A specific secretion system mediates PPE41 transport in pathogenic mycobacteria

Abstract

SummaryMycobacterial genomes contain two unique gene families, the so-called PE and PPE gene families, which are highly expanded in the pathogenic members of this genus. Here we report that one of the PPE proteins, i.e. PPE41, is secreted by pathogenic mycobacteria, both in culture and in infected macrophages. As PPE41 lacks a signal sequence a dedicated secretion system must be involved. A single gene was identified in Mycobacterium marinum that showed strongly reduced PPE41 secretion. This gene was located in a gene cluster whose predicted proteins encode components of an ESAT-6-like secretion system. This cluster, designated ESX-5, is conserved in various pathogenic mycobacteria, but not in the saprophytic species Mycobacterium smegmatis. Therefore, different regions of this cluster were introduced in M. smegmatis. Only introduction of the complete ESX-5 locus resulted in efficient secretion of heterologously expressed PPE41. This PPE secretion system is also involved in the virulence of pathogenic mycobacteria, as the ESX-5 mutant of M. marinum was affected in spreading to uninfected macrophages.

General secretion signal for the mycobacterial type VII secretion pathway

“…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).…”

“…Previously, PPE41 was shown to be exported by ESX-5, both in its native host M. tuberculosis (30) and in M. marinum upon heterologous expression (20). The PPE41 gene forms an operon with PE25 (31) and both proteins form a heterodimer when expressed in Escherichia coli (32).…”

“…Although they are associated with mycobacterial virulence, the function of the PE and PPE proteins is largely unknown (25). 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).…”

iniBAC induction Is Vitamin B12- and MutAB-dependent in Mycobacterium marinum

“…Transposon mutants were assessed for increased mCherry expression, indicating induction of iniBAC by fluorescence microscopy. Colonies that showed a vast increase in fluorescence were isolated, and the transposon insertion site was determined by ligationmediated PCR as described previously (36).…”