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J. Biol. Chem., Vol. 279, Issue 6, 4686-4695, February 6, 2004

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IcmR-regulated Membrane Insertion and Efflux by the Legionella pneumophila IcmQ Protein^*

Guillaume Duménil, Timothy P. Montminy, May Tang, and Ralph R. Isberg, An investigator of the Howard Hughes Medical Institute¶||

From the ^¶Howard Hughes Medical Institute and the Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111

Legionella pneumophila proliferates within alveolar macrophages as a central property of Legionnaires' disease. Intracellular growth involves formation of a replicative phagosome, which requires the bacterial Dot/Icm system, a multiprotein secretion apparatus that translocates proteins from the bacterium across the macrophage plasma membrane. Two components of this system, IcmR and IcmQ, are proposed to exhibit a chaperone/substrate relationship similar to that observed in other protein translocation systems. We report here that IcmQ inserts into lipid membranes and forms pores that allow the efflux of the dye calcein but not Dextran 3000. Both membrane insertion and pore formation were inhibited by IcmR. Trypsin digestion mapping demonstrated that IcmQ is subdivided into two functional domains. The N-terminal region of IcmQ was necessary and sufficient for insertion into lipid membranes and calcein efflux. The C-terminal domain was necessary for efficient association of the protein with lipid bilayers. IcmR was found to bind to the N-terminal portion of the protein thus providing a mechanism for its ability to inhibit IcmQ pore-forming activity. Localization of IcmQ on the surface of the L. pneumophila shortly after infection as well as its pore-forming capacities suggest a role for IcmQ in forming a channel that leads translocated effectors out of the bacterium.

Received for publication, September 5, 2003 , and in revised form, November 14, 2003.

^* 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 Supplementary Data.

An Irvington Institute for Immunological Research Fellow. Current address: INSERM U570, Faculté de Medecine Necker-Enfants Malades, 75015 Paris, France.

^|| To whom correspondence should be addressed. Tel: 617-636-3993; Fax: 617-636-0337; E-mail: ralph.isberg{at}tufts.edu.

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