Molecular characterization of major histocompatibility complex class I genes from the giant panda (Ailuropoda melanoleuca).

The major histocompatibility complex class I genes play crucial roles in the adaptive immune system of vertebrates against intracellular pathogens. To date, no class I genes from the giant panda (Ailuropoda melanoleuca) has been reported, even none from species of Ursidae. In this study, we successfully identified three class I genes from a giant panda bacterial artificial chromosome library and designated them as Aime-128, 152, and 1906, respectively. Pairwise sequence alignments revealed that (1) the Aime-1906 always possessed the lowest identities (52-86%) in different regions compared with the Aime-128 and 152 and (2) the Aime-128 also varied from the Aime-152 in the regions of 5' untranslated region (UTR), 3' UTR, and exon1, whose similarities were 83%, 87%, and 91%, respectively. Comparison of structure characteristics indicated that the Aime-128 possessed all conserved amino acids important to the function of antigen presentation while the Aime-152 and 1906 presented two and five mutated residues. Analysis of phylogenetic trees demonstrated that the Aime-128, 152, and 1906 were clustered into three different branches with 99% or 100% bootstrap values. As a result, these three kinds of evidence supported that the Aime-1906, 152, and 128 should be derived from different loci. Furthermore, in view of a prestop codon in the exon 7 and patterns of amino acid replacement within alleles, the Aime-1906 gene is predicted to be a nonclassical locus, which is most closely related to dog leukocyte antigen 79 in the phylogenetic tree constructed with various mammalian class I loci.