P13423 pXO1-110(pxo1_110) Bacillus anthracis domain:The molecule is folded into four functional domains. Each domain is required for a particular step in the toxicity process. Domain 1 contains two calcium ions and the proteolytic activation site. Cleavage of the PA monomer releases the subdomain 1a, which is the N-terminal fragment of 20-kDa (PA20). The subdomain 1b is part of the remaining 63-kDa fragment (PA63) and contains the binding sites for LP and EF. Domain 2 is a beta-barrel core containing a large flexible loop that has been implicated in membrane insertion and pore formation. There is a chymotrypsin cleavage site in this loop that is required for toxicity. Domain 3 has a hydrophobic patch thought to be involved in protein-protein interactions. Domain 4 appears to be a separate domain and shows limited contact with the other three domains: it would swing out of the

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