The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
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Human PEX12 encodes a multi-pass peroxisomal membrane peroxin that is a core subunit of the PEX2–PEX10–PEX12 RING E3 ubiquitin ligase complex required for peroxisomal matrix protein import. Its primary functional role is in the ubiquitination-dependent recycling (retrotranslocation) of the import receptor PEX5, a step essential for sustained import cycles. A major recent advance (2022–2024) is the demonstration that the PEX2–PEX10–PEX12 complex forms a membrane-spanning pore/channel that serves as the retrotranslocation conduit for PEX5, linking ubiquitination to extraction by the PEX1–PEX6 AAA ATPase. Defects in this machinery cause peroxisome biogenesis disorders (PBD) in the Zellweger spectrum, and modern clinical workflows detect PEX-gene ZSD using newborn screening biomarkers (e.g., C26:0-LPC) and confirmatory biochemical and genetic testing. (feng2022structureandfunction pages 1-3, skowyra2024towardssolvingthe pages 21-27, feng2022aperoxisomalubiquitin pages 1-2, beltran2024newbornscreeningfor pages 2-4)
| Aspect | Key Points | Best Supporting Sources | Source Details |
|---|---|---|---|
| Identity & Domains | Human PEX12 is an integral peroxisomal membrane protein containing a conserved C-terminal RING-finger domain. It forms an obligate heterotrimeric complex with PEX2 and PEX10 in the peroxisomal membrane. | (feng2022structureandfunction pages 1-3, feng2022structureandfunction pages 3-5) | Biochem. Soc. Trans. (Nov 2022) https://doi.org/10.1042/bst20221393; Nature (Jun 2022) https://doi.org/10.1038/s41586-022-04903-x |
| Molecular Function | Functions as a subunit of the peroxisomal E3 ubiquitin ligase complex. The complex catalyzes monoubiquitination of the PEX5 import receptor at a conserved cysteine (C11), a mandatory step for PEX5 recycling to the cytosol. It also polyubiquitinates receptors for degradation when recycling fails (RADAR pathway). | (platta2009pex2andpex12 pages 1-2, liu2012recentadvancesin pages 9-10, feng2022structureandfunction pages 1-3, li2021mechanismsandfunctions pages 2-4) | Mol. Cell. Biol. (Oct 2009) https://doi.org/10.1128/mcb.00388-09; Cells (May 2021) https://doi.org/10.3390/cells10051094 |
| Mechanism & Structure | 2022-2024 Breakthrough: Cryo-EM structures reveal the PEX2–PEX10–PEX12 complex forms a conductive membrane pore (retrotranslocation channel). PEX5 inserts its N-terminus into this pore to be ubiquitinated and is subsequently pulled out by the PEX1–PEX6 AAA+ ATPase. | (skowyra2024towardssolvingthe pages 21-27, feng2022aperoxisomalubiquitin pages 1-2, feng2022structureandfunction pages 5-6, skowyra2024towardssolvingthe pages 6-8) | Trends Cell Biol. (May 2024) https://doi.org/10.1016/j.tcb.2023.08.005; Nature (Jun 2022) https://doi.org/10.1038/s41586-022-04903-x |
| Quality Control & Pexophagy | PEX12 and the RING complex regulate peroxisome homeostasis. Accumulation of ubiquitinated PEX5 or other PMPs on the membrane (often due to recycling failure) recruits autophagy adaptors like NBR1/p62, triggering pexophagy (selective peroxisome degradation). | (li2021mechanismsandfunctions pages 2-4, zientararytter2016autophagicdegradationof pages 4-5, li2021mechanismsandfunctions pages 4-5) | Cells (May 2021) https://doi.org/10.3390/cells10051094; Biochem. Soc. Trans. (Apr 2016) https://doi.org/10.1042/bst20150268 |
| Disease & Diagnostics | Defects in PEX12 cause Peroxisome Biogenesis Disorders (Zellweger Spectrum). Recent studies (2024) highlight detection via newborn screening for elevated C26:0-LPC and confirmation with novel serum bile acid (C27) mass spectrometry assays and sequencing. | (beltran2024newbornscreeningfor pages 2-4, beltran2024newbornscreeningfor pages 1-2, zhang2024tandemmassspectrometry pages 1-2, zhang2024tandemmassspectrometry pages 7-8) | Genes (Jun 2024) https://doi.org/10.3390/genes15070838; J. Mass Spectrom. Adv. Clin. Lab (Nov 2024) https://doi.org/10.1016/j.jmsacl.2024.10.005 |
Table: Overview of PEX12 identity, molecular mechanism as a retrotranslocation channel, and clinical relevance in 2023-2024 literature.
