Resolution of two overlapping neutralizing B cell epitopes within a solvent exposed, immunodominant α-helix in ricin toxin's enzymatic subunit

Residues Y₉₁-T₁₁₆ of ricin toxin's enzymatic subunit (RTA) constitute an immunodominant loop-helix-loop motif that is the target of two potent toxin neutralizing monoclonal antibodies (mAbs), PB10 and R70. To define the exact epitope(s) recognized by these mAbs, we affinity enriched from a phage-displayed peptide library 12 mers that bound one or both of these mAbs. We report that PB10 recognizes a distinct but overlapping epitope with R70, in which residues Q₉₈, E₁₀₂, T₁₀₅, and H₁₀₆ are central to mAb recognition.