| Popis: |
To establish an infection, pathogenic mycobacteria use the Type VII secretion or ESX system to secrete virulence proteins across their cell envelope. The five ESX systems (ESX-1 to ESX-5) have evolved diverse functions in the cell, with the ESX-5 found almost exclusively in pathogens. Here we present a high-resolution cryo-electron microscopy structure of the hexameric ESX-5 Type VII secretion system. This 2.1 MDa membrane protein complex is built by a total of 30 subunits from six protomeric units, which are composed of the core components EccB5, EccC5, two copies of EccD5, and EccE5. The hexameric assembly of the overall ESX-5 complex is defined by specific inter-protomer interactions mediated by EccB5 and EccC5. The central transmembrane pore is formed by six pairs of EccC5 transmembrane helices that adopt a closed conformation in the absence of substrate in our structure. On the periplasmic face of the ESX-5 complex, we observe an extended arrangement of the six EccB5 subunits around a central cleft. Our structural findings provide molecular details of ESX-5 assembly and observations of the central secretion pore, which reveal insights into possible gating mechanisms used to regulate the transport of substrates. |
