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The RickA protein of Rickettsia conorii activates the Arp2/3 complex


Actin polymerization, the main driving force for cell locomotion, is also used by the bacteria Listeria and Shigella and vaccinia virus for intracellular and intercellular movements1,2. Seminal studies have shown the key function of the Arp2/3 complex in nucleating actin and generating a branched array of actin filaments during membrane extension and pathogen movement3. Arp2/3 requires activation by proteins such as the WASP-family proteins or ActA of Listeria. We previously reported that actin tails of Rickettsia conorii, another intracellular bacterium, unlike those of Listeria, Shigella or vaccinia, are made of long unbranched actin filaments apparently devoid of Arp2/3 (ref. 4). Here we identify a R. conorii surface protein, RickA, that activates Arp2/3 in vitro, although less efficiently than ActA. In infected cells, Arp2/3 is detected on the rickettsial surface but not in actin tails. When expressed in mammalian cells and targeted to the membrane, RickA induces filopodia. Thus RickA-induced actin polymerization, by generating long actin filaments reminiscent of those present in filopodia, has potential as a tool for studying filopodia formation.

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Figure 1: The RickA protein of R. conorii activates Arp2/3.
Figure 2: Transfection with ScarWA or the C-terminal part of RickA inhibits actin polymerization.
Figure 3: Arp3 localizes with the bacteria R. conorii and with L. monocytogenes comet tails in Hep-2-infected cells.
Figure 4: Transfection of mammalian cells with RickA-CAAX induces filopodia-like structure.


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We thank L. Blanchoin for help and discussions, L. Machesky for the gift of plasmids Scar-WA and ScarW, and P. Renesto and all members of the Cossart laboratory for discussions and suggestions. This work was supported by the Pasteur Institute, the Ministère de la Recherche et de la Technologie (Programme PRFMMIP) and the Direction Générale des Armées (DGA). C.E. is supported by a Human Frontier Science programme fellowship. P.C. is an international scholar from the Howard Hughes Medical Institute.

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Correspondence to Pascale Cossart.

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Supplementary information

Supplementary Figure 1: The amino acid sequences of RickA from R. conorii and ActA from L. monocytogenes. (PDF 21 kb)


Supplementary Figure 2: The Rickettsia actin tails consist of long filaments. Electron microscopy of myosin S1 decorated actin tails of R. conorii and L. monocytogenes. (JPG 80 kb)


Supplementary Figure 3: The rickettsia actin tails contain the bundling protein fascin. . Vero cells were infected with Rickettsia and transfected 24 hours later with a plasmid expressing GFP-fascin before processing for immunofluorescence 24 hours later, using the R47 antibody. (JPG 60 kb)

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Gouin, E., Egile, C., Dehoux, P. et al. The RickA protein of Rickettsia conorii activates the Arp2/3 complex. Nature 427, 457–461 (2004).

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