Trans-species polymorphism of the major histocompatibility complex-encoded proteasome subunit LMP7 in an amphibian genus, Xenopus.

LMP7 (PSMB8) is a major histocompatibility complex (MHC)-encoded catalytic subunit of 20S immunoproteasome, which is responsible for the production of antigenic peptide to be presented by the MHC class I molecules. Two highly diverged allelic lineages of LMP7, termed LMP7A and LMP7B, have been identified previously in an amphibian, Xenopus laevis. Fourteen Xenopus species were analyzed by genomic Southern hybridization using LMP7A- and LMP7B-specific probes. Ten had both LMP7A and LMP7B, and the other 4 had only LMP7A. Identification of LMP7A and LMP7B was confirmed by reverse transcription-polymerase chain reaction/sequencing analysis of LMP7 mRNA including eight diagnostic amino acid residues that discriminate the two allelic lineages. These data suggest that these two allelic lineages were established more than 80 million years ago, and were transmitted from species to species. Trans-species evolution has so far been reported for MHC class I and II molecules in mammals and teleost fish, and is believed to be a basis for the extraordinary polymorphism of these molecules. A similar mode of evolution of the LMP7 alleles in Xenopus provides a possible explanation for the linkage of the LMP7 gene with the MHC in all vertebrates analyzed to date.