Proteasome activator 11S REG or PA28: recombinant REG alpha/REG beta hetero-oligomers are heptamers.

The proteasome activator 11S REG or PA28 is a conical molecule composed of two homologous subunits, REGα and REGβ. Recombinant REGα forms a heptamer, whereas recombinant REGβ is a monomer. When mixed with REGβ, a monomeric REGα mutant (N50Y) forms an active hetero-oligomer in which the molar ratio of REGβ to REGα(N50Y) is close to 1.3. This apparent stoichiometry is consistent with the REGα(N50Y)/REGβ hetero-oligomer being a heptamer composed of three α and four β subunits. Chemical cross-linking of the α/β oligomers revealed the presence of REGα−REGβ and REGβ−REGβ dimers, but REGα−REGα dimers were not detected. The mass of the REGα(N50Y)/REGβ hetero-oligomer determined by electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) is 194 871 ± 40 Da in good agreement with the theoretical mass of 194 856 Da for an α3β4 heptamer. Hexamers were not observed in the mass spectrum. For wild-type REG subunits coexpressed in bacteria cells at an apparent β/α molar ratio of ∼1.2, the resulting hetero-ol...