Anthrax edema factor

The anthrax toxin is a tripartite toxin and consists ofthe binding component protective antigen (PA), the lethal factor (LF), which is a metalloprotease, and the edema factor (EF), which is a calmodulin-dependent adenylyl-cyclase. Both enzyme components are translocated via PA into target cells. PA is activated by furin-induced cleavage and forms heptamers, which are similar to the binding components of C2 toxin and iota toxin. In the low pH compartment of endosomes, the heptamers form pores to allow translocation of LF and EF. LF cleaves six of the seven MEKs (MAPK-kinases) thereby inhibiting these enzymes. The functional consequence is the blockade of the MAPK pathways that control cell proliferation, differentiation, inflammation, stress response, and survival. Whether this is the reason for the LT-induced cell death of macrophages is not clear [1]. 

The anthrax bioterrorist attacks that followed the events of September 11th 2001 resulted in a renewed interest BadUus anthracis, the causative agent of this disease. Research has focused on the development of better vaccines than the one currently available. It has been estimated that the aerosolized release of 100 kg of anthrax spores upwind of Washington DC would cause mortalities of 130,000-3,000,000 [63]. Nonetheless, wild-type Bacillus anthracis is susceptible to conventional antibiotics, including penicillin, oxyfloxacin and ciprofloxacin. The problem lies not with the bacterial infection itself, but with three proteins released by the bacteria - protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa) and edema factor (EF, 89 kDa) -known as anthrax toxins [63]. 

Anthrax Anthrax is a toxin with three separate components a protective antigen (PA), an edema factor (EF), and a lethal factor (LF). 

Anthrax toxin is a bacterial toxin from Bacillus anthracis consisting of three parts protective antigen (PA), lethal factor (LF) and edema factor (EF). Both LF and EF compete for binding sites on the PA protein. The PA protein binds with high affinity to an as yet unknown receptor on macrophages and related cell types. When PA is internalized by the target cells, it functions as a shuttle protein for either EF or LF. Intracellularly, in the acidic environment of the endosome, EF and LF are capable of entering the cytosol by pH-dependent pore formation [139]. 

Edema factor of anthrax is one of the three exotoxins produced by the bacteria. The primary sequence is only slightly homologous to other adenylyl cyclases. When it enters the host cells, edema factor acts as an adenylyl cyclase and transforms ATP to cAMP efficiently. The structme of CaM-edema factor of anthrax complex has been obtained (Figure 9). In the CaM-edema factor complex, only the C-terminal lobe binds two calcium ions while the N-terminal lobe is empty. CaM remains extended and is deeply inserted between the catalytic and the a-helical domains of this exotoxin with a large number of residues of CaM iuvolved in the interaction. CaM binding results in a large domain reorientation of edema factor. The helical domain of edema factor undergoes a 15 A translation and a 30° rotation. The CaM inserted site is far from the catalytic site and the... 

Lacy, D.B., Mourez, M., Fouassier, A., Collier, R.J. (2002). Mapping the anthrax protective antigen hinding site on the lethal and edema factors. J. Biol. Chem. 277 3006-10. 

Shen, Y., Zhukovskaya, N.L., Guo, Q., Florian, J., Tang, W-J. (2005). Calcium-independent calmodulin binding and two-metal-ion catalytic mechanism of anthrax edema factor. Eur. Mol. Biol. Org. J. 24 929 1. 

Endocytosis may not be required for the entry of an invasive adenylate cyclase from Bordello pertussis (Hanski and Ferfel, 1985 Donovan and Storm, 1990). This is a single chain protein (mol. wt. approx. 200 kDa) which resembles the edema factor from anthrax toxin in that it must interact with calmodulin to become active. In contrast to anthrax toxin, it consists of only one polypeptide which is, however, easily cleaved by proteases and thereby activated. An enzymatically active 45 kDa fragment is not active on whole cells, but it could in conjunction with the rest of the molecule enter the cytosol. The facts that this toxin acts much more rapidly than anthrax toxin, and that it is active even at 4 °C and on erythrocytes that have little, if any, endocytosis, suggest that the toxin is able to penetrate directly through the cell surface membrane. 

Leppla SH (1982) Anthrax toxin edema factor a bacterial adenylate cyclase that increases cAMP concentrations in eucaryotic cells. Proc Natl Acad Sci USA 79 ... 

Anthrax toxin is composed of three proteins protective antigen (PA 83kDa), lethal factor (LF 90kDa), and edema factor (EF 89kDa). Individually, none of the three proteins are toxic but interact synergistically with at least one of the others. PA and LF (called LeTx) can cause lethal shock in experimental animals, and a mixture of PA and EF (edema toxin, EdTx) induces edema at the site of injection. Since two discrete units of the toxin are required for its action, the term binary toxin has been used to this and other bacterial toxins. Anthrax is unique from other binary toxins in that the binary moieties (EF and LF) interact only after being secreted from the bacteria. Further, EF and LF enter the cell via a single PA protein. Assembly of the three toxin proteins is initiated when PA binds to a proteinaceous cellular receptor and is activated by a member of the furin family of cellular proteases. The exact mechanisms of internalization of the toxin moieties are subject of scientific enquiry. Inside the cellular cytoplasm, EF (a calcium and calmodulin-dependent adenylate cyclase) causes a dramatic increase in intracellular cAMP concentrations and LF acts proteolytically to cleave certain MAPK kinases. 

Chen D, Misra M, Sower L et al (2008) Novel inhibitors of anthrax edema factor. Bioorg Med Chem 16 7225-7233... 

Edema Toxin (EdTx) and Lethal Toxin (LeTx) are two toxins with immunomodulatory activity that are produced by A anthracis, the cause of the disease anthrax. Both toxins are composed of a heptameric complex of protective antigen (PA) bound to either edema factor (EF) or lethal factor (LF). " The heptameric complex of PA is responsible for receptor binding and cellular entry, whereas toxicity is associated with both EF, a calmodulin-dependent adenylate cyclase that induces increases in cytosolic cAMP and LF, a metalloprotease that cleaves mitogen-activated protein kinase kinases (MAPKK). ... 

Hoover, D.L., Friedlander, A.M., Rogers, L.C., Yoon, I.K., Warren, R.L., Cross, A.S. (1994). Anthrax edema toxin differentially regulates lipopolysaccharide-induced monocyte production of tumor necrosis factor a and interleukin-6 by increasing intracellular cyclic AMP. Infect. Immun. 62 4432-9. 

B anthracis possesses three known virulence factors an antiphagocytic capsule and two protein exotoxins, called the lethal and the edema toxins. The role of the capsule in pathogenesis was demonstrated in the early 1900s, when anthrax strains lacking a capsule were shown to be avirulent.17 In more recent years, the genes encoding synthesis of the... 

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