Cloning and characterization of the bovine MHC class I-like Fc receptor

In the cow, maternal immunity is exclusively mediated by colostral Igs, but the receptor responsible for the IgG transport has not yet been identified . The role of an IgG-Pe receptor (FcRn) that resembles a class I MHC Ag in transporting IgGs through epithelial cells was recently shown in selected s pecies. We now report the cloning and characterization of the bovine FcRn ( bFcRn), The cDNA and deduced amino acid sequences show high similarity to t he FcRn in other species, and it consists of three extracellular domains, a hydrophobic transmembrane region, and a cytoplasmic tail. Despite the high similarity of the extracellular domains with other species, the bovine cyt oplasmic tan is the shortest thus far analyzed. Aligning the known FcRn seq uences, we noted that the bovine protein shows a 3-aa deletion compared to the rat and mouse sequences in the al loop. Furthermore, we found a shorter transcript of the bFcRn reflecting an exon 6-deleted mRNA, which results f rom an inadequate splice acceptor site in intron 5 and produces a transmemb rane-deficient molecule, as was previously demonstrated in the related MHC class I gene family in mouse and humans. The presence of bFcRn transcripts in multiple tissues, including the mammary gland, suggests their involvemen t both in IgG catabolism and transcytosis.