Key role of Pro230 in the hinge region on the IgG architecture and function

Immunoglobulin G (IgG) is a molecule that plays an important role in biological defense; IgG molecules have been applied as drugs due to their high specificity for antigens and their ability to activate immunity via effector molecules on immune cells. On the other hand, the flexibility of the hinge region makes it difficult to apply conventional structural biology approaches due to its dynamic conformational changes, and the mechanism of action of the molecule as a whole has not been elucidated. Here, we introduced a deletion amino acid mutation in the hinge region to elucidate the role of the hinge region and its effect on the structure and function of the IgG molecule. Deletion of Pro230 resulted in the formation of a half-molecular in which the interaction between heavy chains was lost. We elucidated the mechanism of half-IgG formation by structural analysis using nuclear magnetic resonance (NMR) measurements and by disulfide quantification using peptide mapping using LC-MS/MS. For this purpose, a new NMR stable isotope labeling method was introduced. Finally, cell assay revealed that the IgG half-molecules have specific FcγRI-mediated activity. This report provides new insights into the higher-order structure formation of IgG molecules and is expected to contribute to the elucidation of the molecular basis of the Fcγ receptor-mediated activation mechanism of the immune system. ### Competing Interest Statement The authors have declared no competing interest.