Koseki Y, Yamaguchi Y, Aoyama M, Hiraka K, Tada M, Kodama A, Senoo A, Ishii-Watabe A, Uchihashi T, Murata K, Uchiyama S, Kato K, Yanaka S, J. M. M. Caaveiro

Journal of Medicinal Chemistry,  (2026)

Immunoglobulin G (IgG) is a multifunctional glycoprotein essential for immune defense and widely used as a therapeutic due to its antigen specificity and effector functions. However, the inherent flexibility of its hinge region complicates structural characterization and obscures the molecular basis of its mechanism of action. To clarify the hinge’s role, we performed systematic amino acid substitutions. Notably, deletion of Pro230 led to the formation of a half-IgG1 species lacking inter-heavy chain interactions. Structural analysis using nuclear magnetic resonance (NMR), negative-stain EM, and disulfide bond quantification by LC-MS/MS peptide mapping revealed the mechanism underlying half-IgG1 generation. To enable this, we developed a new stable-isotope labeling method for NMR. Functional assays with FcγR-expressing reporter cells demonstrated that half-IgG1 retained selective FcγRI-mediated activity. These findings provide new insights into higher-order IgG structure and Fcγ receptor-dependent immune activation, offering a basis for designing next-generation antibody therapeutics.