Anthrax toxin prepore. Molecular model of protective antigen (PA) from anthrax (Bacillus anthracis) bacteria complexed with an anthrax toxin receptor, forming a prepore (hole, centre). Anthrax employs a suite of three proteins, collectively termed anthrax toxin, that attack target organisms. Protective antigen is the first of these and works by providing a ready-made pore in its victim's cell membrane, through which the other two proteins, edema factor and lethal factor (not shown), can be delivered into the cell's interior.