Anthrax toxin
Anthrax toxin is a three-protein exotoxin secreted by virulent strains of the bacterium, Bacillus anthracis—the causative agent of anthrax. The toxin was first discovered by Harry Smith in 1954. Anthrax toxin is composed of a cell-binding protein, known as protective antigen, and two enzyme components, called edema factor and lethal factor. These three protein components act together to impart their physiological effects. Assembled complexes containing the toxin components are endocytosed. In the endosome, the enzymatic components of the toxin translocate into the cytoplasm of a target cell. Once in the cytosol, the enzymatic components of the toxin disrupt various immune cell functions, namely cellular signaling and cell migration. The toxin may even induce cell lysis, as is observed for macrophage cells. Anthrax toxin allows the bacteria to evade the immune system, proliferate, and ultimately kill the host animal. Research on anthrax toxin also provides insight into the generation of macromolecular assemblies, and on protein translocation, pore formation, endocytosis, and other biochemical processes.
''Bacillus anthracis'' virulence factors
Anthrax is a disease caused by Bacillus anthracis, a spore-forming, Gram positive, rod-shaped bacterium. The lethality of the disease is caused by the bacterium's two principal virulence factors: the polyglutamic acid capsule, which is anti-phagocytic, and the tripartite protein toxin, called anthrax toxin. Anthrax toxin is a mixture of three protein components: protective antigen, edema factor, and lethal factor.Mechanism of action
Anthrax toxin is an A-B toxin. Each individual anthrax toxin protein is nontoxic. Toxic symptoms are not observed when these proteins are injected individually into laboratory animals. The co-injection of PA and EF causes edema, and the co-injection of PA and LF is lethal. The former combination is called edema toxin, and the latter combination is called lethal toxin. Thus the manifestation of physiological symptoms requires PA, in either case.The PA requirement observed in animal-model experiments demonstrates a common paradigm for bacterial toxins, called the A / B paradigm. The A component is enzymatically active, and the B component is the cell binding component. Anthrax toxin is of the form A2B, where the two enzymes, EF and LF, are the A components and PA is the B component. PA is necessary for the enzymatic components to enter the cell. It does this by the formation of pores that span the cell membrane, allowing the entry of the toxin, though the mechanism is not fully understood. Once in the cytosol, they may then catalyze reactions that disrupt normal cellular physiology.
Anthrax toxin assembly and translocation
Anthrax toxin protein components must assemble into holotoxin complexes to function. In order for LF and EF to function inside a target cell, they must localize to the cell and enter its cytoplasm. Through a series of steps, PA can translocate EF and LF into the cell. This process starts when the 83-kDa form of PA, called PA83, binds to an anthrax toxin receptor. There are two known homologous receptors, which bind to PA83, called tumor endothelium marker-8 and capillary morphogenesis protein 2. Then a 20 kDa fragment is cleaved off PA83's amino terminus by membrane endoproteases from the furin family. When PA20 dissociates, the remaining receptor-bound portion of PA, called PA63, may assemble into either a heptameric or octameric ring-shaped oligomer. This ring-shaped oligomer is often referred to as the pre-pore form of PA, since later in the pathway it will become a translocase pore. The surface of the pre-pore oligomer, which was exposed upon release of the PA20 moiety, can then bind to LF and EF. The heptameric and octameric forms of the PA oligomer may then bind with up to three or four molecules of EF and/or LF, respectively. The cell then endocytoses these assembled complexes and carries them to an acidic compartment in the cell. The low pH encountered in the endosome causes the PA63 pre-channel to convert into a cation-selective channel. EF and LF are driven through the channel by a pH gradient, allowing the enzyme factors to enter the cytosol.Enzyme function of LF and EF
Once in the cytosol, the EF and LF then carry out their respective damage-inducing processes.- EF acts as a Ca2+ and calmodulin dependent adenylate cyclase that greatly increases the level of cAMP in the cell. This increase in cAMP upsets water homeostasis, severely imbalances the intracellular signaling pathways, and impairs macrophage function, allowing the bacteria to further evade the immune system.
- LF also helps the bacteria evade the immune system through killing macrophages. Once in these cells, LF acts as a Zn2+-dependent endoprotease that snips off the N-terminus of mitogen-activated protein kinase kinases. This inhibits these kinases by not allowing them to efficiently bind to their substrates, which leads to altered signaling pathways and ultimately to apoptosis.
