Sequence characterization and N-glycoproteomics of secretory immunoglobulin A from donkey milk

Secretory immunoglobulin A (sIgA) is the major antibody present in the human milk where it confers passive immunity to neonates. Other than human, non-ruminants such as equine, swine etc., also possess sIgA in milk but detailed characterization is limited. In the present study, we characterized sIgA from donkey milk for amino acid sequence and N-glycosylation through LC-MS/MS analysis. The complete amino acid sequence of alpha chain constant region (C_H) was elucidated. The sequence analysis of variable regions (V_H and V_L) and light chain constant region (C_L) showed several amino acid substitutions indicating the presence of diverse immunoglobulin repertoire. Glycoproteomic analysis of secretory component revealed bi-antennary complex and hybrid types with differential core fucosylation at site N₈₃LT, only complex glycans at N₁₃₅GT, N₄₂₃GT and N₅₃₀LT with mainly NeuAc whereas N₂₉₁QT harbors high mannose glycans. Heavy chain possesses majorly bi-antennary complex with differential core fucosylation at sites N₁₃₉AS and N₃₃₈VS, in which N₃₃₈VS shows partial occupancy. Joining chain harbors only complex type at N₇₂IS, with core fucosylation and terminal NeuGc to some extent. N-glycan repertoire in part is similar to human sIgA. This comprehensive analysis of sequence and glycan pattern of donkey milk sIgA would be beneficial for its potential applications.