In order to survive in its host, M. tuberculosis must be able to survive and thrive in the in vivo milieu. M. tuberculosis has evolved many virulence factors to deal with the host response to infection. One important element is the MmpL family of proteins. Several of the MmpLs are required for virulence. The mmpL4- mutant has the most severe defect of all the mmpL- mutants, but the cause of the defect is unknown. Interestingly, the in the absence of interferon-γ, the mmpL4- mutant has no virulence defect, suggesting that the function of MmpL4 is to counteract a host immune response. Our work has revealed that MmpL4, an RND-like efflux pump, is required for virulence by conferring resistance to 4-cholestene-3-one (4c3), a toxic metabolite of cholesterol metabolism. Interestingly, both MmpL4 and cholesterol metabolism are only required when the host interferon-γ signaling pathway is intact.

MmpL4 is a member of a family of related lipid transporters called the MmpLs (Mycobacterial Membrane Protein - Large). This family is homologous to the RND family of transporters found across all kingdoms of life. Several members of the MmpLs have been shown to secrete specific lipids important for virulence. We initially believed this to be the case for MmpL4, but repeated attempts to identify differences in lipid composition in MmpL4- mutant cells were fruitless. We discovered that MmpL4 is only required when host interferon-γ signaling is intact and that this is due to increased toxicity of the mmpL4-γ mutants to 4c3. In addition to 4c3, mmpL4- cells are more sensitive to a variety of xenophobic compounds, raising the possibility that MmpL4 secretes other toxic compounds M. tuberculosis encounters in the host