Structure of the <i>Thomasclavelia ramosa</i> immunoglobulin A protease reveals a modular and minimizable architecture distinct from other immunoglobulin A proteases

Immunoglobulin A proteases (IgAPs) are a diverse group of enzymes secreted from bacteria that inhabit human mucosal tissues. These enzymes have convergently evolved to cleave human immunoglobulin A as a means of modulating and evading host immunity. Only two of three known IgAP families have been biochemically characterized beyond their initial discovery. Here, we show using solution-scattering, steady-state kinetic, and crystallographic approaches that the protease from <i>Thomasclavelia ramosa</i>, representing the uncharacterized third family, has a truly modular and minimizable protein architecture. This analysis also revealed a unique metal-associated domain that likely functions as a molecular spacer and generated a working hypothesis detailing the structural mechanism behind the enzyme's high substrate specificity. Our work provides an in-depth biochemical account of this IgAP family, paving the way for advancing clinically relevant IgAP-related research and our understanding of IgAPs as a whole.