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

Virulent Mycobacterium tuberculosis (MTB) strains cause cell death of macrophages (MO) 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 an 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 undergoes a switch from beta-sheet to helix in response to mutational and environmental stresses. MPT63 in its helical form creates pores in both synthetic and M phi membranes, while the native beta-sheet protein remains inert toward membrane interactions. Using fluorescence correlation spectroscopy and atomic force microscopy, 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 M phi 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 M phi cell death.