Pore-forming Esx proteins mediate toxin secretion by Mycobacterium tuberculosis

Mycobacterium tuberculosis secretes the tuberculosis necrotizing toxin (TNT) to kill host cells. Here, we show that the WXG100 proteins EsxE and EsxF are essential for TNT secretion. EsxE and EsxF form a water-soluble heterodimer (EsxEF) that assembles into oligomers and long filaments, binds to membranes, and forms stable membrane-spanning channels. Electron microscopy of EsxEF reveals mainly pentameric structures with a central pore. Mutations of both WXG motifs and of a GXW motif do not affect dimerization, but abolish pore formation, membrane deformation and TNT secretion. The WXG/GXW mutants are locked in conformations with altered thermostability and solvent exposure, indicating that the WXG/GXW motifs are molecular switches controlling membrane interaction and pore formation. EsxF is accessible on the bacterial cell surface, suggesting that EsxEF form an outer membrane channel for toxin export. Thus, our study reveals a protein secretion mechanism in bacteria that relies on pore formation by small WXG proteins. Tuberculosis necrotizing toxin (TNT) is secreted by Mycobacterium tuberculosis to kill host cells. Here, Tak, Dokland and Niederweis show that proteins EsxE and EsxF form membrane-spanning hetero-oligomeric pores that are important for TNT secretion.

Automated summary

The tuberculosis necrotizing toxin (TNT) is secreted by Mycobacterium tuberculosis to kill host cells, with proteins EsxE and EsxF forming membrane-spanning hetero-oligomeric pores that are essential for TNT secretion.