Peroxisome biogenesis and matrix protein import require peroxins (PEX proteins). Matrix proteins synthesized in the cytosol are targeted to peroxisomes primarily by PEX5, which binds cargos and delivers them to the peroxisomal membrane; after cargo delivery, PEX5 must be recycled to enable repeated rounds of import. Ubiquitination of membrane-associated PEX5 is a key signal controlling whether PEX5 is recycled (mono-ubiquitination) or routed to degradation (poly-ubiquitination) in quality control pathways. (thoms2006peroxisomalmatrixprotein pages 1-2, liu2012recentadvancesin pages 9-10)
PEX12 is part of a conserved triad of RING-finger peroxins (PEX2, PEX10, PEX12) that form a membrane-embedded E3 ubiquitin ligase complex at the peroxisomal membrane. Reviews and mechanistic studies describe PEX12 as an integral membrane peroxin required for receptor ubiquitination and recycling, and as physically/functional integrated with PEX2 and PEX10 in the “RING subcomplex” of the import machinery. (liu2012recentadvancesin pages 9-10, thoms2006peroxisomalmatrixprotein pages 9-9)
PEX12 is not an enzyme that catalyzes a metabolic reaction (peroxisomal metabolism is carried out by matrix enzymes). Instead, its primary “biochemical activity” is as a membrane E3 ligase subunit in a multi-protein complex that coordinates ubiquitin transfer to receptors and provides a protein-conducting pathway for receptor retrotranslocation. (feng2022structureandfunction pages 1-3, feng2022aperoxisomalubiquitin pages 1-2)
Foundational work established that the RING peroxins function as E3s in the matrix import pathway, with PEX12 implicated in PEX5 monoubiquitination (a recycling signal) and PEX2 implicated in PEX5 polyubiquitination (a degradation/quality-control signal) in yeast models, with conservation to mammals suggested by E2 usage differences in humans. (platta2009pex2andpex12 pages 1-2, liu2012recentadvancesin pages 9-10)
Mechanistically, mono-ubiquitination occurs on a conserved N-terminal cysteine of PEX5 (reviewed as C6 in yeast and as C11 in mammals in later summaries). This modification is essential for recognition/extraction by the PEX1–PEX6 AAA ATPase complex that dislocates PEX5 from the membrane back to the cytosol, after which deubiquitination resets PEX5. (liu2012recentadvancesin pages 9-10, li2021mechanismsandfunctions pages 2-4)
A central recent advance is that the PEX2–PEX10–PEX12 ligase complex is not merely a peripheral E3 ligase but forms the retrotranslocation channel for receptor recycling.
Structure/function insight (cryo-EM): Each subunit (PEX2/PEX10/PEX12) contributes five transmembrane segments that assemble into an open pore (~10 Å), while the RING fingers form a cytosolic tower positioned above the pore. This provides a physical basis for how a luminal/matrix-exposed segment of PEX5 can be moved back across the membrane to the cytosol. (feng2022structureandfunction pages 3-5)
Proposed sequence: The N-terminus of PEX5 inserts into the pore from the lumenal side and is monoubiquitinated by a RING finger positioned above the pore; extraction is then driven by the PEX1–PEX6 AAA ATPase. If recycling is compromised, receptors are polyubiquitinated by the other RING fingers and degraded (RADAR-like quality control). (feng2022structureandfunction pages 1-3, feng2022aperoxisomalubiquitin pages 1-2)
A 2024 synthesis highlights a retrotranslocation intermediate in which PEX5 adopts a transmembrane orientation with ~20–30 N-terminal residues exposed to the cytosol, and this intermediate is enriched when the conserved cysteine required for recycling is mutated; immunoprecipitation identifies the PEX2–PEX10–PEX12 complex as major interactors, supporting its assignment as the retrotranslocon. (skowyra2024towardssolvingthe pages 6-8)
A 2024 Trends in Cell Biology review integrates structural and biochemical work into a model in which PEX5 can fully enter the peroxisomal lumen/matrix and then be returned to the cytosol through the PEX2–PEX10–PEX12 pore, with ubiquitination at a conserved cysteine licensing extraction by PEX1–PEX6. This consolidates PEX12’s role as essential for sustained matrix import through receptor recycling. (skowyra2024towardssolvingthe pages 21-27)
Publication details: Skowyra ML, Feng P, Rapoport TA. Trends in Cell Biology (May 2024). URL: https://doi.org/10.1016/j.tcb.2023.08.005 (skowyra2024towardssolvingthe pages 21-27)
PEX5 ubiquitination is increasingly viewed as part of a broader “organelle quality control” logic where persistent ubiquitin signals on peroxisomal membranes can recruit ubiquitin-binding autophagy adaptors (e.g., NBR1/p62) to trigger pexophagy. While some pathways emphasize PEX2 as a key E3 in starvation-induced pexophagy, the core import-associated RING complex (including PEX12) is mechanistically linked to the ubiquitination states that can generate pexophagy signals when recycling is blocked. (li2021mechanismsandfunctions pages 4-5, wang2017roleofpex5 pages 5-6)
The cryo-EM structure and pore model of the Pex2–Pex10–Pex12 complex—foundational to the “channel” concept—are shown in figure panels depicting the transmembrane pore and cytosolic RING tower. These visuals directly support mechanistic claims about pore formation and RING-domain positioning. (feng2022aperoxisomalubiquitin media d4311c14, feng2022aperoxisomalubiquitin media 27c6b8ba)
Publication details: Feng P et al. Nature (June 2022). URL: https://doi.org/10.1038/s41586-022-04903-x (feng2022aperoxisomalubiquitin pages 1-2)
Although newborn screening programs are typically designed for X-linked adrenoleukodystrophy (X-ALD), they can incidentally detect Zellweger spectrum disorder (ZSD) caused by biallelic PEX gene variants (potentially including PEX12) because both can elevate C26:0-lysophosphatidylcholine (C26:0-LPC).
