Identification and linkage of the proteasome activator complex PA28 subunit genes in zebrafish

PA28 is an activator of the latent 20S proteasome, a large multisubunit com plex involved in intracellular proteolysis. Two forms of hexameric PA28 hav e been identified, PA28-(alpha beta)(3) and PA28-(gamma)(6,) of which the f ormer is of immunological importance. Both the PA28-alpha and PA28-beta sub units are inducible by interferon-gamma (IFN-gamma) and the PA28-(alpha bet a)(3) complex enhances the ability of the 20S proteasome to produce peptide s suited for binding to major histocompatibility complex (Mhc) class I mole cules. To identify the homologues of the PA28 subunits in zebrafish we scre ened a cDNA library and obtained full-length cDNA sequences of the genes PS ME1, PSME2 and PSME3 coding for the PA28-alpha, PA28-beta and PA28-gamma su bunits, respectively. Phylogenetic analysis indicates the existence of the ancestors of all three genes prior to the divergence of tetrapods and bony fishes. The IFN-gamma-inducible subunits, PA28-alpha and PA28-beta, evolve faster than the presumably older PA28-gamma subunit. Using zebrafish radiat ion hybrid panels, the genes PSME2 and PSME3 were mapped to linkage group 1 2 and shown to be separated by a distance of less than 2.4 cm. This observa tion suggests that an intrachromosomal duplication event created the precur sor of the IFN-gamma-inducible genes from a PA28-gamma-like ancestor prior to their recruitment into the Mhc class I peptide presentation pathway.