Hydrophobic residues Phe552, Phe554, Ile562, Leu566, and Ile574 are required for oligomerization of anthrax protective antigen

Anthrax protective antigen (PA) plays a central role in facilitating the en try of active toxin components, namely, lethal factor and edema factor, int o the cells. PA is also the main immunogen of both human and veterinary vac cine against anthrax. During host cell intoxication, protective antigen bin ds to the receptors on cell surface, gets proteolytically activated, oligom erizes to form a heptamer and binds to lethal factor or edema factor. The c omplex, formed by binding of lethal factor or edema factor to oligomerized PA, is internalized by receptor-mediated endocytosis. Acidification of the endosome results in the insertion of the heptamer into the membrane, thereb y forming a pore through which lethal factor or edema factor can translocat e into the cytosol. In this study we have identified hydrophobic residues, Phe552, Phe554, Ile562, Leu566, and Ile574, which are required for oligomer ization of anthrax protective antigen. Mutation of these conserved residues to alanine impaired the oligomerization of protective antigen. Consequentl y, these mutants became nontoxic in combination with lethal factor and edem a factor. Therapeutic importance of these mutants and their potential as va ccine candidates is discussed. (C) 2001 Academic Press.