The heterogeneity of bovine IgG2--VIII. The complete cDNA sequence of bovine IgG2a (A2) and an IgG1.

The complete cDNA sequences of two bovine IgGs were obtained by RT-PCR cloning. The first-strand cDNA was prepared from an animal homozygous (A2/A2) for IgG2a; the resulting sequences of the two bovine IgGs reported here were identified as IgG2a(A2) and IgG1. These sequences, and their deduced amino acid sequences, are compared to the previously reported partial protein sequences of IgG2a(A2) and IgG2(A1), two genomic DNA sequences of IgG2a and one genomic DNA sequence of IgG1. Data show that the two IgG2a allotypes (A1 and A2) differ in four regions: (a) region I-the site of the L-H bond in CH1; (b) region II-the middle hinge; (c) region III-a seven amino acid region at the beginning of the intradomain loop in CH3; and (d) region IV-an Arg-to-Glu exchange at the end of the same intradomain loop. The A1 allotype, which so remarkably distinguishes these allotypic variants, must result from differences in regions III and IV. The IgG1 sequence differs in the hinge region from the sequence reported previously and may represent an allotypic variant. We found no evidence to support the hypothesis that similarities between the CH3 domains of IgG2a(A2) and IgG1 result from gene conversion in the C-region of the bovine heavy chain locus.