Extracellular toxin structure-function relationship
The mechanism of anthrax toxin action is the result of the molecular structures of the three toxin proteins in combination with biomolecules of the host cell. The molecular interactions are apparent upon performing a detailed analysis of the structures of PA, EF, LF, and the cellular receptors. Structures for the toxin molecules, the receptor, and the complexes of the molecules all provided insight into the synergistic actions of these proteins. Analyses on binding sites and conformational changes augmented the structural studies, elucidating the functions of each domain of PA, LF, and EF, as briefly outlined in Table 1.The structure of PA was the first to be determined. This structure and that of its cellular receptor shed much light on the specificity of recognition and binding. This specificity of PA and the receptor CMG2 is due to interactions through a metal ion dependent adhesion site, a hydrophobic groove, and a β-hairpin projection. These all contribute to a tight interaction in which much protein surface area on CMG2 is buried.
Image:ANTHRA 1.JPG|right|thumb|200px|Ribbon diagram of a PA63 heptamer forming a pre-pore.
Petosa et al. solved the structure of a PA63 heptamer at 4.5 Å. The structure they solved was of a non-membrane bound pre-pore, the conformation of the heptamer before the complex extends a β-barrel through the plasma membrane to shuttle the LF and EF into the cytosol.
Heptamerization and pore formation is sterically hindered by the PA20 fragment, but when it is removed from the top of the monomer, the pre-pore is quickly formed. The heptamer formation causes no major changes in the conformation of each individual monomer, but by coming together, more than 15400 Å2 of protein surface is buried. This buried surface consists mostly of polar or charged side groups from domains 1 and 2.
PA also forms an octameric pre-channel structure. The octameric form was shown to be more thermostable than the heptameric form, and hence the octameric oligomer can persist in the plasma of the host during an anthrax infection.
During the oligomerization of PA63, molecules of EF and/or LF rapidly and simultaneously bind to the PA prechannel. This binding occurs because after removing the PA20 domain, a large hydrophobic surface is uncovered on domain 1 of PA63. Domain 1 provides a large surface that interacts with the N-terminus of EF and LF, which is almost completely homologous for the first ~36 residues and similar in tertiary structure for the first ~250 residues. Studies on the binding region of LF and EF demonstrated that a large surface area contacts with domain 1 of two adjacent PA63 molecules when in the heptamer conformation. This large binding area explains why previous studies could only bind up to three molecules on a PA63 heptamer. The co-crystal structure of the PA octamer in complex with N-terminal LF revealed that the binding interaction is, in fact, two discontinuous sites. One site, termed the C-terminal subsite, resembles a classic "hot-spot" with predicted salt bridges and electrostatic interactions. The other site, termed the alpha-clamp subsite, is a deep cleft that nonspecifically binds the N-terminal alpha helix and short beta-strand of LF, guiding the N-terminus of the substrate towards the PA prechannel lumen. In this manner, the alpha clamp aids in protein translocation, nonspecifically binding and subsequently unfolding secondary structure as it unfurls from the substrate. The LF/EF binding site is now being utilized for delivery of therapeutics via fusion proteins.
Upon formation of the prepore and attachment of LF and/or EF, the heptamer migrates to a lipid raft where it is rapidly endocytosed. Endocytosis occurs as a result of a series of events. This begins when CMG2 or TEM8 is palmitoylated, which inhibits the association of the receptor with lipid rafts. This inhibits the receptor from being endocytosed before PA83 is cleaved and before LF or EF can associate with the heptamer. Reassociation of the receptor with the cholesterol and glycosphigolipid-rich microdomains occurs when PA63 binds to the receptor and heptamerizes. Once the receptor and PA returns to the lipid raft, E3 ubiquitin ligase Cb1 ubiquitinates the cytoplasmic tail of the receptor, signaling the receptor and associated toxin proteins for endocytosis. Dynamin and Eps15 are required for this endocytosis to occur, indicating that anthrax toxin enters the cell via the clathrin-dependent pathway.
As discussed, each molecule interacts with several others in order to induce the endocytosis of the anthrax toxin. Once inside, the complex is transferred to an acidic compartment, where the heptamer, still in the non-membrane-spanning pre-pore conformation, is prepared for translocation of EF and LF into the cytosol.