Assembly and Disassembly Kinetics of Anthrax Toxin Complexes^†

Proteolytic activation of the protective antigen (PA) component of anthrax toxin allows it to self-associate into a ring-shaped homoheptamer, [PA₆₃]₇, which can bind the enzymatic components lethal factor (LF) and edema factor (EF). [PA₆₃]₇ is a pore-precursor (prepore), and under the low-pH conditions of the endosome, it forms a transmembrane pore that allows LF and EF to enter the cytosol. PA was labeled with donor and acceptor fluorescent dyes, and Förster resonance energy transfer was used to measure the assembly and disassembly kinetics of the prepore complex in solution. The dissociation rate constant for [PA₆₃]₇ was 1 × 10^-6 s^-1 (t_1/2 7 days). In contrast, a ternary complex containing the PA-binding domain of LF (LF_N) bound to a PA₆₃ dimer composed of two nonoligomerizing mutants dissociated rapidly (t_1/2 1 min). Thus, the substantial decrease in the rate of disassembly of [PA₆₃]₇ relative to the ternary complex is due to the cooperative interactions among neighboring subunits in the heptameric ring. Low concentrations of LF_N promoted assembly of the prepore from proteolytically activated PA, whereas high concentrations inhibited assembly of both the prepore and the ternary complex. A self-assembly scheme of anthrax toxin complexes is proposed.