Bacillus anthracis is a Gram-positive bacterial pathogen known to cause anthrax. B. anthracis secretes a three-protein toxin: protective antigen, lethal factor, and edema factor. Protective antigen binds the host cell surface at cell receptor TEM8. Following proteolytic cleavage, protective antigen forms a membrane channel that allows the passage of lethal factor and edema factor into the cytoplasm. Once inside, edema factor acts as an adenylate cyclase, while lethal factor acts as a zinc metalloprotease, both leading to reduced host cell signaling. Anthrax toxin is best described as a(n) .