PEX12

PEX12 is an integral peroxisomal membrane protein essential for peroxisomal matrix protein import, and its loss is causative for the complementation group 3 form of peroxisome biogenesis disorder, with import restored by complementation in patient fibroblasts (PMID:9090384). The protein spans the membrane twice and exposes both its N- and C-terminal cytoplasmic regions—each required for function—with the C-terminus carrying a RING (zinc finger) motif (PMID:9632816). Through this RING domain PEX12 directly binds the PTS1 import receptor PEX5 and the partner membrane protein PEX10, and a patient S320F mutation in the zinc-binding domain weakens both interactions; because loss of PEX12 does not impair PEX5 docking, PEX12 acts downstream of receptor docking (PMID:10562279). Mechanistically, PEX12 functions as a RING-domain E3 ubiquitin ligase that, together with PEX2 and PEX10, drives PEX4 (Ubc10)-dependent monoubiquitination of PEX5 to target the receptor for recycling rather than proteasomal degradation (PMID:19687296). This activity is confined to the recycling step rather than cargo recognition, as the PEX12 RING domain does not alter PEX5 binding to PTS1 peptide (PMID:12456682).

1. 1997 High

   Established that PEX12 is a peroxisomal membrane protein required for matrix protein import and the gene defective in a defined human disease group, answering whether PEX12 is essential for import at all.

   Evidence Functional complementation of PBD complementation group 3 patient fibroblasts with localization by fractionation/immunofluorescence and patient mutation detection

   PMID:9090384

   Open questions at the time

   • Did not define membrane topology
   • Did not assign a biochemical activity to the protein
2. 1998 High

   Defined PEX12 topology, showing two transmembrane segments with both termini cytosolic and a C-terminal RING motif, establishing where its functional domains face and act.

   Evidence Topology mapping via epitope-tagged and truncation mutants with GFP-based peroxisome restoration in CHO mutant cells, plus patient mutation analysis

   PMID:9632816

   Open questions at the time

   • Did not establish the molecular activity of the RING domain
   • Binding partners not identified
3. 1999 High

   Identified PEX5 and PEX10 as direct RING-domain binding partners and placed PEX12 downstream of receptor docking, clarifying its position in the import pathway.

   Evidence Two-hybrid, blot overlay, co-immunoprecipitation, patient S320F mutation analysis, and genetic suppression by PEX5/PEX10 overexpression

   PMID:10562279

   Open questions at the time

   • Did not demonstrate enzymatic activity
   • Functional consequence of the interactions not resolved
4. 2002 Medium

   Showed the PEX12 RING domain does not affect PEX5–PTS1 binding kinetics, excluding a role in initial cargo recognition and localizing PEX12 function to a later step.

   Evidence Fluorescence anisotropy in vitro binding assay with purified recombinant PEX5 and PEX12 RING domain

   PMID:12456682

   Open questions at the time

   • Single lab and single method
   • Negative result; positive function not assigned here
5. 2009 High

   Established PEX12 as a RING E3 ubiquitin ligase driving PEX4/Ubc10-dependent monoubiquitination of PEX5 for receptor recycling, defining its catalytic role in import.

   Evidence In vitro ubiquitin ligase assays and genetic/biochemical dissection of E2 specificity in yeast

   PMID:19687296

   Open questions at the time

   • Precise PEX5 ubiquitination site/structural basis not resolved
   • Stoichiometry within the E3 complex not defined
6. 2025 Medium

   Placed PEX12 within a PEX2/PEX10/PEX12 E3 complex that engages the Pex5/Pex8 module at the translocation-to-recycling transition, structurally framing the assembly that initiates recycling.

   Evidence Cryo-EM/X-ray structure of Pex8–Pex5, yeast translocation assays, and mutagenesis disrupting Pex8–Pex5 (preprint)

   PMID:bio_10.1101_2025.08.30.673231

   Open questions at the time

   • PEX12-specific placement is contextual within a broader study
   • Not independently replicated

• How the PEX2/PEX10/PEX12 E3 complex is structurally organized in the membrane and how it coordinates E2 charging, substrate positioning, and the retrotranslocation machinery remains unresolved.
• No high-resolution structure of the intact human E3 complex in the timeline
• Mechanism linking monoubiquitination to receptor extraction unresolved