Evidence for Pore Formation in Host Cell Membranes by ESX-1-Secreted ESAT-6 and Its Role in Mycobacterium marinum Escape from the Vacuole

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Abstract
The ESX-1 secretion system plays a critical role in the virulence of M. tuberculosis and M. marinum , but the precise molecular and cellular mechanisms are not clearly defined. Virulent M. marinum is able to escape from the Mycobacterium -containing vacuole (MCV) into the host cell cytosol, polymerize actin, and spread from cell to cell. In this study, we have examined nine M. marinum ESX-1 mutants and the wild type by using fluorescence and electron microscopy detecting MCV membranes and actin polymerization. We conclude that ESX-1 plays an essential role in M. marinum escape from the MCV. We also show that the ESX-1 mutants acquire the ability to polymerize actin after being artificially delivered into the macrophage cytosol by hypotonic shock treatment, indicating that ESX-1 is not directly involved in initiation of actin polymerization. We provide evidence that M. marinum induces membrane pores ∼4.5 nm in diameter, and this activity correlates with ESAT-6 secretion. Importantly, purified ESAT-6, but not the other ESX-1-secreted proteins, is able to cause dose-dependent pore formation in host cell membranes. These results suggest that ESAT-6 secreted by M. marinum ESX-1 could play a direct role in producing pores in MCV membranes, facilitating M. marinum escape from the vacuole and cell-to-cell spread. Our study provides new insight into the mechanism by which ESX-1 secretion and ESAT-6 enhance the virulence of mycobacterial infection.

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