Genomic organisation and sequence of the extracellular domain exons of the bovine Fc γRI receptor, and evidence for restricted binding of ruminant IgG to U937 cells

Abstract

Southern blotting of bovine genomic DNA and hybridization with a human Feγ RI cDNA probe, p 135, has identified a single copy of the bovine FcγRI gene. A bovine genomic lymphocyte library in λEMBL3 was screened with probe p135. A positive λ clone, 15.5.4, containing the three extracellular domain exons of Feγ RI, has been cloned, mapped and sequenced. Each extracellular domain is encoded within a single exon. All three domains are assigned to the C-2 set of the Ig superfamily with 58% identity between amino acid residues of bovine, human and mouse Fc γRI. Pairs of cysteine residues are conserved in each domain as potential sites for intra-chain disulphide bonding. Human monocytoid U937 cells were used as a model to test binding homology within the Fc γRI family. The binding of IgG isotypes to IFN-γ stimulated U937 cells was determined by FACScan flow cytometry. U937 cell Fc γRI receptor does not bind bovine or ovine IgG isotypes. On the basis of these studies and by comparison of the Fc determinant region sequences of IgG, the introduction of species specificity in Fc γRI/IcG interaction by evolutionary drift is proposed.