Structure of the conserved Francisella virulence protein FvfA.

Francisella tularensis is a potent human pathogen that invades and survives in macrophage and epithelial cells. Two identical proteins, FTT_0924 from F. tularensis subsp. tularensis and FTL_1286 from F. tularensis subsp. holarctica LVS, have previously been identified as playing a role in protection of the bacteria from osmotic shock and its survival in macrophages. FTT_0924 has been shown to localize to the inner membrane, with its C-terminus exposed to the periplasm. Here, crystal structures of the F. novicida homologue FTN_0802, which we call FvfA, in two crystal forms are reported at 1.8 Šresolution. FvfA differs from FTT_0924 and FTL_1286 by a single amino acid. FvfA has a DUF1471 fold that closely resembles the Escherichia coli outer membrane lipoprotein RscF, a component of a phosphorelay pathway involved in protecting bacteria from outer membrane perturbation. The structural and functional similarities and differences between these proteins and their implications for F. tularensis pathogenesis are discussed.

Morphological analysis of Francisella novicida epithelial cell infections in the absence of functional FipA.

Francisella novicida is a surrogate pathogen commonly used to study infections by the potential bioterrorism agent, Francisella tularensis. One of the primary sites of Francisella infections is the liver where >90% of infected cells are hepatocytes. It is known that once Francisella enter cells it occupies a membrane-bound compartment, the Francisella-containing vacuole (FCV), from which it rapidly escapes to replicate in the cytosol. Recent work examining the Francisella disulfide bond formation (Dsb) proteins, FipA and FipB, have demonstrated that these proteins are important during the Francisella infection process; however, details as to how the infections are altered in epithelial cells have remained elusive. To identify the stage of the infections where these Dsbs might act during epithelial infections, we exploited a hepatocyte F. novicida infection model that we recently developed. We found that F. novicida ΔfipA-infected hepatocytes contained bacteria clustered within lysosome-associated membrane protein 1-positive FCVs, suggesting that FipA is involved in the escape of F. novicida from its vacuole. Our morphological evidence provides a tangible link as to how Dsb FipA can influence Francisella infections.