Properties of the beta subunit of the proteasome activator PA28 (11S REG)

The proteasome activator PA28 (11S REG) is composed of two homologous subun its termed cu and beta. The properties of the recombinant beta -subunit wer e explored and compared to the properties of the recombinant alpha -subunit . PA28 beta produced in an Escherichia coli expression system migrates on a calibrated gel filtration column as an apparent heptamer (M-r = 250,000). Low concentrations of SDS (0.005%), dissociate the protein to a monomer (M- r = 33,000). PA28 beta has a complex effect on proteasome activity. At conc entrations which favor oligomerization (> 2 muM), PA28 beta is a strong pro teasome activator although its affinity for the proteasome is about 10-fold less than recombinant PA28 alpha. The catalytic properties of the PA28 alp ha and PA28 beta -activated proteasome are similar. At low concentrations, PA28 beta is a monomer and a potent allosteric proteasome inhibitor. These studies show that oligomerization of PA28 beta is required for proteasome a ctivation and that PA28 beta monomers are potent proteasome inhibitors. (C) 2000 Academic Press.