FipB, an Essential Virulence Factor of Francisella tularensis subsp. tularensis, Has Dual Roles in Disulfide Bond Formation

FipB, an essential virulence factor of Francisella tularensis, is a lipoprotein with two conserved domains that have similarity to disulfide bond formation A (DsbA) proteins and the amino-terminal dimerization domain of macrophage infectivity potentiator (Mip) proteins, which are proteins with peptidyl-prolyl cis/trans isomerase activity. This combination of conserved domains is unusual, so we further characterized the enzymatic activity and the importance of the Mip domain and lipid modification in virulence. Unlike typical DsbA proteins, which are oxidases, FipB exhibited both oxidase and isomerase activities. FipA, which also shares similarity with Mip proteins, potentiated the isomerase activity of FipB in an in vitro assay and within the bacteria, as measured by increased copper sensitivity. To determine the importance of the Mip domain and lipid modification of FipB, mutants producing FipB proteins that lacked either the Mip domain or the critical cysteine necessary for lipid modification were constructed. Both strains replicated within host cells and retained virulence in mice, though there was some attenuation. FipB formed surface-exposed dimers that were sensitive to dithiothreitol (DTT), dependent on the Mip domain and on at least one cysteine in the active site of the DsbA-like domain. However, these dimers were not essential for virulence, because the Mip deletion mutant, which failed to form dimers, was still able to replicate intracellularly and retained virulence in mice. Thus, the Mip domains of FipB and FipA impart additional isomerase functionality to FipB, but only the DsbA-like domain and oxidase activity are essential for its critical virulence functions.