A comparative study of the actin-based motilities of the pathogenic bacteria Listeria monocytogenes, Shigella flexneri and Rickettsia conorii

Listeria monocytogenes, Shigella flexneri, and Rickettsia conorii are three bacterial pathogens that are able to polymerize actin into 'comet tail' st ructures and move within the cytosol of infected cells. The actin-based mot ilities of L, monocytogenes and S, flexneri are known to require the bacter ial proteins ActA and IcsA, respectively, and several mammalian cytoskeleto n proteins including the Arp2/3 complex and VASP (vasodilator-stimulated ph osphoprotein) for L. monocytogenes and vinculin and N-WASP (the neural Wisk ott-Aldrich syndrome protein) for S. flexneri. In contrast, little is known about the motility of R. conorii. In the present study, we have analysed t he actin-based motility of this bacterium in comparison to that of L. monoc ytogenes and S. flexneri, Rickettsia moved at least three times more slowly than Listeria and Shigella in both infected cells and Xenopus laevis egg e xtracts. Decoration of actin with the S1 subfragment of myosin in infected cells showed that the comet tails of Rickettsia have a structure strikingly different from those of L. monocytogenes or S, flexneri. In Listeria and S higella tails, actin filaments form a branching network while Rickettsia ta ils display longer and not cross-linked actin filaments. Immunofluorescence studies revealed that the two host proteins, VASP and alpha-actinin coloca lized with actin in the tails of Rickettsia but neither the Arp2/3 complex which we detected in the Shigella actin tails, nor N-WASP, were detected in Rickettsia actin tails, Taken together, these results suggest that R, cono rii may use a different mechanism of actin polymerization.