PSMC1

PSMC1 (RPT2/Rpt2) is an AAA+ ATPase subunit of the 19S regulatory particle of the 26S proteasome and is required for proper proteasome assembly and ubiquitin-dependent protein turnover (PMID:22677101). N-myristoylation of the Rpt2 subunit governs nucleo-cytoplasmic localization of the 26S proteasome, directing compartment-specific protein quality control; loss of myristoylation does not impair assembly or peptidase activity but causes nuclear proteasomes to disperse into cytoplasmic aggregates, leading to accumulation of polyubiquitinated nucleo-cytoplasmic proteins and altered Hsp70 chaperone levels (PMID:23102099, PMID:26344132). Beyond its core proteasomal role, PSMC1 physically interacts with the dual ubiquitin/SUMO1 E3 ligase TOPORS and co-localizes with it at the centrosome and at the RPE/photoreceptor outer segment interface (PMID:26872363). The ATP-binding/hydrolysis domain is essential: a homozygous p.I328T variant disrupting this domain's hydrophobic core only partially rescues a Drosophila Rpt2-silencing neurodegeneration phenotype, and causes a recessive neurological syndrome (PMID:35861243). Genetic loss of PSMC1 disrupts 26S proteasome assembly, triggers G2/M cell cycle arrest in dividing cells, and causes neuronal degeneration, with dopaminergic neurons being particularly dependent on this subunit for survival (PMID:22677101, PMID:32715147).

1. 2012 High

   Established that PSMC1/Rpt2 myristoylation controls where the proteasome resides rather than whether it functions, separating localization control from catalytic assembly.

   Evidence Site-directed mutagenesis of the N-myristoylation site with fluorescence localization, peptidase activity assays, and polyubiquitin accumulation readouts in S. cerevisiae

   PMID:23102099

   Open questions at the time

   • Demonstrated in yeast ortholog; mammalian PSMC1 myristoylation not directly tested here
   • Mechanism by which myristoylation drives nuclear import not resolved
2. 2012 High

   Showed that PSMC1 is essential for 26S proteasome assembly in vivo and that its loss produces distinct phenotypes — neurodegeneration in neurons and G2/M arrest in dividing cells.

   Evidence Conditional homozygous and heterozygous Psmc1 knockout in mouse brain with biochemical assembly analysis, cell cycle analysis of primary MEFs, and polyubiquitin immunohistochemistry

   PMID:22677101

   Open questions at the time

   • Molecular basis of the G2/M arrest not defined
   • Why neurons are selectively vulnerable not established
3. 2015 High

   Extended the localization mechanism to a system-level consequence: myristoylation-dependent proteasome compartmentalization shapes the ubiquitinated proteome of nucleo-cytoplasmic substrates and chaperone homeostasis.

   Evidence Quantitative K-ε-GG ubiquitinome mass spectrometry comparing myristoylation mutant versus wild-type yeast, with chaperone protein quantification

   PMID:26344132

   Open questions at the time

   • Performed in yeast ortholog
   • Direct link between specific substrates and physiological outcome not drawn
4. 2016 Medium

   Identified a centrosome-associated, proteasome-independent context by showing PSMC1 binds the dual E3 ligase TOPORS, implicating it in localized ubiquitin/SUMO signaling.

   Evidence Yeast two-hybrid screen of a human retinal cDNA library, co-immunoprecipitation from mammalian cells, and immunofluorescence co-localization in cell lines and mouse retina

   PMID:26872363

   Open questions at the time

   • Functional consequence of the PSMC1-TOPORS interaction not determined
   • Single lab; not independently replicated
   • Whether interaction occurs within or outside the proteasome context unclear
5. 2020 Medium

   Defined a cell-type-specific requirement, showing dopaminergic neurons depend on PSMC1/Rpt2 for survival, behavior, and proteostasis.

   Evidence Gal4/UAS RNAi knockdown of Rpt2 in Drosophila CNS and dopaminergic neurons with proteasome activity assays, insoluble ubiquitin aggregate measurement, behavioral and survival assays

   PMID:32715147

   Open questions at the time

   • Performed in Drosophila ortholog via knockdown, not human PSMC1 loss
   • Single lab
   • Selective dopaminergic vulnerability mechanism not explained
6. 2022 Medium

   Connected PSMC1 dysfunction to human disease and pinpointed the ATP-binding/hydrolysis domain as functionally critical via a hypomorphic variant.

   Evidence Homozygosity mapping and exome sequencing in a recessive neurological syndrome, structural modeling, and Drosophila Rpt2 RNAi rescue with human wild-type versus p.I328T PSMC1

   PMID:35861243

   Open questions at the time

   • Single family/lab; genotype-phenotype breadth unknown
   • Direct ATPase activity measurement of the variant not performed
   • How partial activity loss produces the specific syndrome unresolved

• How myristoylation-dependent compartmentalization, the TOPORS/centrosome interaction, and the selective neuronal (especially dopaminergic) vulnerability mechanistically converge remains unresolved.
• No mammalian validation that PSMC1 myristoylation controls human proteasome localization
• Functional role of the PSMC1-TOPORS centrosomal interaction undefined
• Mechanistic link between proteasome subunit loss and dopaminergic-neuron specificity unestablished