STRUCTURE AND EXPRESSION OF A DIVERGENT CANINE CLASS-I GENE

We have isolated and characterized a canine class I MHC (dog leukocyte Ag, DLA) gene, DLA-79. The deduced protein sequence shares only 65% i dentity with a previously published canine class I cDNA, designated DL A-A, and exhibits 64% amino acid identity with the HLA-A, -B, -C conse nsus. The peptide-binding region of DLA-79 is unusual. Three of four h ighly conserved tyrosine residues (Tyr 7, 59, 159, and 171), proposed to interact with the N terminus of peptide-Ag, are substituted. Additi onally, the long alpha-helix lining the peptide-binding region in the alpha 1 domain contains one more amino acid residue than that observed in typical class I. Together, these features suggest that DLA-79 bind s a distinct subset of peptides or other ligands. This gene has been e xpressed in a class I null human lymphoblastoid cell line, and the enc oded heavy chain associated with beta(2)-microglobulin and was transpo rted to the cell surface. Ribonuclease protection analysis detected lo w levels of gene-specific mRNA in a broad variety of dog tissues. The highest levels were found in skeletal muscle, a tissue expressing rela tively low levels of classical class I Ag. These data suggest that DLA -79 is functional and plays a specialized role in the immune response. Nucleotide sequence analysis of second exon sequences (encoding the a lpha 1 domain) identified only two alleles in five dogs of different b reeds; a third variant was found in a coyote. The divergent structure, relatively low mRNA expression, and limited polymorphism of this gene suggest that DLA-79 is not a classical or class Ia gene, but rather, an analogue of the MHC class Ib genes of humans and rodents.