Toxin, anthrax: The toxic substance secreted by the bacterium Bacillus anthracis, the causative agent of the disease anthrax.
Anthrax toxin is made up of three proteins. One is protective antigen and two are enzymes that are called edema factor and lethal factor. The protective antigen binds to surface receptors on the host's cell membranes. After being cleaved by a protease, protective antigen binds to the two toxic enzymes -- edema factor and lethal factor -- and mediates their transportation into the cytosol (the soluble portion of the cytoplasm) where they exert their pathogenic (disease-causing) effects. Lethal factor is the crucial pathogenic enzyme. It is the killer in the toxin.
Anthrax toxin is not yet fully fathomed. The receptor for protective antigen, named anthrax toxin receptor, has been discovered to be a type I membrane protein with an extracellular Von Willebrand factor A domain that binds directly to protective antigen. And lethal factor has been found to cleave members of what is called the MAPKK (mitogen-activated protein kinase kinase) family and to disrupt cellular signalling.