PSMB7

PSMB7 (β2) is a catalytic subunit of the 20S proteasome core particle that contributes caspase-like (peptidyl-glutamyl peptide-hydrolyzing) proteolytic activity through an N-terminal threonine active-site mechanism (PMID:8811196, PMID:19489727). siRNA-mediated knockdown of PSMB7 in cardiomyocytes selectively abolishes caspase-like activity without affecting chymotrypsin-like or trypsin-like activities, and induces ER stress and autophagy that is exacerbated by anoxia–reoxygenation (PMID:23969338). Upon interferon-γ stimulation, PSMB7 is replaced by the immunoproteasome counterpart PSMB10 (β2i), and cells lacking this replacement accumulate oxidant-damaged protein aggregates and undergo increased apoptosis, indicating that the standard β2-containing proteasome is less efficient than the immunoproteasome at clearing oxidatively damaged substrates (PMID:20723761).

1. 1996 High

   Establishing that PSMB7 resides in the catalytic inner rings of the 20S proteasome and participates in a novel threonine-based proteolytic mechanism resolved the identity of the active-site residue and the multi-activity architecture of the proteasome.

   Evidence Biochemical purification, peptidase activity assays, electron microscopy, and active-site characterization of purified 20S complexes

   PMID:8811196

   Open questions at the time

   • Specific catalytic contribution of the β2 subunit versus β1 and β5 was not individually resolved
   • No knockout or knockdown data for PSMB7 alone
2. 2009 High

   Reconstitution and inhibitor studies clarified that PSMB7 contributes trypsin-like activity and that substrates must be unfolded and translocated by the 19S ATPases before reaching the catalytic chamber, placing PSMB7 function downstream of gated substrate entry.

   Evidence In vitro proteasome reconstitution with active-site inhibitors and structural analysis

   PMID:19489727

   Open questions at the time

   • Relative contribution of β2 to total proteasome flux in intact cells was not quantified
   • Whether β2 activity is rate-limiting for any specific substrate class was unknown
3. 2010 High

   Demonstrating that interferon-induced replacement of PSMB7 by PSMB10 (β2i) is required to prevent accumulation of oxidant-damaged aggregates established a functional distinction between the constitutive and immunoproteasome β2 subunits under oxidative stress.

   Evidence Genetic knockout and RNAi of immunoproteasome subunits, aggresome flow cytometry, viability assays in cell lines and mouse inflammation models

   PMID:20723761

   Open questions at the time

   • Structural basis for the enhanced clearance by β2i versus β2 was not determined
   • Whether PSMB7 itself is actively degraded during the subunit switch or diluted by new assembly was not resolved
4. 2013 Medium

   Selective knockdown of PSMB7 in cardiomyocytes showed it is specifically required for caspase-like proteasome activity and that its loss triggers ER stress and compensatory autophagy, revealing a non-redundant role in cardiac proteostasis.

   Evidence siRNA knockdown in neonatal cardiomyocytes, proteasome peptidase activity assays, RT-qPCR for ER stress markers, LC3 and MDC staining for autophagy

   PMID:23969338

   Open questions at the time

   • Single-lab study; independent replication in additional cardiac or non-cardiac models is lacking
   • Whether the ER stress response is a direct consequence of reduced caspase-like activity or secondary to aggregate accumulation was not dissected
   • In vivo cardiac phenotype of PSMB7 deficiency has not been reported

• The structural basis for PSMB7's selective contribution to caspase-like versus other proteasomal activities, its substrate specificity within the assembled 20S complex, and the in vivo consequences of its loss in adult tissues remain unresolved.
• No conditional knockout phenotype in adult organisms
• No substrate-level proteomics identifying PSMB7-dependent degradation targets
• No structural comparison of β2 versus β2i active-site pockets at atomic resolution