Proteasome activation by REG molecules lacking homolog-specific inserts

The peptidase activities of eukaryotic proteasomes are markedly activated by the 11 S REG or PA28. The three identified REG subunits, designated α, β, and γ, differ significantly in sequence over a short span of 15-30 amino acids that we call homolog-specific inserts. These inserts were deleted from each REG to produce the mutant proteins REGαΔi, REGβΔi, and REGγΔi. The purified recombinant proteins were then tested for their ability to oligomerize and activate the proteasome. Both REGαΔi and REGγΔi formed apparent heptamers and activated human red cell proteasomes to the same extent as their full-length counterparts. By contrast, REGβΔi exhibited, at low protein concentrations, reduced proteasome activation when compared with the wild-type REGβ protein. REGβΔi was able to form hetero- oligomers with a single site, monomeric REGα mutant and with REGαΔi. At low concentrations, the REGαΔi/REGβΔi hetero-oligomers stimulated the proteasome less than REGα/REGβ oligomers formed from wild-type subunits, and the reduced activation by REGαΔi/REGβΔi was due to removal of the REGβ insert, not the REGα insert. These studies demonstrate that the REGα and REGγ inserts play virtually no role in oligomerization or in proteasome activation. By contrast, removal of REGβ insert reduces binding of this subunit and REGα/REGβ oligomers to proteasomes. On the whole, however, our findings show that REG inserts are not required for binding and activating the proteasome. We speculate that they serve to localize REG-proteasome complexes within cells, possibly by binding components in endoplasmic reticulum membranes.