Reconstitution of a minimal ESX-5 type VII secretion system suggests a role for PPE proteins in outer membrane transport of proteins

Abstract

Mycobacteria utilize type VII secretion systems (T7SSs) to secrete proteins across their highly hydrophobic and diderm cell envelope. Pathogenic mycobacteria have up to five different T7SSs, called ESX-1 to ESX-5, which are crucial for growth and virulence. Here, we use a functionally reconstituted ESX-5 system in the avirulent species Mycobacterium smegmatis that lacks ESX-5, to define the role of each gene of the esx-5 locus in ESX-5 functionality. By creating an array of gene deletions and assessing ESX-5 component protein levels and membrane complex assembly, we observed that only the five components of the inner membrane complex are required for the assembly of this complex. However, in addition to these five core members, active secretion also depends on both the Esx proteins and the PE/PPE substrates. Tagging the PPE substrates followed by subcellular localization, membrane extraction and surface labeling showed that these proteins localize to the cell envelope, indicating that they could play a role in secretion across the mycobacterial outer membrane. These results provide a first blueprint on the role of each component within mycobacterial T7SSs.

A marine sponge-associated mycobacterium closely related toMycobacterium tuberculosis

“…Overall, the pattern of FSD4b-SM PE/PPE proteins follows that in other mycobacteria, with multiple, high-identity paralogues. The roles these PE/PPE proteins play in FSD4b-SM physiology remain to be discovered, but a role in protein export across the highly hydrophobic mycobacterial cell envelope is one candidate function (81).…”