PPE and PE_PGRS proteins of Mycobacterium marinum are transported via the type VII secretion system ESX‐5

Abstract

SummaryESX-5 is one of the five type VII secretion systems found in mycobacteria. These secretion systems are also known as ESAT-6-like secretion systems. Here, we have determined the secretome of ESX-5 by a proteomic approach in two different strains of Mycobacterium marinum. Comparison of the secretion profile of wild-type strains and their ESX-5 mutants showed that a number of PE_PGRS and PPE-MPTR proteins are dependent on ESX-5 for transport. The PE and PPE protein families are unique to mycobacteria, are highly expanded in several pathogenic species, such as Mycobacterium tuberculosis and M. marinum, and certain family members are cell surface antigens associated with virulence. Using a monoclonal antibody directed against the PGRS domain we showed that nearly all PE_PGRS proteins that are recognized by this antibody are missing in the supernatant of ESX-5 mutants. In addition to PE_PGRS and PPE proteins, the ESX-5 secretion system is responsible for the secretion of a ESAT-6-like proteins. Together, these data show that ESX-5 is probably a major secretion pathway for mycobacteria and that this system is responsible for the secretion of recently evolved PE_PGRS and PPE proteins.

General secretion signal for the mycobacterial type VII secretion pathway

“…Immunoblot analysis showed that both PE25-HA and PPE41 were secreted in the WT strain, but hardly in the ESX-5 mutant (Fig. 1A), similar to what has been described for PPE41 (20,21). This secretion defect was not accompanied by an increase in cellular levels of PE25 and PPE41.…”

“…Consistently, deletion of ESX-1 genes from the genomes of M. tuberculosis or M. bovis results in diminished virulence (18,19). Of the other ESX loci, both ESX-3, which is involved in iron acquisition, and ESX-5 have been demonstrated to encode functional secretion systems (20)(21)(22).…”

“…Consistently, deletion of ESX-1 genes from the genomes of M. tuberculosis or M. bovis results in diminished virulence (18,19). Of the other ESX loci, both ESX-3, which is involved in iron acquisition, and ESX-5 have been demonstrated to encode functional secretion systems (20)(21)(22).Four of the ESX clusters contain members of the pe and ppe gene families (9, 10), both of which are unique for mycobacteria and some closely related species. Intriguingly, whereas nonpathogenic mycobacteria generally only carry a small number of pe/ppe genes, these gene families are highly expanded in the genomes of M. tuberculosis and the fish pathogen Mycobacterium marinum (23, 24).…”

“…Most members of the large PE and PPE protein families are transported across the mycobacterial cell wall by the specialized T7S system ESX-5 (20,21). Similar to the substrates of other Sec-independent secretion systems (39,40), most PE and PPE proteins lack recognizable signal peptides.…”

“…Many PE and PPE proteins are exported to the bacterial cell surface or the culture supernatant (20,21,(26)(27)(28)(29). The vast majority of these intriguing proteins are exported by the most recently evolved T7S system, i.e., ESX-5 (21). Like other T7S substrates, PE and PPE proteins lack classical signal peptides (4).…”