Salmonella SipA Polymerizes Actin by Stapling Filaments with Nonglobular Protein Arms

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Science  26 Sep 2003:
Vol. 301, Issue 5641, pp. 1918-1921
DOI: 10.1126/science.1088433

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Like many bacterial pathogens, Salmonella spp. use a type III secretion system to inject virulence proteins into host cells. The Salmonella invasion protein A (SipA) binds host actin, enhances its polymerization near adherent extracellular bacteria, and contributes to cytoskeletal rearrangements that internalize the pathogen. By combining x-ray crystallography of SipA with electron microscopy and image analysis of SipA-actin filaments, we show that SipA functions as a “molecular staple,” in which a globular domain and two nonglobular “arms” mechanically stabilize the filament by tethering actin subunits in opposing strands. Deletion analysis of the tethering arms provides strong support for this model.

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