Structural and phylogenetic analysis of the MHC class I-like Fc receptor gene

The intestinal epithelium of neonatal mice and rats expresses an Fc receptor that mediates selective uptake of IgG in mothers' milk. This receptor (FcRn), which helps newborn animals to acquire passive immunity, is an MHC class I-like heterodimer made up of a heavy chain and beta 2-microglobulin. In the present study, we determined the genomic structure of a mouse gene (Fcrn) encoding the heavy chain of FcRn. The overall exon-intron organization of the Fcrn gene was similar to that of the MHC class I gene, thus providing structural evidence that Fcrn is a bona fide class I gene. The 5'-flanking region of the Fcrn gene contained the binding motifs for two cytokine-inducible transcription factors, NF-IL6 and NF1. However, regulatory elements found in MHC class I genes (enhancer A, enhancer B, and the IFN response element) were absent. Phylogenetic tree analysis suggested that, like the MICA, AZGP1, and CD1 genes, the Fcrn gene diverged from MHC class I genes after the emergence of amphibians but before the split of placental and marsupial mammals. Consistent with this result, Southern blot analysis with a mouse Fcrn cDNA probe detected cross-hybridizing bands in various mammalian species and chickens. Sequence analysis of the Fcrn gene isolated from eight mouse strains showed that the membrane-distal domain of FcRn has at least three amino acid variants. The fact that Fcrn is a single copy gene indicates that it is expressed in both the neonatal intestine and the fetal yolk sac.