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Protective Antigen

Protective antigen (PA). PA is not toxic by itself but is the essential component for EF and LF to generate their toxic effect. PA binds to to specific receptors on the host cell surface and after activation by a furin-like protease (8) forms a heptamer complex and creates a binding site for either EF or LF. The complex (either PA+LF or PA+EF) is then taken into the cell via clathrin coated endocytosis. Upon acidification in the endosomes, the heptamer changes conformation, inserts in the membrane forming a channel (as seen in the ribbon model to the right) and allows the two factors to enter the cytosol.

Ribbon Model of Heptamer (3)