Crystal structures reveal how the multispecific antibody <scp>G2</scp> achieves binding to different peptides

Monoclonal antibody G2, obtained via immunization with the chicken prion protein (ChPrP), has unique antigen-binding specificity. G2 specifically binds to the ChPrP-derived peptide (Pep18mer) as expected, but also to three other peptides. In this study, we determined the crystal structures of G2 single-chain Fv antibodies covalently linked to Pep18mer and another peptide, PepH4P6. Both bound peptides formed similar U-shaped structures that stuck into the G2 antigen-binding pocket. Their three-dimensional structures were stabilized by interactions within the peptides, and the structure of the bound Pep18mer was similar to that of the corresponding ChPrP region. G2 acquired the binding ability to both Pep18mer and PepH4P6 via deletion of the 95th residue of the light chain during affinity maturation, consistent with our structural analysis.