Structure of a PE–PPE–EspG complex fromMycobacterium tuberculosisreveals molecular specificity of ESX protein secretion

Abstract

Nearly 10% of the coding capacity of the Mycobacterium tuberculosis genome is devoted to two highly expanded and enigmatic protein families called PE and PPE, some of which are important virulence/immunogenicity factors and are secreted during infection via a unique alternative secretory system termed "type VII." How PE-PPE proteins function during infection and how they are translocated to the bacterial surface through the five distinct type VII secretion systems [ESAT-6 secretion system (ESX)] of M. tuberculosis is poorly understood. Here, we report the crystal structure of a PE-PPE heterodimer bound to ESX secretion-associated protein G (EspG), which adopts a novel fold. This PE-PPE-EspG complex, along with structures of two additional EspGs, suggests that EspG acts as an adaptor that recognizes specific PE-PPE protein complexes via extensive interactions with PPE domains, and delivers them to ESX machinery for secretion. Surprisingly, secretion of most PE-PPE proteins in M. tuberculosis is likely mediated by EspG from the ESX-5 system, underscoring the importance of ESX-5 in mycobacterial pathogenesis. Moreover, our results indicate that PE-PPE domains function as cis-acting targeting sequences that are read out by EspGs, revealing the molecular specificity for secretion through distinct ESX pathways.tuberculosis | protein secretion | antigenic variation | virulence factor | host-pathogen interactions

Separable roles for Mycobacterium tuberculosis ESX-3 effectors in iron acquisition and virulence

“…Our data also clearly support the observation that specific PE-PPE pairs are targeted to specific ESX systems (58,59). To our knowledge, PE15-PPE20 is the first PE-PPE pair encoded outside an esx locus for which the data support secretion by an ESX system other than ESX-5.…”

The Enigmatic Esx Proteins: Looking Beyond Mycobacteria

“…A second protein, EsaE, binds to the 13 non-nuclease part of EsaD and appears to target the EsaDG complex to the 14 secretion machinery [31], analogous to the chaperone EspG that interacts 15 with large PE/PPE substrate proteins in pathogenic mycobacteria [55,56]. 16…”

Structural variability of EspG chaperones from mycobacterial ESX-1, ESX-3 and ESX-5 type VII secretion systems

“…However, in addition a "wing-shaped dimer" similar to the EspG 3msm (PDB ID 4L4W) structure is present in the crystal lattice, with 2054 Å 2 buried surface area. Furthermore, the asymmetric unit of EspG 3msm (PDB ID 4W4J [27]) structure also contains a similar wing-shaped dimer with substantial buried surface area as well as the β8-mediated dimer (Fig. 4).…”

“…In order to produce an atomic rigid body model of the PE5-PPE4-EspG 3mtu complex, 300 decoy structures were generated using a molecular docking approach [35] and the crystallographic EspG 3mtu (PDB ID 4W4I, [27]) structure together with homology models of a complex of full length PE5 and the N-terminal domain of PPE4 (residues 1-178) from M.…”

“…The crystal structure of the heterotrimeric PE25-PPE41-EspG 5 protein complex revealed that EspG 5 interacts with a PE25-PPE41 heterodimer by binding to a hydrophobic patch at the tip of PPE41 [26,27]. The general YxxD/E secretion motif [28,29] at the distal end of PE25 is free to interact with the ESX-5 secretion machinery in the inner membrane, probably by interaction with the Ftsk/SpoIIIE-like ATPase EccC 5 [30,31].…”