A 2024 California case series (2016–2022) describes a two-tier approach (tier 1 FIA-MS/MS; tier 2 LC-MS/MS) for C26:0-LPC, followed by ABCD1 sequencing; among nine infants ultimately diagnosed with non–X-ALD conditions, 7/9 had ZSD due to biallelic PEX variants, confirmed by targeted ZSD gene panels or whole-exome sequencing and confirmatory biochemical markers. (beltran2024newbornscreeningfor pages 1-2)
Publication details: Beltran CFM et al. Genes (June 2024). URL: https://doi.org/10.3390/genes15070838 (beltran2024newbornscreeningfor pages 1-2)
A 2024 clinical-lab mass spectrometry study reports a serum assay for C27 cholestanoic bile acids (DHCA/THCA and conjugates) with clear separation between controls and peroxisomal-disorder patients.
Publication details: Zhang W et al. Journal of Mass Spectrometry and Advances in the Clinical Lab (Nov 2024). URL: https://doi.org/10.1016/j.jmsacl.2024.10.005 (zhang2024tandemmassspectrometry pages 1-2)
In the California NBS ZSD series, 4 infants received cholic acid therapy initiated between 2–8 months, illustrating that earlier molecular diagnosis via screening can accelerate initiation of supportive/interventional management (though responses were mixed and genotype-specific conclusions were limited). (beltran2024newbornscreeningfor pages 2-4)
Recent expert reviews interpret the PEX2–PEX10–PEX12 complex as a peroxisome analog of retrotranslocation/ubiquitin systems (conceptually similar to ERAD, but adapted for peroxisomal receptor cycling). In this view, ubiquitination is not merely a tag but is physically and temporally coupled to receptor movement through a dedicated pore formed by the ligase complex. (feng2022structureandfunction pages 5-6, skowyra2024towardssolvingthe pages 21-27)
Earlier mechanistic assignments implicated PEX12 in monoubiquitination (recycling) and PEX2 in polyubiquitination (degradation) in yeast, whereas structural/review syntheses emphasize that RF2 (PEX2) is positioned above the pore and may catalyze monoubiquitination in the channel model; the conceptual consensus is that the complex is required as a whole, while catalytic contributions may be organism- and context-dependent and remain an active area of refinement. (platta2009pex2andpex12 pages 1-2, feng2022structureandfunction pages 3-5)
In the 2024 California study of nine infants with elevated C26:0-LPC who were not X-ALD:
These values quantify the screening signal strength and outcomes for infants in whom downstream workup identified ZSD due to biallelic PEX variants. (beltran2024newbornscreeningfor pages 2-4)
The 2024 serum C27 assay reports linear quantification range 20–2,500 ng/mL and intra-/inter-assay imprecision <20% CV, supporting feasibility for clinical lab workflows, and provides disease vs control distributions (above). (zhang2024tandemmassspectrometry pages 1-2)
PEX12 is a peroxisomal membrane component of the import machinery, functioning as part of the membrane-embedded ubiquitin ligase/retrotranslocon that acts at the cytosolic face (RING tower) and across the membrane (pore). (feng2022structureandfunction pages 3-5, feng2022aperoxisomalubiquitin pages 1-2)
PEX12 is positioned at the critical interface between:
This coupling provides a mechanistic explanation for why loss-of-function in PEX12 disrupts peroxisomal matrix protein import globally. (liu2012recentadvancesin pages 9-10, skowyra2024towardssolvingthe pages 21-27)
Mutations in components of the PEX2–PEX10–PEX12 ligase/channel cause peroxisome biogenesis disorders; structural reviews explicitly connect disease-causing point mutations to transmembrane segments and RING domains, linking protein architecture to pathology. (feng2022structureandfunction pages 3-5, feng2022aperoxisomalubiquitin pages 1-2)
Recent clinical implementation papers (newborn screening and biomarker assays) support a modern diagnostic pipeline for PEX-gene ZSD that would include PEX12 as a candidate gene in sequencing panels/WES follow-up and biochemical confirmation. (beltran2024newbornscreeningfor pages 1-2, zhang2024tandemmassspectrometry pages 1-2)
The most detailed mechanistic and structural sources focus on the PEX2–PEX10–PEX12 complex rather than isolating PEX12-only functions, and recent (2023–2024) human-PEX12-specific patient variant series were not directly retrieved in the available full texts. Thus, ZSD statistics are presented for PEX-gene ZSD broadly, with PEX12 placed mechanistically in the causal pathway. (beltran2024newbornscreeningfor pages 2-4, feng2022structureandfunction pages 3-5)
References
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