Partial Characterization of Immunoglobulin C Gene of Water Buffalo (Bubalus bubalis) Predicts Distinct Structural Features of C1q-Binding Site in C3 Domain
Partial characterization of immunoglobulin C gene of water buffalo (Bubalus bubalis) revealed high amino acid sequence identity with C of cattle (94.28%) and sheep (91.71%). Four amino acid replacements (Met-301, Val-310, Asn-331, and Thr-432) in C 2, C 3, and C 4 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 C 4. Three N-linked glycosylation (Asn-X-Thr/Ser) sites (one at position 325–327 in C 2; two at positions 372–374 and 394–396 in C 3) differentiate buffalo IgM from cattle and sheep. Similar to cattle, buffalo IgM has fewer prolines in C 2, which acts as hinge, which restricts Fab arm flexibility. Increased structural flexibility of the C1q-binding site in C 3 compensates for the rigid buffalo C 2 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. 1. Introduction The water buffalo (Bubalus bubalis), member of family Bovidae domesticated approximately 5000 years ago in Asia, is raised for milk, meat, and draught purposes. Approximately, 170 million water buffaloes are mainly found in Asia (97%), but their number is growing across Africa, Australia, Europe and South America [1]. India possesses the best of dairy breeds (Murrah, Nili-Ravi, and Surti) that produce 72 million tones of milk annually, 5% of world’s total milk output [1, 2]. Buffalo milk is rich in fat, protein, and minerals but low in cholesterol [3] and is, thus, perfect source of good quality dairy products, especially the traditional Italian mozzarella cheese [4]. The demand for buffalo meat is high as it is relatively lean with low fat and high mineral content as compared to beef or pork. Buffaloes provide an excellent source of draught power in more than 50 countries [1, 2]. Buffalo utilizes poorly digestible feeds better than cattle and, therefore, can be maintained on low quality fodder and crop [1]. Importantly, buffaloes are resistant to common diseases, ticks, and external parasites that commonly afflict cattle [5]. Little is known about the structural and functional features of the immune system of
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