Crystal Structure of the Human Igg4 C(H)3 Dimer Reveals the Role of Arg409 in the Mechanism of Fab-Arm Exchange.

Abstract Antibodies of the human IgG4 subclass uniquely undergo a process of Fab-arm exchange in which the heavy-chains of antibodies of different specificities can dissociate and then recombine. The mechanism by which the resulting functionally monovalent but bi-specific antibodies are formed is not only key to understanding their biological role, but is also important for the design of therapeutic monoclonal antibodies. Both the hinge region and the C H 3 domain interface are known to be involved, and of the residues that differ between human IgG1 and IgG4 in C H 3, residue 409, the only difference at the interface itself, has been implicated. We report the high resolution (1.8 A) structure of the C H 3 domain dimer of IgG4, and find that Arg409 in IgG4, when compared with Lys409 observed in high resolution IgG1 structures, disrupts a network of water-mediated hydrogen bonding that is conserved in IgG1. Other conformational differences were detected that are a consequence of the presence of Arg409, such as a widening of the separation between residues Asn390 in one domain and Ser 400 in the other, which opens up a groove at the edge of the interface in IgG4 compared with IgG1. The effect of all these differences on the C H 3 interface, doubled as a result of the interface's two-fold symmetry, is weakening of the inter-domain interaction in IgG4 compared with IgG1. This suggests a mechanism by which Arg409 weakens the C H 3 interface in IgG4, predisposing this human antibody subclass to Fab-arm exchange.