Unravelling ESX-5, a novel bacterial protein transport system specific for pathogenic mycobacteria.

Protein secretion is an essential characteristic for most bacterial pathogens; they need to secrete virulence factors in order to manipulate and/or damage the host or to escape destruction by the immune system. Different secretion systems have been identified, such as the well-known type III secretion system in pathogenic Gram-negative bacteria. An important group of pathogens that have only recently been shown to secrete virulence factors are the mycobacteria. Bacteria of this genus, which includes Mycobacterium tuberculosis and the fish pathogen Mycobacterium marinum, secrete proteins across their complex cell wall via a novel protein transport pathway: the ESAT-6 secretion pathway or ESX-1. Genome analysis shows that homologues of ESX-1 are also present in other Gram-positive pathogens and ESX might turn out to be the Gram-positive counterpart of type III secretion. However, thus far virtually nothing is known about the mechanism of any of the ESX secretion systems. Genome analysis furthermore shows that M. tuberculosis contains five ESX-like gene clusters. In addition to ESX-1, another ESX locus has recently been shown to be functional: ESX-5 secretes the PPE41 protein, a potential virulence factor. Strikingly, ESX-5 is only present in pathogenic mycobacteria and plays a role in the macrophage infection cycle. The goal of this research project is to study the ESX-5 secretion system in more detail by proteomic, biochemical and molecular techniques in order to answer the following important questions: which proteins are secreted via ESX-5, what is the secretion signal, what is the composition of the secretion machinery and what is the role of ESX-5 in virulence? Understanding ESX-5 will not only clarify how hydrophilic proteins can be transported across complex, hydrophobic layers, but will also increase our understanding of mycobacterial virulence. This knowledge will be useful to develop new intervention strategies for this group of important pathogens.