Oligomerization of anthrax toxin protective antigen and binding of lethal factor during endocytic uptake into mammalian cells

The protective antigen (PA) protein of anthrax toxin binds to a cellular re ceptor and is cleaved by cell surface furin to produce a 63-kDa fragment (P A63). The receptor-bound PA63 oligomerizes to a heptamer and acts to transl ocate the catalytic moieties of the toxin, lethal factor (LF) and edema fac tor (EF), from endosomes to the cytosol. In this report, we used nondenatur ing gel electrophoresis to show that each PA63 subunit in the heptamer can bind one LF molecule. Studies using PA immobilized on a plastic surface sho wed that monomeric PA63 is also able to bind LF. The internalization of PA and LF by cells was studied,vith radiolabeled and biotinylated proteins. Up take was relatively slow, with a half-time of 30 min. The number of moles o f LF internalized was nearly equal to the number of moles of PA subunit int ernalized. The essential role of PA oligomerization in LF translocation was shown with PA protein cleaved at residues 313-314. The oligomers formed by these proteins during uptake into cells were not as stable when subjected to heat and detergent as were those formed by native PA. The results show t hat the structure of the toxin proteins and the kinetics of proteolytic act ivation, LF binding, and internalization are balanced in a way that allows each PA63 subunit to internalize an LF molecule, This set of proteins has e volved to achieve highly efficient internalization and membrane translocati on of the catalytic components, LP and EF.