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Anthrax lethal toxin assembles at the surface of mammalian cells when the lethal factor (LF) binds via its amino-terminal domain, LF(N), to oligomeric forms of activated protective antigen (PA). LF x PA complexes are then trafficked to acidified endosomes, where PA forms heptameric pores in the bounding membrane and LF translocates through these pores to the cytosol. We used enhanced peptide amide hydrogen/deuterium exchange mass spectrometry and directed mutagenesis to define the surface on LF(N) that interacts with PA. A continuous surface encompassing one face of LF(N) became protected from deuterium exchange when LF(N) was bound to a PA dimer. Directed mutational analysis demonstrated that residues within this surface on LF(N) interact with Lys-197 on two PA subunits simultaneously, thereby showing that LF(N) spans the PA subunit:subunit interface and explaining why heptameric PA binds a maximum of three LF(N) molecules. Our results elucidate the structural basis for anthrax lethal toxin assembly and may be useful in developing drugs to block toxin action.