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Cell biology

Pathogen propulsion

Bacterial pathogens have a cunning way of moving about inside the cells they infect — they harness components of the host's cytoskeletal machinery, in particular, the protein actin.

One such pathogen, Rickettsia conorii, is transmitted to humans through tick bites, and causes Mediterranean spotted fever. This bacterium assembles an elaborate ‘tail’ made of actin filaments to propel itself through the cytoplasm of the infected host cell and to invade neighbouring cells. Rickettsia tails consist of parallel, unbranched filaments that closely resemble those present in thread-like cellular projections called filopodia.

Elsewhere in this issue, Pascale Cossart and colleagues shed light on the molecular mechanism that generates Rickettsia tails (E. Gouin et al. Nature 427, 457–461; 2004). The authors had previously identified a Rickettsia protein, RickA, that is structurally similar to members of the WASP family found in higher organisms. WASPs are potent activators of the Arp2/3 complex, which nucleates actin filaments.

In the latest development, Cossart's group has found that RickA occurs on the bacterial surface, where actin filaments are generated. Using an in vitro actin polymerization assay, they go on to show that although RickA on its own cannot form actin filaments, it can activate the Arp2/3 complex — thereby stimulating actin polymerization. They extend these studies to show that Rickettsia uses the Arp2/3 complex in vivo to form actin tails, and that RickA can induce the formation of filopodium-like projections when it is targeted to the plasma membrane. So future work on Rickettsia motility might help to clarify the poorly understood process of filopodium formation.

Rickettsia actin tails are very different to those created by another pathogen, Listeria monocytogenes, which consist of shorter, highly branched arrays of actin filaments. Nevertheless, the Listeria tails are also formed by the Arp2/3 complex, which the bacterium recruits from the host cell using the surface protein ActA. So pathogens may have evolved different ways to induce actin polymerization using the same effector, the Arp2/3 complex.

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Nath, D. Pathogen propulsion. Nature 427, 407 (2004). https://doi.org/10.1038/427407a

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  • DOI: https://doi.org/10.1038/427407a

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