PXMP4

PXMP4 (PMP24) is a peroxisomal integral membrane protein that functions as a key signal for selective autophagy of damaged peroxisomes (pexophagy) and contributes to peroxisomal metabolic homeostasis. Upon peroxisomal damage, TBK1 is activated by ROS accumulation and phosphorylates the E3 ubiquitin ligase MARCHF7, which ubiquitinates PXMP4 at lysine 20 on the peroxisomal membrane; this ubiquitin tag recruits the autophagy receptor NBR1 to initiate pexophagy, and a K20R ubiquitination-defective mutant impairs pexophagic flux (PMID:41267209). PXMP4 knockout mice are viable but exhibit mildly elevated phytanic/pristanic acid levels and reduced hepatic ether lipids enriched in polyunsaturated fatty acids, establishing a role in peroxisomal α-oxidation and ether lipid metabolism (PMID:35169201). PXMP4 interacts with the peroxisomal biogenesis chaperone PEX19, and its expression is regulated by CpG methylation at a critical intronic site whose hypermethylation silences the gene in prostate cancer cells, where re-expression suppresses proliferation (PMID:39205283, PMID:14712230, PMID:20054818).

1. 2004 Medium

   The discovery that PXMP4 is epigenetically silenced in prostate cancer by CpG island hypermethylation, and that its re-expression suppresses proliferation, established it as a functionally relevant peroxisomal membrane protein with growth-regulatory consequences — moving it from a poorly characterized peroxisomal protein to one with a defined cellular phenotype upon loss.

   Evidence Bisulfite sequencing, 5-aza-dC demethylation reactivation, and transient transfection with colony formation assays in prostate cancer cell lines

   PMID:14712230

   Open questions at the time

   • Mechanism by which PXMP4 suppresses proliferation was not identified
   • Whether PXMP4 silencing is a driver or passenger event in prostate tumorigenesis was not resolved
   • No direct link to a specific peroxisomal metabolic pathway was established
2. 2008 Low

   Identification of PEX19 as a binding partner of PXMP4 placed PXMP4 within the peroxisomal membrane protein import pathway, providing the first molecular interaction context for its biogenesis.

   Evidence Congenic mouse strain analysis with candidate gene binding partner identification

   PMID:18714012

   Open questions at the time

   • PEX19 interaction was referenced from prior data without independent validation in this study
   • No direct functional test of PXMP4 in NKT cell biology was performed
   • The mechanism by which PEX19 chaperones PXMP4 to the peroxisomal membrane was not characterized
3. 2010 Medium

   Fine-mapping of PXMP4 epigenetic regulation to a single intronic CpG dinucleotide that controls DNA-protein binding revealed the precise mechanism of its transcriptional silencing, explaining how methylation at one site can shut down expression.

   Evidence Limited 5-aza-dC demethylation, EMSA gel shift assays, methylated oligonucleotide transfection, bisulfite sequencing of microdissected specimens

   PMID:20054818

   Open questions at the time

   • The transcription factor binding at the critical intronic CpG was not identified
   • Whether this epigenetic mechanism operates in tissues beyond prostate was not tested
4. 2016 Low

   Computational classification of PXMP4 as a Tim17 protein family member suggested it may function as a channel or transporter in the peroxisomal membrane, providing a structural hypothesis for its molecular activity.

   Evidence Phylogenetic and sequence analysis across 5631 proteomes

   PMID:27760563

   Open questions at the time

   • No experimental validation of channel or transporter activity was performed
   • No substrate or solute specificity was tested
   • Structural prediction has not been confirmed by biochemical reconstitution or structural biology
5. 2022 High

   Generation of PXMP4 knockout mice resolved the longstanding question of whether the protein is essential for peroxisome biogenesis (it is not) and instead revealed a specific metabolic role in ether lipid synthesis and α-oxidation capacity.

   Evidence CRISPR/Cas9 knockout mouse model with lipidomic analysis, plasma metabolite profiling, and dietary challenge studies

   PMID:35169201

   Open questions at the time

   • The direct molecular mechanism by which PXMP4 supports ether lipid metabolism is unknown
   • Whether PXMP4 functions as a metabolite transporter in the peroxisomal membrane was not tested
   • Tissue-specific contributions beyond liver were not deeply characterized
6. 2024 Medium

   Demonstration that influenza A virus M2 protein disrupts PEX19–PXMP4 binding provided independent confirmation of the PEX19 interaction and showed it is biologically targetable by pathogens to impair peroxisome biogenesis.

   Evidence Co-immunoprecipitation, PEX19 knockdown, viral growth assays, peroxisome quantification in IAV-infected cells

   PMID:39205283

   Open questions at the time

   • The stoichiometry and affinity of the PEX19–PXMP4 interaction were not measured
   • Whether PXMP4 loss specifically mediates the antiviral signaling defect was not isolated from other PEX19 clients
7. 2025 High

   The discovery that MARCHF7 ubiquitinates PXMP4 at K20 downstream of TBK1-ROS signaling to recruit NBR1 for pexophagy established the first complete signaling pathway for selective peroxisome turnover through PXMP4, defining its primary molecular function as a pexophagy signal.

   Evidence Functional screening, siRNA knockdown, K20R mutant reconstitution, co-immunoprecipitation, pexophagic flux assays, and TBK1 phosphorylation assays in PEX1-deficient cells

   PMID:41267209

   Open questions at the time

   • Whether PXMP4-dependent pexophagy operates in physiological contexts beyond PEX1 deficiency is not established
   • Whether other peroxisomal membrane proteins serve redundant pexophagy signals is unknown
   • The structural basis for MARCHF7 recognition of PXMP4 K20 has not been determined

• It remains unknown whether PXMP4 functions as a membrane channel or transporter as predicted by its Tim17 family membership, and whether its metabolic role in ether lipid synthesis is mechanistically linked to or independent of its pexophagy signaling function.
• No reconstituted transport or channel activity has been demonstrated
• The relationship between PXMP4's metabolic and pexophagy functions has not been dissected
• No structural data for PXMP4 exists