Anthrax PA (Protective Antigen) and Anthrax LF (Lethal Factor), along with Anthrax EF (Edema Factor), are toxins produced by Bacillus anthracis. All three of the proteins must be present for toxic entry into cells. Anthrax PA is secreted from Bacillus anthracis as an 83 kDa (PA83) protein which is cleaved by furin protease, releasing a 20 kD fragment. The 63 kD portion (PA63) of the cleavage product forms a heptamer, which binds the toxins LF and EF. This complex enters via receptor-mediated endocytosis where LF and EF exert their toxic effects.
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