Frutalin as an Affinity Tool for sIgA1: Biophysical and Structural Characterization of the Lectin–Antibody Complex

Secretory immunoglobulin A1 (sIgA1) is the predominant antibody in mucosal secretions and human colostrum, where it plays critical roles in immune exclusion and microbiota regulation. Due to its high degree of glycosylation, oligomeric variability, and structural flexibility, selective purification of sIgA1 remains a technical challengeyet essential for biomedical, immunological, and biotechnological applications, including diagnostic platforms and passive immunotherapy. In this study, frutalin (FTL), a galactose-specific lectin from <i>Artocarpus incisa</i>, was used as both a selective ligand and a structural probe to investigate its interaction with native sIgA1. FTL was immobilized on CNBr-activated agarose to generate an affinity matrix (frutalin-agarose), enabling the efficient purification of sIgA1 from human colostrum. The antibody was further refined by size exclusion chromatography to isolate the dimeric form, which was used for complex formation under native conditions. The resulting FTL-sIgA1 complex was characterized by dynamic light scattering (DLS), biolayer interferometry (BLI), and hemagglutination assays. DLS titration revealed cooperative multivalent binding, with a dominant population centered at ∼32 nm. BLI confirmed high-affinity interaction (<i>K</i> _D = 3.58 nM), modeled as a bivalent analyte system. Hemagglutination assays showed partial retention of lectin activity, indicating selective but nonsaturating engagement of carbohydrate recognition domains. Negative-staining TEM and cryo-EM provided preliminary structural evidence of complex formation. These findings highlight FTL as a stable glycan-specific ligand for mucosal antibodies and support its application in glycoprotein purification, immunoaffinity systems, and structural glycobiology.