Master control of protein secretion by Mycobacterium tuberculosis

Tuberculosis is the leading cause of death from a single infectious disease. Mycobacterium tuberculosis secretes proteins using five ESX systems with distinctive functions essential for its growth and virulence. Here we show that a non-canonical supercomplex of the EsxU-EsxT proteins, encoded in the esx-4 locus, with the orphan EsxE-EsxF proteins, encoded in the cpnT operon, is required for toxin secretion by M. tuberculosis. Surprisingly, the outer membrane localization of all Esx proteins and their secretion into the cytosol of infected macrophages also depend on the EsxEF-EsxUT supercomplex and ESX-4. These results not only demonstrate that the Esx proteins have dual functions as the long-sought outer membrane components of ESX systems and as secreted effector proteins, but also reveal a novel master control mechanism of protein secretion in M. tuberculosis. The mutual dependency of EsxEF and EsxUT on each other synchronizes ESX effector protein secretion, enabling M. tuberculosis to block phagosomal maturation and to permeabilize the phagosomal membrane only when it is capable of killing host cells by toxin secretion. The requirement of the ESX-4 system for general protein secretion is a critical vulnerability which could be targeted by drugs and/or vaccines to simultaneously block many virulence factors of M. tuberculosis.