For example VgrG-1, which is a component of the Vibrio cholerae T6SS, contains a C-terminal domain that can enter macrophages where it cross-links actin [38]. Overall however, the GF120918 concentration identities and functions of T6SS effectors are still poorly understood. Type VII secretion system Although Gram-positive bacteria have only a single membrane, some species, most notably the mycobacteria, have a cell wall that is heavily modified by lipids, called a mycomembrane. As a result, the genomes of these species encode a family of specialized secretion systems collectively called type VII section

systems (T7SS) (reviewed in [39]). The presence of the T7SS was initially predicted bioinformatically based on clustering of genes encoding secreted proteins that lacked signal sequences with those encoding membrane proteins, ATPases and/or chaperones. Sequencing of the BIBF 1120 cell line Mycobacterium bovis BCG vaccine strain, and mutational analysis of the ESX-1 cluster in M. tuberculosis confirmed

the hypothesis. ESX-1 is also required for virulence and hemolysis in the fish pathogen Mycobacterium marinum, and for conjugation in the non-pathogenic species Mycobacterium smegmatis [39]. Mycobacterial genomes contain up to five T7SS gene clusters that do not functionally complement one another. T7SS gene clusters are also found in the closely related pathogens Corynebacterium diphtheriae and Nocardia [39]. More distantly related gene clusters are also found in the genomes of pathogenic and non-pathogenic Gram-positive species that lack mycomembranes such as Streptomyces species and firmicutes such as Bacillus and Clostridium spp., Staphylococcus aureus, Streptococcus agalactiae and Listeria monocytogenes. The T7SS is required for virulence in Staphylococcus aureus but not in Listeria monocytogenes [39]. The structure and operation of the T7SS are still being pieced together. Current models [39] suggest an inner membrane translocation channel formed by the integral membrane protein Rv3877, and a separate channel in the mycomembrane

composed of as yet unknown protein(s). Chaperone-like tetracosactide ATPases anchored to the inner membrane bind the C-termini of effectors, which are invariably secreted as heterodimers. How the Gene Ontology addresses secretion systems In this section we review the GO terms that were specifically created by the PAMGO project for secretion systems. Many of the functions and processes of proteins related to secretion systems (for example effectors) can be described with GO terms from other parts of the GO hierarchy; those are not covered here in detail. We also note that many additional terms are still needed in this area, especially for secretion systems that are not central to bacteria-host interactions and which therefore have received less attention from the PAMGO consortium.