Origin of bovine IgM structural variants

The structure of IgM determined from two cDNAs isolated from a Holstein (BL V7G1) and an Angus x Hereford cross-bred (B5D8) cow reveals high sequence s imilarity both at nucleotide (98.7%) and amino acid (97.9%) level and is cl osest to sheep (89.4%). Three bovine IgM allotypes. designated as IgMa, IgM b and IgMc, are classified based on nucleotide substitutions in all the C m u exons resulting in amino acid replacements. Further, insertion of three i n-frame codons at C mul and C mu2 junction of B5D8 IgM from the intervening intron. via cleavage of pre-mRNA at an alternate cryptic 5' splice donor s ite, leads to generation of additional bovine IgM variants. The Clq-binding site, involved in classical complement pathway. is identified in bovine Ig M where ten amino acids are conserved across species. Interestingly, bovine IgM has the lowest number of proline residues (5) in the C mu2 domain in c omparison to other species (7-9) and this is likely to impose structural co nstraints on mobility of Fab arms of the bovine IgM during antigen recognit ion. The rigidity in the bovine IgM C mu2 domain may, however, facilitate e xposure of Clq-binding site subsequent to antigen binding and enhance its c omplement fixing ability. The restricted mobility of bovine IgM Fab arms ma y possibly favor generation of an antigen-combining site requiring an unusu ally long third complementarity determining region of the heavy chain (CDR3 H), apart from antigen selection of variable domains. This is consistent wi th the fact that an exceptionally long CDR3H has not been observed in bovin e IgG which bears a long and more flexible hinge region. Additional hydroph ilic threonine and serine residues in the C mu2 domain of bovine IgM. as co mpared to other species, however, enhance its ability to extend into the so lvent. Finally, restricted fragment length polymorphism analysis of genomic DNA from four cattle breeds reveals the presence of, at least, four alleli c variants of bovine Cp gene. (C) 2001 Published by Elsevier Science Ltd.