UC Berkeley

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' containing long, unbranched actin filaments. Despite this distinct organization, it was proposed that actin in Rickettsia comet tails was nucleated by the host Arp2/3 complex and the bacterial protein RickA, which assemble branched actin networks. To identify additional rickettisal proteins that might function in actin assembly, we searched translated Rickettsia genome databases for proteins with WASP homology 2 (WH2) motifs, which are actin-binding peptides found in many cellular proteins. We identified WH2 motifs in Rickettsia Sca2 (surface cell antigen 2), a protein of the autotransporter family. Here, we demonstrate that R. parkeri Sca2 represents a new class of bacterial actin assembly factor that functionally mimics the eukaryotic formin family of actin nucleators. R. parkeri Sca2 nucleates unbranched actin filaments, processively associates with growing barbed ends, requires profilin for efficient elongation, and inhibits the activity of filament capping proteins, all properties shared with formins. Sca2 localizes to the R. parkeri surface, is enriched at the actin tail interface, and is sufficient to promote the assembly of long actin filaments in cytoplasmic extract. There is a strong correlation between the domain organization of Sca2 and the actin-based motility phenotype in diverse Rickettsia species. 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.