Composition of the type VII secretion system membrane complex

Pathogenic mycobacteria require type VII secretion (T7S) systems to transport virulence factors across their complex cell envelope. These bacteria have up to five of these systems, termed ESX‐1 to ESX‐5. Here, we show that ESX‐5 of Mycobacterium tuberculosis mediates the secretion of EsxN, PPE and PE_PGRS proteins, indicating that ESX‐5 is a major secretion pathway in this important pathogen. Using the ESX‐5 system of Mycobacterium marinum and Mycobacterium bovis BCG as a model, we have purified and analysed the T7S membrane complex under native conditions. blue native‐PAGE and immunoprecipitation experiments showed that the ESX‐5 membrane complex of both species has a size of ～ 1500 kDa and is composed of four conserved membrane proteins, i.e. EccB₅, EccC₅, EccD₅ and EccE₅. Subsequent limited proteolysis suggests that EccC₅ and EccE₅ mostly reside on the periphery of the complex. We also observed that EccC₅ and EccD₅ expression is essential for the formation of a stable membrane complex. These are the first data on a T7S membrane complex and, given the high conservation of its components, our data can likely be generalized to most mycobacterial T7S systems.