Identification and characterization of the mammalian association and actin-nucleating domains in the Rickettsia conorii autotransporter protein, Sca2.

Establishment of infection by spotted fever group rickettsial species is dependent on the ability of these bacteria to adhere to and invade the host endothelium. Recent studies have attributed these processes to a handful of rickettsial surface proteins from the surface cell antigen (sca) family of autotransporters. A rickettsial autotransporter from Rickettsia conorii, Sca2, has been shown to be sufficient to mediate both adherence and invasion of human endothelial cells and to participate in intracellular actin-based motility. Here we identify a region of Sca2 capable of interacting with the mammalian cell surface and show that this function of Sca2 is independent and separable from its actin nucleation activity. Furthermore, pre-incubation of mammalian cells with the Sca2 mammalian association region prior to R. conorii infection can competitively inhibit rickettsial invasion, suggesting that Sca2 plays an important role in the initial interaction with mammalian cells. Together, our results demonstrate that the Sca2 autotransporter protein in R. conorii contains distinct functional domains that likely are involved in mediating cellular interactions at the plasma membrane and the host cytosol.