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Functional analysis of ESAT-6 and EspB, two virulence proteins secreted by the ESX-1 system in Mycobacterium marinum
Smith, Jennifer Ann
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<italic>Mycobacterium tuberculosis</italic> (<italic>Mtb</italic>) and <italic>Mycobacterium marinum</italic> (<italic>Mm</italic>) are able to persist inside host cell macrophages by modulating the phagosome environment. ESX-1 is a specialized secretion system that is required for virulence. Two of the proteins secreted by ESX-1 are ESAT-6 and EspB. They are codependent for secretion and are important virulence effectors, though their specific functions are not known. <italic>Mm</italic> is able to escape from the phagosome into the host cell cytosol where it can initiate actin-based motility. <italic>Mm</italic> escape is dependent on a functional ESX-1 system. I show that the ESAT-6 protein is able to form pores in host cell membranes which may play a role in <italic>Mm</italic> escape from the phagosome. I also dissect the <italic>Mm</italic> EspB protein and show that cleavage of EspB is required for growth inside RAW cells, virulence in zebrafish, and for modulating ESAT-6 secretion. The resulting C-terminal 11 kDa fragment is sufficient for the codependent secretion of ESAT-6; while the 50 kDa N-terminal fragment seems to be somewhat dispensable for ESAT-6 secretion but is definitely required for virulence. When EspB is expressed as a full-length protein, the highly conserved WXG motif in the N-terminal fragment is involved in the codependent secretion of the two proteins since secretion is reduced when this motif is mutated. Interestingly, when the N-terminal fragment is expressed without the C-terminal fragment it can secrete independent of the ESX-1 system, indicating that the C-terminus confers specificity for EspB secretion through ESX-1. I show that the virulence function of the EspB N-terminal fragment is dependent on the secretion of ESAT-6, and EspB must be expressed in its full-length form in order to be fully functional. These results indicate that EspB function is dependent on a close association with ESAT-6. It is possible that the N-terminus is translocated into the host cell cytosol through the ESAT-6 formed pore.