From Proteopedia

One of the CBI Molecules being studied in the University of Massachusetts Amherst Chemistry-Biology Interface Program at UMass Amherst and on display at the Molecular Playground

Protective Antigen

Anthrax toxin complex consists of three nontoxic proteins: protective antigen (), lethal factor (LF), and endema factor (EF). PA forms pore and delivers LF and EF, which lead to lethal shock and edema of the host separately, to the cytosol. Formation of the toxin complex starts when PA binds to the cellular receptors and is cleaved into two fragments. The smaller fragment (20 kDa) dissociates and the remaining receptor-bound fragment (63 kDa) self-assemblies to a ring-shaped heptameric oligomer, a pore precursor (). LF and/or EF binds to the PA63 prepore to form a toxic complex. Oligomerization of PA-63 causes the endocytosis and the complex is trafficked to an acidic compartment. The PA-63 heptamer converts from the prepore to the pore state under a low pH condition and translocates LF/EF into the cell.

Reference

1. R.John Collier and John A.T.Young. (2003) Anthrax Toxin., Annu. Rev. Cell Dev. Biol.19:45-70