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 (CH) was elucidated. The sequence analysis of variable regions (VH and VL) and light chain constant region (CL) 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.