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  • 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.
    Author: Li J, Gao X, Joss L, Rechsteiner M.
    Journal: J Mol Biol; 2000 Jun 09; 299(3):641-54. PubMed ID: 10835274.
    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.
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