β-Subunit appendages promote 20S proteasome assembly by overcoming an Ump1-dependent checkpoint

Proteasomes are responsible for most intracellular protein degradation in eukaryotes. The 20S proteasome comprises a dyad-symmetric stack of four heptameric rings made from 14 distinct subunits. How it assembles is not understood. Most subunits in the central pair of β-subunit rings are synthesized in precursor form. Normally, the β5 (Doa3) propeptide is essential for yeast proteasome biogenesis, but overproduction of β7 (Pre4) bypasses this requirement. Bypass depends on a unique β7 extension, which contacts the opposing β ring. The resulting proteasomes appear normal but assemble inefficiently, facilitating identification of assembly intermediates. Assembly occurs stepwise into precursor dimers, and intermediates contain the Ump1 assembly factor and a novel complex, Pba1-Pba2. β7 incorporation occurs late and is closely linked to the association of two half-proteasomes. We propose that dimerization is normally driven by the β5 propeptide, an intramolecular chaperone, but β7 addition overcomes an Ump1-dependent assembly checkpoint and stabilizes the precursor dimer.