B. anthracis PA

Bacillus anthracis is a Gram-positive, nonmotile, aerobic, facultatively anaerobic, spore-forming rod-shaped bacterium, which is the etiological agent of anthrax. Bacillus anthracis secretes an exotoxin composed of three distinct proteins: protective antigen (PA), edema factor (EF), and lethal factor (LF).
The mature PA is 735 amino-acids and its molecular weight is 82,684. The crystal structure of monomeric PA has been determined. The molecule is folded into four functional domains. Each domain is required for a specific step in the intoxication process. Domain 1 ranges from residue1 to residue 249. It contains the proteolytic activation site. Cleavage of the PA monomer, in vitro by trypsin or in vivo by cellular proteases, releases the sub-domain 1a, which is a 20kD fragment. The remaining part is a 63kD fragment. Domain 2 of PA ranges from residue 250 to 487. It is a β barrel core containing a large flexible loop which has been implicated in pore formation.
Protective antigen functions as a relevant antigen and as an immunogen, providing protection from anthrax infection in immunized animals. Moreover, PA is an essential component of anthrax vaccine.