Partial Characterization of Immunoglobulin Cμ Gene of Water Buffalo (Bubalus bubalis) Predicts Distinct Structural Features of C1q-Binding Site in Cμ3 Domain

Abstract

Partial characterization of immunoglobulin Cµ gene of water buffalo (Bubalus bubalis) revealed high amino acid sequence identity with Cµ of cattle (94.28%) and sheep (91.71%). Four amino acid replacements (Met-301, Val-310, Asn-331, and Thr-432) in Cµ2, Cµ3, and Cµ4 of buffalo IgM are distinct, however. Unlike cattle, a codon deletion (GTG encoding valine at position 507 in cattle) and an insertion (GGC encoding glycine at position 532) occur in buffalo Cµ4. Three N-linked glycosylation (Asn-X-Thr/Ser) sites (one at position 325–327 in Cµ2; two at positions 372–374 and 394–396 in Cµ3) differentiate buffalo IgM from cattle and sheep. Similar to cattle, buffalo IgM has fewer prolines in Cµ2, which acts as hinge, which restricts Fab arm flexibility. Increased structural flexibility of the C1q-binding site in Cµ3 compensates for the rigid buffalo Cµ2 domain. Secondary structure of C1q-binding site is distinct in buffalo and cattle IgM where long alpha-helical structure is predominant that may be relevant to complement fixation function. Conserved protein motif “Thr-Cys-Thr-Val-Ala-His” provides protein signatures of C1q-binding region of ruminant species. The distinct structural features of C1q-binding site of buffalo and cattle IgM seem to be of functional significance and, therefore, useful in designing antibody based therapeutics.