EsxA

EsxA is a gene found in Mycobacterium tuberculosis and related mycobacteria that encodes the secreted protein EsxA, also known as early secreted antigenic target 6 kDa (ESAT-6). EsxA is a small, roughly 95–amino-acid protein and is a key substrate of the ESX-1 (type VII) secretion system, typically produced together with EsxB (CFP-10) as a 1:1 partner.

EsxA and EsxB form a tight heterodimer that is secreted by the ESX-1 machinery. Once outside the bacterial cell, EsxA is thought to contribute to virulence by influencing host cell membranes and promoting disruption of the phagosomal membrane, which can enable bacterial escape into the cytosol. The precise molecular mechanism remains an area of active research, but EsxA is widely regarded as a major virulence factor in pathogenic mycobacteria.

In addition to its role in pathogenesis, EsxA is a prominent antigen that elicits strong T cell responses in infected individuals. Because of its immunogenicity, EsxA is used in tuberculosis diagnostics, such as interferon-gamma release assays, and is a focal point in TB vaccine research. Genetically, esxA is located within the ESX-1 gene cluster and is often annotated as esxA (Rv3875 in Mycobacterium tuberculosis H37Rv). Homologs of EsxA are present in other mycobacteria and in related ESX secretion systems (ESX-2 to ESX-5), reflecting a conserved role in secretion and host interaction.