Speaker
Description
Immunoglobulin A (IgA) is the principal guardian of mucosal surfaces, acting as the primary defense against pathogens. To successfully colonize the human body, several bacteria have evolved peptidases that specifically cleave antibodies, uncoupling antigen recognition from immune effector functions. While most of these enzymes exclusively target the hinge region of the IgA1 subclass, the metallopeptidase IgAse from the human gut commensal Thomasclavelia ramosa (formerly Clostridium ramosum) uniquely cleaves both IgA1 and the recalcitrant IgA2 subclass. Until now, the highly transient nature of this enzyme-substrate interaction has precluded structural characterization, leaving the mechanism of this striking subclass-independent recognition elusive.
To overcome this hurdle, we employed an integrative structural biology approach. Guided by AI-driven conformational landscape sampling (AlphaFold3), we engineered a strategically disulfide-stabilized complex to trap the peptidase-antibody interaction. Utilizing high-resolution single-particle cryo-electron microscopy (cryo-EM), alongside macromolecular crystallography (MX) and molecular dynamics simulations, we successfully captured two consecutive states along the reaction coordinate: a non-productive 'recognition' complex and a subsequent 'precatalytic' complex.
Our integrative analysis reveals a previously undescribed, CH1-mediated ‘knob-in-hole’ gating mechanism. We demonstrate how the antibody heavy-chain CH1 loop docks into a preformed pocket of the peptidase and, through a 180° rotation, physically delivers the antibody hinge into the active-site cleft for cleavage. Together, these results establish the definitive structural paradigm for subclass-independent IgA cleavage. Beyond explaining a fundamental host-microbiome interaction at atomic resolution, our findings provide a vital structural framework for the rational design of IgA-directed biotherapeutics and treatments for IgA-deposition diseases, such as IgA nephropathy.