Trans-species polymorphism of the major histocompatibility complex-encodedproteasome 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 ident ified previously in an amphibian, Xenopus laevis. Fourteen Xenopus species were analyzed by ge nomic Southern hybridization using LMP7A- and LMP7B-spe cific probes. Ten had both LMP7A and LMP7B, and the other 4 had only LMP7A. Identification of LMP7A and LMP7B was confirmed by reverse transcription-p olymerase chain reaction/sequencing analysis of LMP7 mRNA including eight d iagnostic amino acid residues that discriminate the two allelic lineages. T hese data suggest that these two allelic lineages were established more tha n 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 molecule s in mammals and teleost fish, and is believed to be a basis for the extrao rdinary 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.