Characterization of the gene for the membrane and secretory form of the IgM heavy‐chain constant region gene (Cμ) of the cow (Bos taurus)

Our present understanding of the evolution of immunoglobulins is derived from a few vertebrate species. In order to obtain additional information on the development of the humoral immune system, we cloned and determined the nucleotide sequence of the bovine cDNA and genomic IgM heavy‐chain constant region gene (Cμ). The gene contains four constant region domain‐encoding exons (CH1 to CH4) and two exons encoding the transmembrane domain (TM1, TM2), expressed in the membrane‐bound receptor form of the IgM. The sequence of a cDNA clone encoding the 3′ portion of the membrane form of the μ‐chain revealed that the TM1 exon is spliced to the CH4 exon, as occurs in other mammals. Comparison of deduced amino acid sequence data from different vertebrates revealed a high similarity to sheep Cμ (88%) and a lower degree of similarity to pig (62%), rat (62%), rabbit (58%) human (56%), hamster (55%), mouse (54%), chicken (28%) and horned shark (22%) Cμ.