The experiment was repeated at least three times and a representative example is shown. Importantly, Hcp secretion as well as VipB production was efficiently restored upon expression of wild-type VipA in trans (Figure 4). To determine whether the drastic phenotypes of some of the mutants could be explained by a reduction in

VipA stability, we used immunoblot analysis and commercially available anti-His antibodies. By this approach, reduced levels of mutants Δ104-113, D104A and E112A were consistently detected (Figure 4). Of these, only Δ104-113 exhibited a null mutant-like phenotype with respect to Hcp secretion and VipB production. No obvious reduction in the total protein levels of any of AZ 628 in vivo the other mutants exhibiting a null phenotype was observed (Figure 4). To further analyze the stability of the VipA mutants, we used a protein stability assay. The ΔvipA mutant or ΔvipA expressing wild-type or mutated vipA in trans were grown in LB overnight SBI-0206965 and subcultured into fresh

medium supplemented with IPTG to induce VipA production. After addition of chloramphenicol to stop de novo protein synthesis, bacteria were collected at different time points and subjected to immunoblotting with antisera recognizing His6 (i.e. VipA) or VipB. In ΔvipA expressing wild-type VipA in trans, both VipA and VipB were very stable over a period of 240 min (Figure 5, top panel). In contrast, in the non-complemented ΔvipA mutant, VipB was barely detected in the time zero sample. We also expressed His6-tagged VipB in ΔvipA or ΔvipB mutant backgrounds and used anti-His antibodies to determine VipB stability. The overall levels of VipB were significantly lower in the ΔvipA strain, which was also reflected by a decrease in VipB stability over time after chloramphenicol addition (data not shown). In order to understand the effects of VipA on VipB, we also analyzed transcriptional stability of the vipA mutant, however, it produced

vipB transcripts at levels similar to the parental strain A1552, -1.77 ± 0.68 (P = 0.17). Thus, the extreme instability of VipB in the absence of VipA is most likely due to degradation by endogenous proteases. Similar results have also been found for homologous IglA/IglB of F. tularensis[6]. As already observed upon analyzing the Calpain pellet samples (above), mutant Δ104-113 was significantly less stable also in the protein stability assay; it did not support VipB stability and had essentially disappeared 120 min after stopping de novo protein synthesis. In comparison to wild-type VipA, some of the point mutants appeared less stable over time, especially D104A and E112A, although this did not affect VipB stability (Figure 5). In contrast, none of the double, triple, or quadruple mutants appeared to be affected for VipA stability; still, VipB was very unstable in these mutant backgrounds (Figure 5).