1. Unité des Interactions Bactéries-Cellules, Institut Pasteur, 28 Rue du Docteur Roux, Paris 75015, France
2. Harvard Medical School, Department of Cell Biology, Boston, Massachusetts 02115, USA
3. Cleveland Clinic Foundation, Department of Cell Biology, Cleveland, Ohio 44195, USA
4. Massachusetts Institute of Technology, Department of Biology, Cambridge, Massachusetts 02139, USA

Correspondence to: Pascale Cossart¹ Correspondence and requests for materials should be addressed to P.C. (Email: pcossart@pasteur.fr). The sequence for RickA has the GenBank accession number RCO909.

Abstract

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 movements^{1, 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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