Alanine-scanning mutations in domain 4 of anthrax toxin protective antigen reveal residues important for binding to the cellular receptor and to a neutralizing monoclonal antibody

A panel of variants with alanine substitutions in the small loop of anthrax toxin protective antigen domain 4 was created to determine individual amino acid residues critical for interactions with the cellular receptor and with a neutralizing monoclonal antibody, 14B7. Substituted protective antigen proteins were analyzed by cellular cytotoxicity assays, and their interactions with antibody were measured by plasmon surface resonance and analytical ultracentrifugation. Residue Asp(683) was the most critical for cell binding and toxicity, causing an similar to1000-fold reduction in toxicity, but was not a large factor for interactions with 14B7. Substitutions in residues Tyr(681), Asn(682), and Pro(686) also reduced toxicity significantly, by 10-100-fold. Of these, only Asn(682) and Pro(686) were also critical for interactions with 14B7. However, residues Lys(684), Leu(685), Leu(687), and Tyr(688) were critical for 14B7 binding without greatly affecting toxicity. The K684A and L685A variants exhibited wild type levels of toxicity in cell culture assays; the L687A and Y688A variants were reduced only 1.5- and 5-fold, respectively.