Anthrax toxin complexes: heptameric protective antigen can bind lethal factor and edema factor simultaneously.

The 83 kDa protective antigen (PA(83)) component of anthrax toxin, after proteolytic activation, self-associates to form ring-shaped heptamers (PA(63)) that bind and aid delivery of the Edema Factor (EF) and Lethal Factor (LF) components to the cytosol. Here we show using fluorescence (Förster) resonance energy transfer that a molecule of PA(63) can bind EF and LF simultaneously. We labeled EF and LF with an appropriate donor/acceptor pair and found quenching of the donor and an increase in sensitized emission of the acceptor when, and only when, a mixture of the labeled proteins was combined with PA(63). Addition of unlabeled PA(63)-binding domain of LF to the mixture competitively displaced labeled EF and LF, causing a loss of energy transfer. In view of the known maximum occupancy of 3 ligand molecules per PA(63), these findings indicate that PA, EF, and LF can form mixtures of liganded toxin complexes containing both EF and LF.