A secreted protein that plays a number of roles in modulating the host's immune response to infection as well as being responsible for bacterial escape into the host cytoplasm. Acts as a strong host (human) T-cell antigen (PubMed:7729876, PubMed:11940590). Inhibits IL-12 p40 (IL12B) and TNF-alpha expression by infected host (mouse) macrophages, reduces the nitric oxide response by about 75% (PubMed:14557536). In mice previously exposed to the bacterium, elicits high level of IFN-gamma production by T-cells upon subsequent challenge by M.tuberculosis, in the first phase of a protective immune response (PubMed:7897219, PubMed:7729876). Higher levels (1.6-3.3 uM) of recombinant protein inhibit IFN-gamma production by host (human) T-cells and also IL-17 and TNF-alpha production but not IL-2; decreases expression of host ATF-2 and JUN transcription factors by affecting T-cell receptors signaling downstream of ZAP70, without cytotoxicity or apoptosis (PubMed:19265145). EsxA inhibits IFN-gamma production in human T-cells by activating p38 MAPK (MAPK14), p38 MAPK is not responsible for IL-17 decrease (PubMed:21586573). Binds host (mouse) Toll-like receptor 2 (TLR2) and decreases host MYD88-dependent signaling; binding to TLR2 activates host kinase AKT and subsequently inhibits downstream activation of NF-kappa-B; the C-terminal 20 residues (76-95) are necessary and sufficient for the TLR2 inhibitory effect (PubMed:17486091). Required for induction of host (human) IL-1B maturation and release by activating the host NLRP3/ASC inflammasome; may also promote access of other tuberculosis proteins to the host cells cytoplasm (PubMed:20148899). Induces IL-8 (CXCL8) expression in host (human) lung epithelial cells (PubMed:23867456). Exogenously applied protein, or protein expressed in host (human and mouse), binds beta-2-microglobulin (B2M) and decreases its export to the cell surface, probably leading to defects in class I antigen presentation by the host cell (PubMed:25356553). Responsible for mitochondrial fragmention, redistribution around the cell nucleus and decreased mitochondrial mass; this effect is not seen until 48 hours post-infection (PubMed:26092385). Able to disrupt artificial planar bilayers in the absence of EsxB (CFP-10) (PubMed:14557547). Native protein binds artificial liposomes in the absence but not presence of EsxB and is able to rigidify and lyse them; the EsxA-EsxB complex dissociates at acidic pH, EsxB might serve as a chaperone to prevent membrane lysis (PubMed:17557817). Recombinant protein induces leakage of phosphocholine liposomes at acidic pH in the absence of ExsB, undergoes conformational change, becoming more alpha-helical at acidic pH (PubMed:23150662, PubMed:25645924). The study using recombinant protein did not find dissociation of EsxA-EsxB complex at acidic pH (PubMed:23150662). Involved in translocation of bacteria from the host (human) phagolysosome to the host cytoplasm (PubMed:17604718, PubMed:22319448). Translocation into host cytoplasm is visible 3 days post-infection using cultured human cells and precedes host cell death (PubMed:22319448). Recombinant protein induces apoptosis in host (human) differentiated cell lines, which is cell-line dependent; bacteria missing the ESX-1 locus do not induce apoptosis (PubMed:17298391). Host (human) cells treated with EsxA become permeable to extracellular dye (PubMed:17298391). EsxA and EsxA-EsxB are cytotoxic to pneumocytes (PubMed:19906174). ESX-1 secretion system-induced host (mouse) cell apoptosis, which is probably responsible for infection of new host cells, might be due to EsxA (PubMed:23848406). EsxA induces necrosis in aged neutrophils (PubMed:25321481). May help regulate assembly and function of the type VII secretion system (T7SS) (By similarity). EsxA disassembles pre-formed EccC-EsxB multimers, possibly by making EccC-EsxA-EsxB trimers instead of EccC-EsxB-EsxB-EccC tetramers (By similarity). May be critical in pro-bacteria versus pro-host interactions; ESX-1 mediates DNA mediated export (maybe via EsxA). The DNA interacts with host (human) cGAS, leading to cGAMP production and activation of the host STING-TBK-1-IRF-3 signaling pathway that leads to IFN-beta which is thought to be 'pro-bacteria'. Mycobacterial dsDNA also interacts with AIM2-NLRP3-ASC to activate an inflammasome, leading to the 'pro-host' IL-1-beta (PubMed:26048138, PubMed:26048136). Forms a tight 1:1 complex with EsxB (CFP-10) (PubMed:11940590, PubMed:14557536, PubMed:16048998, PubMed:16973880, PubMed:19854905, PubMed:19906174, PubMed:23150662, PubMed:26260636, PubMed:20085764, PubMed:15973432, PubMed:24586681). The complex persists even after secretion (PubMed:16048998). Recombinant His-tagged protein forms dimers and higher order multimers; how the protein is prepared influences its multimerization and its subsequent properties in vitro (PubMed:26260636). In vitro EsxB only interacts with non-acetylated EsxA; it also interacts with C-terminally truncated EsxA (missing the last 10 residues) (PubMed:15378760). The native EsxA-EsxB complex dissociates at pH 4.0, and EsxA may then be freed to lyse (host) membranes (PubMed:17557817). Another study using recombinant protein did not find dissociation at acidic pH (PubMed:23150662). Recombinant heterodimer (with a His tag on EsxB) can be dissociated by the detergents