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family of the cytolysin A (ClyA) family alpha pore-forming toxins (alpha-PFT) including Bacillus cereus HblB, Aeromonas hydrophila AhlB, Bacillus thuringiensis Cry6Aa and similar proteins This family belongs to the ClyA family of alpha-PFT bacterial toxins. PFTs form the major group of virulence factors in many pathogenic bacteria and in general are critical components of the molecular offensive and defensive machinery of cells in all kingdoms of life. Bacterial PFTs facilitate the takeover of host resources by puncturing holes in the membrane. PFTs can be classified as alpha-PFTs and beta-PFTs depending on the secondary structures of their membrane component. Alpha-PFTs use a ring of amphipathic helices while beta-PFTs use a beta-barrel to construct the pore. Members of this family include the toxins: Bacillus cereus hemolysin binding component B (HblB or HBL-B) of the diarrheal enterotoxin hemolysin BL, Aeromonas hydrophila hemolytic (Ahl) component B (AhlB) of the tripartite AhlABC toxin, Vibrio cholerae cytotoxin motility associated killing factor A (MakA) cytotoxin, Xenorhabdus nematophila alpha-xenorhabdolysin (XaxA), Bacillus thuringiensis crystal 6Aa (Cry6Aa) parasporal crystal (Cry) toxin, and Bacillus cereus non-hemolytic enterotoxin (Nhe) component A (NheA) of the non-hemolytic enterotoxin Nhe, which, despite its name, is hemolytic, among others. In solution, ClyA proteins have an elongated, almost entirely alpha-helical structure, except for a short hydrophobic beta-hairpin known as the beta-tongue. Pore formation by ClyA requires circular oligomerization of the toxin by a sequential mechanism. This, in turn, concentrates the amphipathic helices in the center of the ring-like structure, forming a helical barrel that inserts into the membrane by a wedge-like mechanism. Compared with ClyA, NheA is almost entirely alpha-helical with an enlarged "head" domain, and an enlarged beta-tongue; it has been proposed that NheA could even form beta-barrel pores. Alpha-PFTs with similar structures are increasingly being found in eukaryotes, in particular as components of the immune systems of animals. This family may be distantly related to Escherichia coli alpha-PFT hemolysin E (HlyE, also known as ClyA or SheA). |
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