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

Abstract

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 b-subunit rings are synthesized in precursor form. Normally, the b5 (Doa3) propeptide is essential for yeast proteasome biogenesis, but overproduction of b7 (Pre4) bypasses this requirement. Bypass depends on a unique b7 extension, which contacts the opposing b 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.

Involvement of Bag6 and the TRC pathway in proteasome assembly

“…Half-CPs then dimerize on the incorporation of b7 to form mature CPs, accompanied by cleavage of b-subunit propeptides and degradation of UMP1 and PAC1-PAC2 ( Supplementary Fig. S1) 14,26,[31][32][33][34][35][36] .…”

Comparative Oncogenomics Identifies PSMB4 and SHMT2 as Potential Cancer Driver Genes

“…There is precedence that such an approach is feasible, as CRBN has been shown to directly interact with PSMB4, negatively regulating proteasome activity in vivo, possibly through interfering with the assembly of the 20S proteasome (39). Because proteasomal assembly requires the 15-amino acid C-terminal tail of PSMB4 to intercalate into a groove between the PSMB6 and PSMB7 subunits, interfering with this interaction offers a potential therapeutic opportunity (40). Despite PSMB4 being amplified and overexpressed in cancer (a prerequisite for being screened in this project), tumor cell sensitivity to loss of PSMB4 did not correlate with gene copy number or gene expression.…”

The Ubiquitin–Proteasome System of Saccharomyces cerevisiae

“…Interestingly, the propeptide of b5 is essential for this subunit's incorporation into the CP (Chen and Hochstrasser 1996). The b5 propeptide also interacts physically with Ump1 (Heink et al 2005), a chaperone that suppresses halfmer dimerization until the b ring is complete (Li et al 2007c). The b ring is completed with the addition of the b7 subunit (Marques et al 2007).…”

“…The proteolytic sites of the CP are held in an inactive state until the a 7 b 7 b 7 a 7 complex is fully assembled, so that the proteolytic sites are never active unless sequestered from the cytoplasm. This pathway is ordered through the action of five dedicated assembly chaperones (Table 5) (Ramos et al 1998;Le Tallec et al 2007;Li et al 2007c; reviewed by .…”