The protective
antigen (PA) moiety of
anthrax toxin binds to cellular receptors and mediates entry of the two enzymatic moieties of the toxin into the cytosol. Two PA receptors,
anthrax toxin receptor (ATR)/
tumor endothelial marker 8 (TEM8) and capillary morphogenesis
protein 2 (CMG2), have been identified. We expressed and purified the von Willebrand A (VWA) domain of CMG2 and examined its interactions with monomeric and heptameric forms of PA. Monomeric PA bound a stoichiometric equivalent of CMG2, whereas the heptameric prepore form bound 7 eq. The Kd of the VWA domain-PA interaction is 170 pm when liganded by Mg2+, reflecting a 1000-fold tighter interaction than most VWA domains with their endogenous
ligands. The dissociation rate constant is extremely slow, indicating a 30-h lifetime for the CMG2.PA monomer complex. CMG2
metal ion-dependent adhesion site (MIDAS) was studied kinetically and thermodynamically. The association rate constant (approximately 10(5) m(-1) s(-1)) is virtually identical in the presence or absence of Mg2+ or Ca2+ , but the dissociation rate of
metal ion liganded complex is up to 4 orders of magnitude slower than
metal ion free complex. Residual affinity (Kd approximately 960 nm) in the absence of divalent
metal ions allowed the free energy for the contribution of the
metal ion to be calculated as 5 kcal mol(-1), demonstrating that the
metal ion-dependent adhesion site is directly coordinated by CMG2 and PA in the binding interface. The high affinity of the VWA domain for PA supports its potency in neutralizing
anthrax toxin, demonstrating its potential utility as a novel therapeutic for
anthrax.