Monitoring anthrax toxin receptor dissociation from the protective antigen by NMR

The binding of the Bacillus anthracis protective antigen (PA) to the host cell receptor is the first step toward the formation of the anthrax toxin, a tripartite set of proteins that include the enzymatic moieties edema factor (EF), and lethal factor (LF). PA is cleaved by a furin‐like protease on the cell surface followed by the formation of a donut‐shaped heptameric prepore. The prepore undergoes a major structural transition at acidic pH that results in the formation of a membrane spanning pore, an event which is dictated by interactions with the receptor and necessary for entry of EF and LF into the cell. We provide direct evidence using 1‐dimensional ¹³C‐edited ¹H NMR that low pH induces dissociation of the Von‐Willebrand factor A domain of the receptor capillary morphogenesis protein 2 (CMG2) from the prepore, but not the monomeric full length PA. Receptor dissociation is also observed using a carbon‐13 labeled, 2‐fluorohistidine labeled CMG2, consistent with studies showing that protonation of His‐121 in CMG2 is not a mechanism for receptor release. Dissociation is likely caused by the structural transition upon formation of a pore from the prepore state rather than protonation of residues at the receptor PA or prepore interface.