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J. Biol. Chem., Vol. 280, Issue 43, 36293-36300, October 28, 2005

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The PscE-PscF-PscG Complex Controls Type III Secretion Needle Biogenesis in Pseudomonas aeruginosa^*

Manuelle Quinaud1, Jacqueline Chabert, Eric Faudry, Emmanuelle Neumann, David Lemaire, Alexandrine Pastor, Sylvie Elsen, Andréa Dessen, and Ina Attree2

From the Biochimie et Biophysique des Systèmes Intégrés, UMR 5092 CNRS/Commissariat à l'Energie Atomique (CEA)/Université Joseph Fourier (UJF), Département de Réponse et Dynamique Cellulaires, CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble cedex 09 and the Institut de Biologie Structurale Jean-Pierre Ebel, UMR 5075 CNRS/CEA/UJF, Grenoble, France

Type III secretion (T3S) systems play key roles in pathogenicity of many Gram-negative bacteria and are employed to inject toxins directly into the cytoplasm of target cells. They are composed of over 20 different proteins that associate into a basal structure that traverses both inner and outer bacterial membranes and a hollow, needle-like structure through which toxins travel. The PscF protein is the main component of the Pseudomonas aeruginosa T3S needle. Here we demonstrate that PscF, when purified on its own, is able to form needle-like fibers of 8 nm in width and >1 µm in length. In addition, we demonstrate for the first time that the T3S needle subunit requires two cytoplasmic partners, PscE and PscG, in P. aeruginosa, which trap PscF in a ternary, 1:1:1 complex, thus blocking it in a monomeric state. Knock-out mutants deficient in PscE and PscG are non-cytotoxic, lack PscF, and are unable to export PscF encoded extrachromosomally. Temperature-scanning circular dichroism measurements show that the PscE-PscF-PscG complex is thermally stable and displays a cooperative unfolding/refolding pattern. Thus, PscE and PscG prevent PscF from polymerizing prematurely in the P. aeruginosa cytoplasm and keep it in a secretion prone conformation, strategies which may be shared by other pathogens that employ the T3S system for infection.

Received for publication, July 25, 2005 , and in revised form, August 18, 2005.

^* This work was supported in part by the French cystic fibrosis association "Vaincre la Mucoviscidose." 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 supplemental Table IS.

¹ Recipient of a CFR fellowship from the Commissariat à l'Energie Atomique (CEA).

² To whom correspondence should be addressed: Tel.: 33-438-783-483; Fax: 33-438-784-499; E-mail: iattreedelic{at}cea.fr.

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