Structures of EccB(1) and EccD(1) from the core complex of the mycobacterial ESX-1 type VII secretion system

BACKGROUND: The ESX-1 type VII secretion system is an important determinant of virulence in pathogenic mycobacteria, including Mycobacterium tuberculosis. This complicated molecular machine secretes folded proteins through the mycobacterial cell envelope to subvert the host immune response. Despite its important role in disease very little is known about the molecular architecture of the ESX-1 secretion system. RESULTS: This study characterizes the structures of the soluble domains of two conserved core ESX-1 components – EccB(1) and EccD(1). The periplasmic domain of EccB(1) consists of 4 repeat domains and a central domain, which together form a quasi 2-fold symmetrical structure. The repeat domains of EccB(1) are structurally similar to a known peptidoglycan binding protein suggesting a role in anchoring the ESX-1 system within the periplasmic space. The cytoplasmic domain of EccD(1)has a ubiquitin-like fold and forms a dimer with a negatively charged groove. CONCLUSIONS: These structures represent a major step towards resolving the molecular architecture of the entire ESX-1 assembly and may contribute to ESX-1 targeted tuberculosis intervention strategies. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12900-016-0056-6) contains supplementary material, which is available to authorized users.