Properties of the Beta Subunit of the Proteasome Activator PA28 (11S REG)

The proteasome activator PA28 (11S REG) is composed of two homologous subunits termed α and β. The properties of the recombinant β-subunit were explored and compared to the properties of the recombinant α-subunit. PA28β produced in an Escherichia coli expression system migrates on a calibrated gel filtration column as an apparent heptamer (Mr = 250,000). Low concentrations of SDS (0.005%), dissociate the protein to a monomer (Mr = 33,000). PA28β has a complex effect on proteasome activity. At concentrations which favor oligomerization (> 2 μM), PA28β is a strong proteasome activator although its affinity for the proteasome is about 10-fold less than recombinant PA28α. The catalytic properties of the PA28α and PA28β-activated proteasome are similar. At low concentrations, PA28β is a monomer and a potent allosteric proteasome inhibitor. These studies show that oligomerization of PA28β is required for proteasome activation and that PA28β monomers are potent proteasome inhibitors.