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Background
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Anthrax infection is initiated by the inhalation, ingestion, or cutaneous contact with Bacillus anthracis endospores. B. anthracis produces three polypeptides that comprise the anthrax toxin: protective antigen (PA), lethal factor (LF), and edema factor (EF). PA binds to two related proteins on the cell surface, these are termed tumor epithelial marker 8 (TEM8)/anthrax toxin receptor (ATR) and capillary morphogenesis protein 2 (CMG2). PA is cleaved into two fragments by a furin-like protease after receptor binding. The bound fragment binds both LF and EF, the resulting complex is then endocytosed into the cell which allows the release of LF and EF into the cytoplasm. These toxins are usually sufficient to cause rapid cell death, and often the death of the infected organism. LF is the primary toxin of anthrax and functions as a highly specific protease that cleaves members of the mitogen-activated protein kinase kinase (MAPKK) family near their amino terminus, interfering with MAPK signaling and inducing apoptosis . EF is a calmodulin and Ca++-dependent adenylate cyclase responsible for the edema seen in the disease. It is thought to benefit the B. anthracis bacteria by inhibiting cells of the host immune system. The Anthrax toxin receptor (ATR) was initially discovered as the tumor endothelial marker 8 (TEM8). This protein, which exists in three isoforms (36, 40, and 60 kDa), is highly expressed in tumor vessels as well as in the vasculature of developing embryos, suggesting that it may normally play a role in angiogenesis in addition to its role as the anthrax toxin receptor.
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