Mycobacterial Virulence Factors: Surface-Exposed Lipids and Secreted Proteins.

The clinically important () and related mycobacterial pathogens use various virulence mechanisms to survive and cause disease in their hosts. Several well-established virulence factors include the surface-exposed lipids in the mycobacterial outer membrane, as well as the Esx family proteins and the Pro-Glu (PE)/ Pro-Pro-Glu (PPE) family proteins secreted by type VII secretion systems (T7SS). Five ESX T7SS exist in and three-EsxA secretion system-1 (ESX-1), ESX-3, and ESX-5-have been implicated in virulence, yet only the structures of ESX-3 and ESX-5 have been solved to date. Here, we summarize the current research on three outer membrane lipids-phthiocerol dimycocerosates, phenolic glycolipids, and sulfolipids-as well as the secretion machinery and substrates of three mycobacterial T7SS-ESX-1, ESX-3, and ESX-5. We propose a structural model of the ESX-1 system based on the latest structural findings of the ESX-3 and ESX-5 secretion apparatuses to gain insight into the transport mechanism of ESX-associated virulence factors.