title: The proteasome activator 11 S REG or PA28: chimeras implicate carboxyl-terminal sequences in oligomerization and proteasome binding but not in the activation of specific proteasome catalytic subunits.
abstract: The REG homologs, alpha, beta and gamma, activate mammalian proteasomes in distinct ways. REGalpha and REGbeta activate the trypsin-like, chymotrypsin-like and peptidylglutamyl-preferring active sites, whereas REGgamma only activates the proteasome's trypsin-like subunit. The three REG homologs differ in carboxyl-terminal sequences that are located next to activation loops on their proteasome binding surface. To assess the importance of these carboxyl-terminal sequences in the activation of specific proteasome beta catalytic subunits, we characterized chimeras in which 8 or 12 residues were exchanged among the three proteins. Like the wild-type molecule, REGalpha chimeras activated all three proteasome catalytic subunits regardless of the carboxyl-terminal sequence. However, REGalpha-beta chimeras activated the proteasome at lower concentrations than wild-type REGalpha and higher levels of REGalpha-gamma chimeras were needed for maximal activation because exchanged carboxyl-terminal sequences can stabilize (REGalpha-beta) or destabilize (REGalpha-gamma) the REGalpha heptamer. REGgamma chimeras were equivalent to REGgamma in their activation properties, but they bound the proteasome less tightly than the wild-type molecule. REGbeta chimeras also bound the proteasome more weakly than wild-type REGbeta and were virtually unable to activate it. Our findings demonstrate that the carboxyl-terminal sequences of REG subunits can affect heptamer stability and proteasome affinity, but they do not determine which proteasome beta subunits become activated.
authors.affiliation: Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT, 84132, USA.
authors.forename: J
authors.initials: J
authors.lastname: Li
authors.forename: X
authors.initials: X
authors.lastname: Gao
authors.forename: L
authors.initials: L
authors.lastname: Joss
authors.forename: M
authors.initials: M
authors.lastname: Rechsteiner
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D001324:Autoantigens; D001667:Binding, Competitive; D020134:Catalytic Domain; D003546:Cysteine Endopeptidases / Q000737:chemistry* / Q000378:metabolism*; D004789:Enzyme Activation; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000378:metabolism*; D009124:Muscle Proteins; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D011506:Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D013379:Substrate Specificity; D013816:Thermodynamics; D014461:Ultracentrifugation
chemical_list: D001324:Autoantigens; C099558:Ki antigen; D009097:Multienzyme Complexes; D009124:Muscle Proteins; C085047:PSME1 protein, human; D011506:Proteins; D011993:Recombinant Fusion Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex