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) was elucidated. The sequence analysis of variable regions (V and V) and light chain constant region (C) 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 NLT, only complex glycans at NGT, NGT and NLT with mainly NeuAc whereas NQT harbors high mannose glycans. Heavy chain possesses majorly bi-antennary complex with differential core fucosylation at sites NAS and NVS, in which NVS shows partial occupancy. Joining chain harbors only complex type at NIS, 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.