Content area
Full Text
L E T T E R S
Rickettsia Sca2 is a bacterial formin-like mediator of actin-based motility
Cat M. Haglund1, Julie E. Choe1, Colleen T. Skau2, David R. Kovar2 and Matthew D. Welch1,3
Diverse intracellular pathogens subvert the host actinpolymerization 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, unbranchedactin 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 networks 3,4. However, a second bacterial gene, sca2, was recently implicated in actin-tail formation by R. rickettsii5. Here, we demonstrate that Sca2is 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.
The geometry of actin filaments in eukaryotic cells is specified by nucleation and elongation factors. These include the Arp2/3 complex and its nucleation-promoting factors (NPFs) that assemble branched networks, and formins and tandem-monomer-binding proteins that assemble unbranched filaments6. The Arp2/3 complex is essential for the motility of diverse microbial pathogens, including Listeria monocytogenes, Shigella flexneri and vaccinia virus, which each express a factor that mimics or recruits host NPFs7. These pathogens have been useful tools for investigating the assembly of branched actin arrays, such as those in cellular lamellipodia. Although many Rickettsia species express the NPF RickA3,4, the failure to observe Arp2/3 complex subunits inRickettsia comet tails1,810, the ability of Rickettsia to undergo motility
in cells in which Arp2/3 is inhibited810 and the unbranched organization of Rickettsia tails, suggest an Arp2/3-independent polymerization
mechanism. Other bacterial pathogens express tandem-monomer-binding nucleators, including VopF from Vibrio cholerae11, VopL fromVibrio parahaemolyticus12, and TARP from Chlamydia trachomatis13.However, these secreted effectors are not implicated in actin-based motility. Because SFG...