EVOLUTION OF PROTEASOME SUBUNIT-DELTA AND SUBUNIT-LMP2 - COMPLEMENTARY-DNA CLONING AND LINKAGE ANALYSIS WITH MHC IN LOWER-VERTEBRATES

The class II region of the mammalian MHC harbors two proteasome subuni t genes, LMP2 and LMP7. These genes are induced by IFN-gamma, and thei r products are incorporated into proteasomes substituting for their cl osest relatives, the delta and X subunits, respectively, This substitu tion is believed to change the proteolytic specificity of proteasomes, making it more suitable for generation of peptides to be presented by class I molecules, To elucidate the phylogenetic origin of LMP2 and t he linkage of its gene with the MHC, reverse transcriptase-PCR amplifi cation of Xenopus laevis and lamprey liver mRNA was performed with pri mers designed to amplify both the mammalian LMP2 and delta sequences. Both LMP2 and delta were amplified from X. laevis, whereas only delta was amplified from lamprey, suggesting that delta/LMP2 gene duplicatio n occurred after divergence of cyclostomes but before divergence of am phibians, The linkage between the LMP2 gene and the MHC was observed i n a diploid Xenopus species, Xenopus tropicalis, but not in a tetraplo id species, X. laevis, indicating that this linkage was established be fore the divergence of amphibian from higher vertebrates, but that thi s linkage was lost in X. laevis, probably by a gene reorganization acc ompanying the tetraploidization, The X. laevis LMP2 and LMP7 mRNA show ed a similar tissue distribution, indicating that the genetic linkage is not required for apparently coordinated tissue-specific expression of these genes, Sequence and linkage analyses suggest that LMP2 may no t play as vital a role as LMP7 in Ag presentation.