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J. Biol. Chem., Vol. 281, Issue 50, 38189-38199, December 15, 2006

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TorT, a Member of a New Periplasmic Binding Protein Family, Triggers Induction of the Tor Respiratory System upon Trimethylamine N-Oxide Electron-acceptor Binding in Escherichia coli^*

Claudine Baraquet¹, Laurence Théraulaz, Marianne Guiral, Daniel Lafitte^¶, Vincent Méjean, and Cécile Jourlin-Castelli²

From the Laboratoire de Chimie Bactérienne, Laboratoire de Bioénergétique et Ingénierie des Protéines, Institut de Biologie Structurale et Microbiologie, Centre National de la Recherche Scientifique, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20 and ^¶Cismet, FRE CNRS 2737, Aix Marseille Université, 27 boulevard Jean Moulin, 13385 Marseille cedex 5, France

In anaerobiosis, Escherichia coli can use trimethylamine N-oxide (TMAO) as a terminal electron acceptor. Reduction of TMAO in trimethylamine (TMA) is mainly performed by the respiratory TMAO reductase. This system is encoded by the torCAD operon, which is induced in the presence of TMAO. This regulation involves a two-component system comprising TorS, an unorthodox histidine kinase, and TorR, a response regulator. A third protein, TorT, sharing homologies with periplasmic binding proteins, plays a key role in this regulation because disruption of the torT gene abolishes tor expression. In this study we showed that TMAO protects TorT against degradation by the GluC endoproteinase and modifies its temperature-induced CD spectrum. We also isolated a TorT negative mutant that is no longer protected by TMAO from degradation by GluC. Isothermal titration calorimetry confirmed that TorT binds TMAO with a binding constant of 150 µM. Therefore, we conclude that TorT binds TMAO and that this binding promotes a conformational change of TorT. We also showed that TorT interacts with the periplasmic domain of TorS in both the presence and absence of TMAO but the TorT-TMAO complex induces a higher GluC protection of TorS than TorT alone. These results support the idea that TMAO binding to TorT induces a cascade of conformational changes from TorT to TorS, leading to TorS activation. We identified several homologues of the TorT protein that define a new family of periplasmic binding proteins. We thus propose that the members of this family bind TMAO or related compounds and that they are involved in signal transduction or even substrate transport.

Received for publication, May 5, 2006 , and in revised form, October 3, 2006.

^* This work was supported in part by grants from the Centre National de la Recherche Scientifique and the Université delaMéditerranée. 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.

The on-line version of this article (available at http://www.jbc.org) contains Table 3.

¹ Supported by a grant from the Conseil Régional PACA (Provence-Alpes-Côte d'Azur).

² To whom correspondence should be addressed: 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20, France. Tel.: 33-4-91-16-40-32; Fax: 33-4-91-71-89-14; E-mail: jourlin{at}ibsm.cnrs-mrs.fr.

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