

From the Center for Membrane Biology, Department of Biochemistry and Molecular Biology, and Department of Microbiology and Molecular Genetics, University of Texas Medical School at Houston, Houston, Texas 77030

Transducer-free sensory rhodopsins carry out light-driven proton transport in Halobacterium salinarum membranes. Transducer binding converts the proton pumps to signal-relay devices in which the transport is inhibited. In sensory rhodopsin I (SRI) binding of its cognate transducer HtrI inhibits transport by closing a cytoplasmic proton-conducting channel necessary for proton uptake during the SRI photochemical reaction cycle. To investigate the channel closure, a series of HtrI mutants truncated in the membrane-proximal cytoplasmic portion of an SRI-HtrI fusion were constructed and expressed in H. salinarum membranes. We found that binding of the membrane-embedded portion of HtrI is insufficient for channel closure, whereas cytoplasmic extension of the second HtrI transmembrane helix by 13 residues blocks proton conduction through the channel as well as full-length HtrI. Specifically the closure activity is localized in this 13-residue membrane-proximal cytoplasmic domain to the 5 final residues, each of which incrementally contributes to reduction of proton conductivity. Moreover, these same residues in the dark incrementally and proportionally increase the pK_a of the Asp-76 counterion to the protonated Schiff base chromophore in the membrane-embedded photoactive site. We conclude that this critical region of HtrI alters the dark conformation of SRI as well as light-induced channel opening. The 5 residues in HtrI correspond in position to 5 residues demonstrated on the homologous NpHtrII to interact with the E-F loop of its cognate receptor NpSRII in the accompanying article (Yang, C.-S., Sineshchekov, O., Spudich, E. N., and Spudich, J. L. (2004) J. Biol. Chem. 279, 42970–42976). These results strongly suggest that the membrane-proximal region of Htr proteins interact with their cognate sensory rhodopsin cytoplasmic domains as part of the signal-relay coupling between the proteins.

Received for publication, June 11, 2004

^* This work was supported by National Institutes of Health Grant R37GM27750 and the Robert A. Welch Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Submitted in partial fulfillment of requirements for a Ph.D. degree from the University of Texas Graduate School of Biomedical Sciences, Houston, TX.

To whom correspondence should be addressed. Tel.: 713-500-5473; Fax: 713-500-0545; E-mail: John.L.Spudich{at}uth.tmc.edu.

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