Conformational Switch Driven Membrane Pore Formation by Mycobacterium Secretory Protein MPT63 Induces Macrophage Cell Death.

Virulent Mycobacterium tuberculosis (MTB) strains create cell death of macrophages (Mϕ) inside TB granuloma using a mechanism, which is not well understood. Many bacterial systems utilize toxins to induce host cell damage, which occurs along with immune evasion. These toxins often use chameleon sequences to generate environment sensitive conformational switch facilitating the process of infection. The presence of toxins is not yet known for MTB. Here, we show that MTB secreted immunogenic MPT63 protein adopts β-sheet to helix switch in response to mutational and environmental stresses. MPT63 in its helical form creates pores both in synthetic and Mϕ membranes, while the native β-sheet protein remains inert towards membrane interactions. Using fluorescence correlation spectroscopy and AFM, we show further that the helical form undergoes self-association to produce toxic oligomers of different morphology. Trypan blue and flow cytometry analyses reveal that the helical state can be utilized by MTB for killing Mϕ cells. Collectively, our study emphasizes for the first time a toxin-like behavior of MPT63 induced by an environment-dependent conformational switch, resulting in membrane pore formation by toxic oligomers and Mϕ cell death.