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Rickettsia Sca2 is a bacterial formin-like mediator of actin-based motility

Abstract

Diverse intracellular pathogens subvert the host actin-polymerization machinery to drive movement within and between cells during infection. Rickettsia in the spotted fever group (SFG) are Gram-negative, obligate intracellular bacterial pathogens that undergo actin-based motility and assemble distinctive 'comet tails' that consist of long, unbranched actin filaments1,2. Despite this distinct organization, it was proposed that actin in Rickettsia comet tails is nucleated by the host Arp2/3 complex and the bacterial protein RickA, which assemble branched actin networks3,4. However, a second bacterial gene, sca2, was recently implicated in actin-tail formation by R. rickettsii5. Here, we demonstrate that Sca2 is a bacterial actin-assembly factor that functionally mimics eukaryotic formin proteins. Sca2 nucleates unbranched actin filaments, processively associates with growing barbed ends, requires profilin for efficient elongation, and inhibits the activity of capping protein, all properties shared with formins. Sca2 localizes to the Rickettsia surface and is sufficient to promote the assembly of actin filaments in cytoplasmic extract. These results suggest that Sca2 mimics formins to determine the unique organization of actin filaments in Rickettsia tails and drive bacterial motility, independently of host nucleators.

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Figure 1: Sca2 shares sequence motifs with actin assembly factors.
Figure 2: Sca2 nucleates actin filaments.
Figure 3: Sca2 is a profilin-dependent actin-filament-elongation factor that protects barbed ends from capping protein.
Figure 4: Sca2 processively associates with growing filament barbed ends and elongation is accelerated by profilin.
Figure 5: Sca2 localizes to actin-associated bacterial surfaces and is sufficient to promote actin polymerization in cell extracts.

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Acknowledgements

We thank T. Hackstadt and T. Clark for reagents, C. Paddock for the R. parkeri strain, the R. Heald lab for Xenopus egg extracts, B. Scott for help with alignments, Y. Li for technical assistance, and E. Benanti, K. Campellone, A. Serio, S. Reed and T. Ohkawa for comments on the manuscript. This work was funded by NIH-NIAID grant R01 AI074760 (to M.D.W.) and NIH grant R01 GM079265 (to D.R.K.).

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C.M.H., D.R.K. and M.D.W. designed the experiments. C.M.H., D.R.K, C.T.S. and J.E.C. performed the experiments. C.M.H. and M.D.W. wrote the manuscript with input from D.R.K.

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Correspondence to Matthew D. Welch.

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Haglund, C., Choe, J., Skau, C. et al. Rickettsia Sca2 is a bacterial formin-like mediator of actin-based motility. Nat Cell Biol 12, 1057–1063 (2010). https://doi.org/10.1038/ncb2109

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