amidosulfobetaine-14 and lauryldimethylamine N-oxide (PubMed:26260636). Interacts with EspF (PubMed:19682254). An artificial EsxB-EsxA heterodimer interacts with EspA, EccB1, EccCa1, EccCb1, EspI, EspJ, EccA2 and EccE2; the latter 2 are from the adjacent ESX-2 locus (PubMed:19854905). Contributes to the heterodimer's interaction with EccCb1 (PubMed:19854905). Interacts with host (mouse and human) TLR2 (PubMed:17486091, PubMed:20800577). Interacts with host (human) beta-2-microglobulin (B2M) alone and in complex with EsxB; only binds free B2M and not B2M in complex with HLA-I (PubMed:25356553). The EsxA-EsxB-B2M complex can be detected in the host endoplasmic reticulum (PubMed:25356553). The B2M-EsxA complex can be detected in patients with pleural tuberculosis and is stable from pH 4.0 to 8.0 and in the presence of 2M NaCl (PubMed:25356553). Probably secreted via the ESX-1 / type VII secretion system (T7SS) (PubMed:19876390). Secreted protein binds to bacterial cell wall (PubMed:19906174). Binds to host (mouse) TLR2 on the cell surface (PubMed:17486091). Binds to CD4+, CD8+, CD14+ and CD19+ host (human) cells (PubMed:19265145). Localized on the cell surface of host (human) cell monocytes and macrophages (often in patches), but not fibroblasts (PubMed:15973432). Exogenous EsxA and EsxA-EsxB complex can enter host (human and mouse) endoplasmic reticulum, where they bind beta-2-microglobulin (PubMed:25356553). At acidic pH able to form pores in artificial membranes, might form pores in host phagosome membranes in absence of EsxB, allowing escape of bacteria into the host cytoplasm (PubMed:25645924, PubMed:26801203). The central pair of alpha helices probably insert into the membrane while the N- and C-terminal arms do not, but may rest on the membrane surface as they are required for membrane insertion (PubMed:25645924). Constitutively expressed in culture (at protein level) (PubMed:9846755, PubMed:23848406). Up-regulated in infected human pneumonocytes (PubMed:19906174). Zinc increases secreted levels of this protein; 0.5 mM Zn(2+), the physiological concentration in macrophages, induces 6-fold more secreted protein (PubMed:25299337). Part of the esxB-esxA operon (PubMed:9846755). May be secreted as a 4 coiled-coil complex with EsxB (PubMed:16048998). Upon purification from strain ATCC 27294 a C-terminally truncated peptide (missing residues 85-95) has been found; it is not clear if this is physiologically relevant (PubMed:15378760). An additional unknown modification on peptide Thr-86-Ala-95 has also been seen (PubMed:15378760). Bacteria no longer translocate from the phagolysosome to the cytosol of host (human) cells; bacteria replicate only in host phagolysosome rather than cytoplasm, decreased apoptosis of infected host (human) dendritic cells (PubMed:17604718). Bacteria missing the RD1 locus do not gain access to host (human) cytoplasm; complementation with the RD1 locus restores access, but if EsxA is missing the last 12 residues cytoplasmic access is not restored although truncated EsxA is secreted by bacteria (PubMed:22319448). Loss of ability to lyse host (human) lung epithelial cells, possibly due to polar effects from the upstream esxB gene; in BALB/c-infected mice bacteria are not as invasive and cause decreased lung disease (PubMed:14557547). No growth in the human macrophage-like cell line THP-1, no cytotoxicity (PubMed:14756778). Severely attenutated infection in mice, nearly 1000-fold less bacteria in lung and spleen of C57BL/6 (PubMed:14557536, PubMed:14756778). Inactivation leads to absence of EsxA and EsxB from cell lysates (PubMed:14756778, PubMed:16368961). No secretion of EspA (PubMed:16030141). No longer decreases expression of IL-12 p40 and TNF-alpha by infected murine macrophages, while the nitric oxide response is only partially reduced (PubMed:14557536). Significantly decreased production of IL-1 beta (IL1B), decreased activation of host (human) CASP-1 in response to bacterial infection (PubMed:20148899). Mitochondrial morphology is no longer perturbed in human alveolar epithelial cell line A549 (PubMed:26092385). A good candiate for vaccine development (PubMed:7897219). It has been tested in a number of experimental situations. Genes esxA and esxB are part of RD1 (part of a 15-gene locus known as ESX-1), a section of DNA deleted in the M.bovis BCG strain used for vaccination. Deletion of this region is thought to be largely responsible for the attenuation of BCG, and esxA and esxB in particular are very important in this effect (PubMed:14557547, PubMed:14756778, PubMed:16368961, PubMed:17298391, PubMed:22319448, PubMed:22524898). Secretion of EspA, EsxA and EsxB is mutually dependent (PubMed:16030141). To improve expression in E.coli the proteins were cloned as a single protein in the order esxB-esxA with a cleavable thrombin tag (PubMed:19854905). 2 inhibitors of ESX-1 secretion decrease secretion of this protein, without being bacteriocidal. BTP15 inhibits autophosphorylation of MprB with subsequent up-regulation of espA and decreased secretion of EspB and EsxA. BBH7 also inhibits TAT-dependent secretion of (at least) Ag85C (fbpC) and up-regulates members of the ESX-5 locus as well as other genes that are involved in cell wall biogenesis and metal ion homeostasis (PubMed:25299337). Belongs to the WXG100 family. ESAT-6 subfamily.