Partial Characterization of Immunoglobulin Cμ Gene of Water Buffalo (Bubalus Bubalis) Predicts Distinct Structural Features of C1 q-Binding Site in Cμ3 Domain

The Partial characterization of immunoglobulin Cμ gene of water buffalo (Bubalus bubalis) has revealed unique features of the IgM isotype. Although, buffalo Cμ domain shares high amino acid sequence similarity with Cμ of cattle (94.28%) and sheep (91.71%), four distinct amino acid replacements (Met-301, Val-310, Asn-331 and Thr- 432) spread across Cμ2, Cμ3 and Cμ4 diverge from those in cattle and sheep. As compared to cattle, buffalo Cμ gene has a codon deletion at position 507 (GTG encoding valine present in cattle) and insertion of GGC encoding glycine at position 532 in the Cμ4 domain. Unlike cattle and sheep, buffalo IgM has three potential N-linked glycosylation (Asn-X-Thr/Ser) sites, one at position 325-327 in the Cμ2 domain and two at positions 372-374 and 394-396 in the Cμ3 domain. Similar to cattle, buffalo IgM has fewer proline residues in Cμ2 domain that acts as hinge resulting in restricted flexibility of Fab arms. The increased structural flexibility in the C1q-binding site in Cμ3 may compensate for the rigid Cμ2 domain of buffalo IgM. The secondary structure of C1q binding site reveals its distinct features in buffalo and cattle IgM where a long alpha-helical structure is predominant which seems to be relevant to complement fixation function. The conserved protein motif ‘Thr- Cys-Thr-Val-Ala-His’ provides protein signatures of C1q binding region across ruminant species. The distinct structural features of C1q binding site of buffalo IgM, closest to cattle, are likely to be of functional significance relevant to designing antibody based therapeutics and diagnostics.