PA63 is the carboxyl-terminal 63- kDa fragment obtained by the proteolytic cleavage of the receptor-bound protective antigen (PA), one of the three proteins that comprise anthrax toxin. Unlike native PA, PA 63 oligomerizes to form a ring-shaped heptamer that binds up to three copies of EF and/or LF competitively and with high affinity (kDa ~1 nM). Whereas native PA persists on the cell surface, the heptamer is endocytosed, presumably because oligomerization aggregates anthrax toxin receptor. The endocytosed toxic complexes are trafficked to endosomal compartments where the low pH causes the PA63 heptamer to insert into the membrane and form a water-filled channel. Alternative Names: Bacillus Anthracis Protein, Bacillus anthracis Protective Nicked Antigen, Anthrax Protein, Recombinant B Anthracis