Protein: 6 kDa early secretory antigenic target (ESAT-6) (P9WNK7)
Organism: Mycobacterium tuberculosis (strain ATCC 25618 / H37Rv)
Function: 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. Inhibits IL-12 p40 (IL12B) and TNF-alpha expression by infected host (mouse) macrophages, reduces the nitric oxide response by about 75%. 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. 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. EsxA inhibits IFN-gamma production in human T-cells by activating p38 MAPK (MAPK14), p38 MAPK is not responsible for IL-17 decrease. 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. 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. Induces IL-8 (CXCL8) expression in host (human) lung epithelial cells. 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. Responsible for mitochondrial fragmention, redistribution around the cell nucleus and decreased mitochondrial mass; this effect is not seen until 48 hours post-infection. Able to disrupt artificial planar bilayers in the absence of EsxB (CFP-10). 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. 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. The study using recombinant protein did not find dissociation of EsxA-EsxB complex at acidic pH. Involved in translocation of bacteria from the host (human) phagolysosome to the host cytoplasm. Translocation into host cytoplasm is visible 3 days post-infection using cultured human cells and precedes host cell death. 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. Host (human) cells treated with EsxA become permeable to extracellular dye. EsxA and EsxA-EsxB are cytotoxic to pneumocytes. ESX-1 secretion system-induced host (mouse) cell apoptosis, which is probably responsible for infection of new host cells, might be due to EsxA. EsxA induces necrosis in aged neutrophils. 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).
Subcellular location: Secreted {ECO:0000269|PubMed:14557536, ECO:0000269|PubMed:14557547, ECO:0000269|PubMed:15378760, ECO:0000269|PubMed:16030141, ECO:0000269|PubMed:16048998, ECO:0000269|PubMed:7729876, ECO:0000269|PubMed:7897219}. Secreted, cell wall {ECO:0000269|PubMed:19906174}. Host cell surface {ECO:0000269|PubMed:15973432, ECO:0000269|PubMed:17486091, ECO:0000269|PubMed:19265145}. Host cytoplasm {ECO:0000269|PubMed:20148899}. Host endoplasmic reticulum {ECO:0000269|PubMed:25356553}. Host cell membrane {ECO:0000305|PubMed:25645924, ECO:0000305|PubMed:26801203}. Note=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). {ECO:0000269|PubMed:15973432, ECO:0000269|PubMed:17486091, ECO:0000269|PubMed:19265145, ECO:0000269|PubMed:19876390, ECO:0000269|PubMed:19906174, ECO:0000269|PubMed:25356553, ECO:0000269|PubMed:25645924, ECO:0000305|PubMed:25645924, ECO:0000305|PubMed:26801203}.
InterPro domains:
- IPR010310: Type VII secretion system ESAT-6-like (family) [5-89]
- IPR036689: ESAT-6-like superfamily (homologous_superfamily) [5-94]
Sequence:
MTEQQWNFAGIEAAASAIQGNVTSIHSLLDEGKQSLTKLAAAWGGSGSEAYQGVQQKWDATATELNNALQNLARTISEAGQAMASTEGNVTGMFA
Predicted GO terms:
Molecular Function (MF): GO:0003674 (molecular function), GO:0005488 (binding), GO:0005515 (protein binding)
Biological Process (BP): GO:0008150 (biological process), GO:0009987 (cellular process), GO:0051179 (localization), GO:0051234 (establishment of localization), GO:0051641 (cellular localization), GO:0055085 (transmembrane transport), GO:0140352 (export from cell), GO:0033036 (macromolecule localization), GO:0070727 (cellular macromolecule localization), GO:0032940 (secretion by cell), GO:0045184 (establishment of protein localization), GO:0071806 (protein transmembrane transport), GO:0006810 (transport), GO:0035592 (establishment of protein localization to extracellular region), GO:0071702 (organic substance transport), GO:0046903 (secretion), GO:0071705 (nitrogen compound transport), GO:0008104 (protein localization), GO:0009306 (protein secretion), GO:0015031 (protein transport), GO:0071692 (protein localization to extracellular region)
Cellular Component (CC): GO:0005575 (cellular component), GO:0110165 (cellular anatomical entity), GO:0005622 (intracellular anatomical structure), GO:0043226 (organelle), GO:0005829 (cytosol), GO:0005737 (cytoplasm), GO:0031974 (membrane-enclosed lumen), GO:0043233 (organelle lumen), GO:0031410 (cytoplasmic vesicle), GO:0043229 (intracellular organelle), GO:0043227 (membrane-bounded organelle), GO:0030139 (endocytic vesicle), GO:0043231 (intracellular membrane-bounded organelle), GO:0070013 (intracellular organelle lumen), GO:0031982 (vesicle), GO:0045335 (phagocytic vesicle), GO:0097708 (intracellular vesicle), GO:0071682 (endocytic vesicle lumen)