Figure 3. The SifA C-term is accountable for the SKIP-independent function of SifA in virulence.

(A–H) Schematic representation of the SifA status in mixed infections of C57BL/6 (light, upper right part) or SKIP^−/− (shadowed lower left part) mice. The two-domain protein SifA is symbolized by a blue cylinder (N-term domain) connected to a green parallelepiped (C-term). The C-terminal CAAX motif is lipidated (red square bracket). A ∆sifA strain does not express the SifA protein (colourless/dotted line). SifA^L130D (colourless N-term) carries a mutation (L130D, in orange) that prevents the binding to SKIP. The strain sifA(1-136) expresses only the SifA N-term domain. As far as the interaction with SKIP is concerned this domain is not functional in SKIP^−/− mice (in grey). The different mixed infections were used to analyse the : (A) contribution of SifA; (B & G) SKIP-dependent contribution of SifA; (C) contribution of the SifA C-term; (D & E) SKIP-independent contribution of SifA; (F) contribution of the SifA N-term; (H) SKIP-independent contribution of the SifA N-term - to Salmonella virulence. (I–N) C57BL/6 or SKIP^−/− mice were inoculated I.P. or P.O. with 10⁵ CFU of various combinations of two Salmonella strains as indicated. CIs were determined two (I.P.) or five (P.O.) days post-inoculation. Each symbol represents one mouse. Mean CI ± SD values are represented and indicated on the top of each mouse group. (I) A ∆sifA strain is significantly attenuated in SKIP^−/− mice. (J) A ∆sseG strain presents the same attenuation of virulence in C57BL/6 and SKIP^−/− mice. (K & L) SifA plays a SKIP-independent function in virulence. (M) The C-term domain of SifA supports its SKIP-independent function in virulence. (N) sifA and sseJ contribute independently to Salmonella virulence. A one-sample t-test was used to determine whether a CI was significantly different of one, and unpaired t-tests to compare two values. P values: ns, not significant; *P < 0.05; **P < 0.01; ***P < 0.001.