Anthrax toxin is a three-protein exotoxin secreted by virulent strains of the bacterium, Bacillus anthracis-the causative agent of anthrax. Anthrax toxin is composed of a cell-binding protein, known as protective antigen (PA), and two enzyme components, called edema factor (EF) and lethal factor (LF). These three protein components act together to impart their physiological effects. Assembled complexes containing the toxin components are endocytosed.