Journal
EMBO JOURNAL
Volume 26, Issue 9, Pages 2339-2349Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/sj.emboj.7601681
Keywords
proteasome; ubiquitin
Categories
Funding
- NIGMS NIH HHS [R01 GM053756, GM053756, GM046904, R01 GM046904, R37 GM046904] Funding Source: Medline
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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 beta-subunit rings are synthesized in precursor form. Normally, the beta 5 ( Doa3) propeptide is essential for yeast proteasome biogenesis, but overproduction of beta 7 ( Pre4) bypasses this requirement. Bypass depends on a unique b7 extension, which contacts the opposing beta 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. b7 incorporation occurs late and is closely linked to the association of two half-proteasomes. We propose that dimerization is normally driven by the b5 propeptide, an intramolecular chaperone, but b7 addition overcomes an Ump1-dependent assembly checkpoint and stabilizes the precursor dimer.
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