Antigen clasping by two antigen-binding sites of an exceptionally specific antibody for histone methylation

Extensive studies of the structure–function relationship of antibodies have established that conventional immunoglobulins contain two copies of the antigen-binding fragment (Fab), each of which serves as an autonomous and complete unit for recognizing an antigen. In this paper, we report a previously unidentified mode of antibody–antigen recognition, dubbed “antigen clasping,” where two antigen-binding sites cooperatively clasp one antigen, and the design of a long-neck antibody format that facilitates antigen clasping. Antigen clasping led to recombinant antibodies for histone posttranslational modifications with extraordinarily high specificity, valuable tools for epigenetic research. This study substantially broadens the long-standing paradigm for antibody–antigen recognition

Antigen clasping by two antigen-binding sites of an exceptionally specific antibody for histone methylation

Antibodies have a well-established modular architecture wherein the antigen-binding site residing in the antigen-binding fragment (Fab or Fv) is an autonomous and complete unit for antigen recognition. Here, we describe antibodies departing from this paradigm. We developed recombinant antibodies to trimethylated lysine residues on histone H3, important epigenetic marks and challenging targets for molecular recognition. Quantitative characterization demonstrated their exquisite specificity and high affinity, and they performed well in common epigenetics applications. Surprisingly, crystal structures and biophysical analyses revealed that two antigen-binding sites of these antibodies form a head-to-head dimer and cooperatively recognize the antigen in the dimer interface. This “antigen clasping” produced an expansive interface where trimethylated Lys bound to an unusually extensive aromatic cage in one Fab and the histone N terminus to a pocket in the other, thereby rationalizing the high specificity. A long-neck antibody format with a long linker between the antigen-binding module and the Fc region facilitated antigen clasping and achieved both high specificity and high potency. Antigen clasping substantially expands the paradigm of antibody–antigen recognition and suggests a strategy for developing extremely specific antibodies