Anthrax protective antigen molecule. Computer model showing the structure of a molecule of protective antigen (PA) produced by anthrax (Bacillus anthracis) bacteria. 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, oedema factor and lethal factor (not shown), can be delivered into the cell's interior.