PEX7

PEX7 is the cytosolic receptor for peroxisomal matrix proteins bearing the type-2 peroxisome targeting signal (PTS2), essential for importing enzymes required for plasmalogen biosynthesis, phytanic acid alpha-oxidation, and fatty acid beta-oxidation into peroxisomes. PEX7 recognizes the amphipathic helical PTS2 motif via a complementary binding groove on its WD40-repeat surface, forms a trimeric complex with PEX5 and PTS2 cargo, and shuttles into the peroxisomal lumen in a PEX5- and cargo-dependent manner upstream of PEX5 monoubiquitination; cargo is released at the trans-side of the membrane while PEX7 remains at the docking/translocation machinery and is subsequently exported back to the cytosol in a process dependent on, but not strictly coupled to, PEX5 recycling (PMID:24865970, PMID:26138649, PMID:22057399). Loss-of-function mutations in PEX7 cause rhizomelic chondrodysplasia punctata (RCDP) and a subset of Refsum disease, with disease severity correlating with residual PEX7 activity; knockout and hypomorphic mouse models confirm that PEX7 is required for normal brain development, endochondral ossification, and lens integrity (PMID:9090381, PMID:12522768, PMID:12915479, PMID:20060764). The PTS2-receptor function of PEX7 is evolutionarily conserved from trypanosomes and Drosophila to humans (PMID:17320990, PMID:30389805).

1. 1997 High

   Identification of PEX7 as the PTS2 receptor resolved the molecular basis of RCDP and revealed how PTS2-bearing matrix proteins are recognized for peroxisomal import.

   Evidence Complementation cloning and expression of PEX7 in RCDP patient fibroblasts rescued PTS2 protein import; mutations identified in 36 probands

   PMID:9090381 PMID:9090383

   Open questions at the time

   • Mechanism by which PEX7 delivers cargo to the peroxisomal membrane was unknown
   • Structural basis of PTS2 recognition by PEX7 was not determined
   • Relationship between PEX7 and PEX5 during import was undefined
2. 2000 Medium

   Characterization of PEX7 gene structure and alternative splice forms established that only the full-length transcript encodes a functional PTS2 receptor.

   Evidence Expression of eight alternative PEX7 splice variants in RCDP fibroblasts; none rescued PTS2 import

   PMID:10673331

   Open questions at the time

   • Single study; biological role of non-functional splice forms remains unclear
   • Whether alternative transcripts have dominant-negative effects was not tested
3. 2002 High

   Genotype-phenotype correlation across dozens of RCDP alleles demonstrated that residual PEX7 activity is the primary determinant of clinical severity, establishing a functional continuum from null to hypomorphic alleles.

   Evidence Expression of 24 mutant PEX7 alleles in RCDP fibroblasts with PTS2 import quantification; correlation with clinical phenotype in 60 probands

   PMID:11781871 PMID:12325024

   Open questions at the time

   • Molecular basis of partial activity for mild alleles not structurally resolved
   • Tissue-specific thresholds of PEX7 activity for normal function were unknown
4. 2003 High

   Discovery that PEX7 mutations also cause Refsum disease broadened the phenotypic spectrum and linked PEX7 to phytanic acid metabolism, while the Pex7 knockout mouse confirmed the gene's requirement for plasmalogen synthesis, neuronal migration, and endochondral ossification in vivo.

   Evidence Genetic linkage and mutation analysis in Refsum patients lacking PHYH mutations; Pex7-null mice with biochemical, neuronal birthdating, and skeletal analyses

   PMID:12522768 PMID:12915479

   Open questions at the time

   • Whether plasmalogen deficiency alone or combined metabolite disruption drives each tissue phenotype was not resolved
   • No therapeutic rescue demonstrated in the mouse model
5. 2007 Medium

   Conservation of PEX7 function in Trypanosoma brucei demonstrated that PTS2 receptor activity is an ancestral eukaryotic feature and revealed unexpected cross-talk between PTS1 and PTS2 import pathways in some organisms.

   Evidence RNAi knockdown of T. brucei PEX7; immunofluorescence and fractionation showing mislocalization of PTS2 and (in procyclic cells) PTS1 proteins

   PMID:17320990

   Open questions at the time

   • Mechanism of PTS1 pathway disruption upon PEX7 depletion not clarified
   • Single-organism RNAi study; not confirmed by complementation
6. 2009 High

   A Pex7 hypomorphic mouse producing <5% normal transcript identified the lens and skeleton as tissues most sensitive to reduced PEX7 activity, linking plasmalogen deficiency to cataract formation.

   Evidence Hypomorphic Pex7 mouse with biochemical, histological, and dietary intervention analyses

   PMID:20060764

   Open questions at the time

   • Whether dietary plasmalogen supplementation can prevent cataracts was not conclusively shown
   • Threshold level of PEX7 activity sufficient for each tissue was not quantified
7. 2011 High

   Structural modeling of PEX7 and characterization of the PTS2 signal as an amphipathic helix, validated by compensatory mutagenesis, defined the molecular recognition interface and enabled prediction of novel PTS2 cargo proteins.

   Evidence 3D modeling of PEX7 WD40 domain; site-directed mutagenesis and mammalian two-hybrid cross-complementation; computational PTS2 prediction

   PMID:22057399

   Open questions at the time

   • No experimentally determined crystal or cryo-EM structure of the PEX7-PTS2 complex
   • Predicted novel cargoes (e.g., KChIP4) not confirmed by in vivo import assays
8. 2014 High

   Reconstituted import/export assays resolved the PEX7 shuttling cycle: PEX7 enters the peroxisomal lumen as part of a trimeric PEX5-PEX7-cargo complex upstream of PEX5 ubiquitination and is exported back to the cytosol in a PEX5-dependent but temporally uncoupled manner.

   Evidence In vitro co-import/export assays with protease protection, dominant-negative blocks, and time-course fractionation

   PMID:24865970

   Open questions at the time

   • Identity of machinery mediating PEX7-specific export step unknown
   • Whether PEX7 is ubiquitinated during cycling was not addressed
9. 2015 High

   Demonstration that PEX7 export competence depends on the specific PEX5 molecule that co-imported it established that PEX7 remains tethered to the docking/translocation machinery during the intra-peroxisomal phase, and that PTS2 cleavage is dispensable for cargo release.

   Evidence In vitro assays with pre-assembled trimeric complexes; biochemical fractionation showing PEX7 retention at DTM

   PMID:26138649

   Open questions at the time

   • Structural basis of PEX7 retention at the DTM not determined
   • How PEX7 releases cargo at the trans-side mechanistically is unresolved
10. 2018 Medium

    Drosophila Pex7 cross-complements human PEX7-deficient cells despite the absence of identified PTS2 cargo in flies, confirming deep evolutionary conservation of PTS2 receptor mechanism.

    Evidence Expression of Drosophila Pex7 in human RCDP fibroblasts rescues thiolase import; Drosophila Pex7 mutant lipid and developmental phenotyping

    PMID:30389805

    Open questions at the time

    • No PTS2 cargo identified in Drosophila; endogenous substrates remain unknown
    • Whether fly Pex7 forms a trimeric complex with fly Pex5 analogous to the mammalian system was not tested

• An experimentally determined high-resolution structure of the PEX7-PTS2 complex, the identity of the machinery that mediates PEX7-specific export from the peroxisomal membrane, and the full complement of PTS2 cargo proteins in different organisms remain to be established.
• No crystal or cryo-EM structure of PEX7 bound to PTS2 cargo
• Mechanism and components of PEX7 export from peroxisomal membrane unknown
• Complete PTS2 proteome not validated in any organism