PXMP4

PXMP4 (PMP24) is a peroxisomal membrane protein that functions both as a metabolic contributor to peroxisomal lipid processing and as a regulatory node in selective peroxisome autophagy (PMID:41267209, PMID:35169201). In pexophagy, PXMP4 serves as a ubiquitination substrate within a defined stress-responsive cascade: upon PEX1 depletion and ROS accumulation, TBK1 is phosphorylated at serine 172 and activates the E3 ubiquitin ligase MARCHF7, which binds PXMP4 and ubiquitinates it at lysine 20; the ubiquitin mark recruits the autophagy receptor NBR1 to peroxisomes to drive their selective degradation (PMID:41267209). Loss of Pxmp4 in mice is compatible with normal peroxisome number and morphology but reduces hepatic alkyldiacylglycerol ether lipids and elevates phytanic/pristanic acid levels, indicating a role in ether lipid metabolism and peroxisomal α-oxidation capacity (PMID:35169201). PXMP4 is a binding partner of PEX19, the chaperone for peroxisomal membrane protein insertion, placing it within the PEX19-dependent membrane assembly machinery (PMID:39205283). In cancer contexts, PXMP4 expression is silenced by methylation of a single intronic CpG dinucleotide that disrupts DNA-protein binding at an intron 1 hypersensitive site (PMID:20054818), and restored PXMP4 expression suppresses prostate cancer cell growth and anchorage-independent colony formation (PMID:14712230); in gastric cancer cells it modulates proliferation, invasion, and EMT upstream of PI3K/AKT signaling (PMID:38401002).

1. 2004 Medium

   Established that PXMP4 has anti-proliferative activity, addressing whether the gene product had any functional consequence when re-expressed in cells that had silenced it.

   Evidence Transient transfection of PMP24 into prostate cancer cells lacking endogenous expression, with growth and soft-agar colony assays

   PMID:14712230

   Open questions at the time

   • No molecular mechanism for the growth suppression identified
   • Overexpression in two cell lines only, single lab
   • Does not link the phenotype to peroxisomal function
2. 2008 Low

   Flagged PXMP4 as a differentially expressed candidate within an NKT-cell-linked locus, raising the question of whether peroxisomal lipid handling influences glycolipid antigen presentation.

   Evidence Microarray expression analysis of a NOD congenic mouse strain

   PMID:18714012

   Open questions at the time

   • Expression correlation only, no functional test
   • PEX19 binding partner cited as prior knowledge, not demonstrated here
   • No causal link between PXMP4 and NKT phenotype
3. 2010 Medium

   Defined the mechanism of PXMP4 silencing in cancer, answering how loss of expression is established at the DNA level.

   Evidence Site-specific methylated-oligonucleotide transfection, gel shift, bisulfite sequencing in prostate tissue

   PMID:20054818

   Open questions at the time

   • Identity of the DNA-binding factor displaced by methylation not determined
   • Single intronic CpG mechanism not validated in vivo across tumors
   • Does not address PXMP4 protein function
4. 2016 Low

   Proposed by sequence homology that PXMP4 belongs to the Tim17 translocase family, framing a hypothesis that it acts as a peroxisomal membrane channel/transporter.

   Evidence Comprehensive phylogenetic analysis across 5631 proteomes

   PMID:27760563

   Open questions at the time

   • No experimental transport activity demonstrated for PXMP4
   • Substrate(s) unknown
   • Family assignment computational only
5. 2022 High

   Determined the physiological consequence of PXMP4 loss in vivo, resolving whether the protein is required for peroxisome integrity versus specific metabolic functions.

   Evidence CRISPR/Cas9 Pxmp4 knockout mouse with lipidomics and plasma metabolite profiling under dietary challenge

   PMID:35169201

   Open questions at the time

   • Molecular basis linking PXMP4 to ether lipid and α-oxidation changes not defined
   • No direct transport/enzymatic substrate identified
   • Hepatic focus; other tissues not characterized
6. 2024 Medium

   Provided direct evidence that PXMP4 interacts with PEX19, anchoring it within the peroxisomal membrane insertion machinery and showing the interaction is virally targetable.

   Evidence Co-IP between PEX19, PMP24, PEX14 and influenza M2 protein, with PEX19 knockdown and viral titer assays

   PMID:39205283

   Open questions at the time

   • Single Co-IP showing disruption; no reciprocal validation of functional consequence for PXMP4 insertion
   • Does not map the PEX19-binding region of PXMP4
   • Single lab
7. 2024 Medium

   Placed PXMP4 upstream of PI3K/AKT signaling in gastric cancer, connecting its expression to a defined oncogenic pathway and EMT.

   Evidence siRNA knockdown/overexpression with LY294002 epistasis and EMT/pathway Western blots in gastric cancer lines

   PMID:38401002

   Open questions at the time

   • Mechanism by which PXMP4 activates PI3K/AKT not defined
   • Single lab, no in vitro reconstitution
   • Relationship to PXMP4's peroxisomal function unclear
8. 2025 High

   Resolved a specific regulatory role for PXMP4 as the ubiquitination substrate that licenses pexophagy, defining a complete TBK1-MARCHF7-PXMP4-NBR1 signaling axis.

   Evidence Functional screening, reciprocal binding and ubiquitination assays, K20 site mutagenesis, reconstitution, and pexophagic flux assays in PEX1-deficient cells

   PMID:41267209

   Open questions at the time

   • How widely the axis operates beyond PEX1-deficient/ROS contexts unknown
   • Structural basis of MARCHF7-PXMP4 recognition not resolved
   • Relationship between PXMP4 ubiquitination and its metabolic role undefined

• Whether PXMP4's proposed channel/transporter activity, its ether-lipid metabolic role, and its function as a pexophagy substrate represent one integrated biochemical activity or distinct roles remains unresolved.
• No demonstrated transport substrate for PXMP4
• No structural model of the protein
• Link between metabolic phenotypes and the ubiquitination/autophagy axis not established