PA28 is an activator of the latent 20S proteasome, a large multisubunit complex involved in intracellular proteolysis. Two forms of hexameric PA28 have been identified, PA28-(αβ)3 and PA28-(γ)6, of which the former is of immunological importance. Both the PA28-α and PA28-β subunits are inducible by interferon-γ (IFN-γ) and the PA28-(αβ)3 complex enhances the ability of the 20S proteasome to produce peptides suited for binding to major histocompatibility complex (Mhc) class I molecules. To identify the homologues of the PA28 subunits in zebrafish we screened a cDNA library and obtained full-length cDNA sequences of the genes PSME1, PSME2 and PSME3 coding for the PA28-α, PA28-β and PA28-γ subunits, respectively. Phylogenetic analysis indicates the existence of the ancestors of all three genes prior to the divergence of tetrapods and bony fishes. The IFN-γ- inducible subunits, PA28-α and PA28-β, evolve faster than the presumably older PA28-γ subunit. Using zebrafish radiation hybrid panels, the genes PSME2 and PSME3 were mapped to linkage group 12 and shown to be separated by a distance of less than 2.4 cm. This observation suggests that an intrachromosomal duplication event created the precursor of the IFN-γ- inducible genes from a PA28-γ-like ancestor prior to their recruitment into the Mhc class I peptide presentation pathway.
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