Type VII Secretion: A Highly Versatile Secretion System.

Type VII secretion (T7S) systems of mycobacteria secrete substrates over the unusual diderm cell envelope. Furthermore, T7S gene clusters are present throughout the phylum Actinobacteria, and functional T7S-like systems have been identified in Firmicutes. Most of the T7S substrates can be divided into two families: the Esx proteins, which are found in both Firmicutes and Actinobacteria, and the PE and PPE proteins, which are more mycobacterium-specific. Members of both families have been shown to be secreted as folded heterodimers, suggesting that this is a conserved feature of T7S substrates. Most knowledge of the mechanism of T7S and the roles of T7S systems in virulence comes from studies of pathogenic mycobacteria. These bacteria can contain up to five T7S systems, called ESX-1 to ESX-5, each having its own role in bacterial physiology and virulence. In this article, we discuss the general composition of T7S systems and the role of the individual components in secretion. These conserved components include two membrane proteins with (predicted) enzymatic activities: a predicted ATPase (EccC), likely to be required for energy provision of T7S, and a subtilisin-like protease (MycP) involved in processing of specific substrates. Additionally, we describe the role of a conserved intracellular chaperone in T7S substrate recognition, based on recently published crystal structures and molecular analysis. Finally, we discuss system-specific features of the different T7S systems in mycobacteria and their role in pathogenesis and provide an overview of the role of T7S in virulence of other pathogenic bacteria.