Structure and dynamics of the ESX-5 type VII secretion system of <i>Mycobacterium tuberculosis</i>

Abstract Mycobacterium tuberculosis causes one of the most important infectious diseases in humans, leading to 1.5 million deaths every year. Specialized protein transport systems, called type VII secretion systems (T7SSs), are central for its virulence, but also crucial for nutrient and metabolite transport across the mycobacterial cell envelope. Here we present the first structure of an intact T7SS inner membrane complex of M. tuberculosis . We show how the 2.32 MDa, 165 transmembrane helices-containing ESX-5 assembly is restructured and stabilized as a trimer of dimers by the MycP 5 protease. A trimer of MycP 5 caps a central periplasmic dome-like chamber formed by three EccB 5 dimers, with the proteolytic sites facing towards the cavity. This chamber suggests a central secretion and processing conduit. Complexes without MycP 5 show disruption of the EccB 5 periplasmic assembly and increased flexibility, highlighting the importance of this component for complex integrity. Beneath the EccB 5 -MycP 5 chamber, dimers of the EccC 5 ATPase assemble into three four-transmembrane helix bundles, which together seal the potential central secretion channel. Individual cytoplasmic EccC 5 domains adopt two distinctive conformations, likely reflecting different secretion states. Our work suggests a novel mechanism of protein transport and provides a structural scaffold to aid drug development against the major human pathogen.