PEX16

PEX16 is an integral peroxisomal membrane protein that initiates and maintains peroxisome biogenesis from the endoplasmic reticulum (PMID:16717127, PMID:25002403). Cloned as the gene defective in complementation group D Zellweger syndrome, human PEX16 restores peroxisome biogenesis in patient fibroblasts, and a C-terminal nonsense mutation (R176ter) abolishes this activity, establishing both its disease relevance and the functional importance of its C-terminal half (PMID:9837814). PEX16 is cotranslationally inserted into the ER, where it recruits other peroxisomal membrane proteins—including PEX3 and PMP34/MMP34—to ER membranes, thereby seeding de novo peroxisome formation and supplying a constitutive ER-to-peroxisome membrane trafficking route that maintains pre-existing peroxisomes (PMID:16717127, PMID:25002403). Export of PEX16 (and the PEX3 it carries) from the ER to peroxisomes requires Sec16B, defining a COPII-related exit pathway (PMID:21768384). Systematic mutagenesis resolves PEX16 into separable domains controlling its own ER-to-peroxisome trafficking versus its PMP-recruitment activity, the latter conserved in plant PEX16 (PMID:25903784). PEX16 also restrains peroxisome turnover, as its depletion lowers peroxisome abundance by activating ATG5-dependent pexophagy (PMID:34360754), and CRISPR knockout cells reveal that PEX16 accelerates but is not strictly required for de novo membrane formation, while a patient-derived mutant acts dominant-negatively (PMID:35437598). Through its control of peroxisome number, PEX16 supports peroxisomal fatty-acid oxidation and adipocyte differentiation downstream of PPARγ (PMID:28017862).

1. 1997 Medium

   Established that Pex16p is a membrane-associated peroxisomal protein whose loss/overexpression alters peroxisome morphology, raising the question of its role in organelle biogenesis.

   Evidence Subcellular fractionation, epitope tagging, and C-terminal motif mutagenesis in Yarrowia lipolytica

   PMID:9182661

   Open questions at the time

   • Mechanism of membrane recruitment not defined
   • Ortholog behavior may differ from mammalian PEX16
   • No molecular partners identified
2. 1998 High

   Identified human PEX16 as the gene mutated in complementation group D Zellweger syndrome and showed its C-terminal half is required for function, linking the protein causally to peroxisome biogenesis disease.

   Evidence cDNA cloning, complementation rescue in patient fibroblasts, and sequencing of the R176ter mutation

   PMID:9837814

   Open questions at the time

   • Molecular function of PEX16 not yet defined
   • Which PMPs it acts on unknown
   • Subcellular trafficking route unresolved
3. 2006 High

   Defined PEX16's molecular role: cotranslational ER insertion followed by recruitment of other PMPs to initiate de novo peroxisome formation from the ER.

   Evidence Photoactivatable-GFP pulse-chase live imaging plus fractionation in normal and peroxisome-deficient mammalian cells

   PMID:16717127

   Open questions at the time

   • ER exit machinery not identified
   • Domains responsible for distinct functions not mapped
   • Direct PMP-binding partners not biochemically defined
4. 2011 High

   Identified the ER-exit machinery for PEX16, showing Sec16B (not Sec16A) drives PEX16/PEX3 export from ER to peroxisomes via a COPII-like route.

   Evidence RNAi knockdown with RNAi-resistant rescue and overexpression imaging in mammalian cells

   PMID:21768384

   Open questions at the time

   • Direct PEX16–Sec16B interaction not shown biochemically
   • COPII coat involvement inferred, not fully reconstituted
5. 2011 Medium

   Revealed evolutionary divergence and a tissue-context role, showing pex16 is dispensable for residual peroxisome-like granules in Drosophila and required in somatic cyst cells for germline maturation.

   Evidence Drosophila genetic knockout with tissue-specific rescue transgenes

   PMID:21826223

   Open questions at the time

   • Mammalian requirement may differ from fly
   • Paracrine signaling mechanism not defined
   • Molecular function unaddressed in this model
6. 2014 High

   Extended PEX16 function beyond de novo biogenesis to constitutive maintenance, showing it traffics PEX3 and PMP34 from ER to pre-existing peroxisomes.

   Evidence Quantitative time-lapse imaging, ER-targeted PEX3 reporter, and gain/loss-of-function with fractionation

   PMID:25002403

   Open questions at the time

   • Cargo selectivity rules unknown
   • Whether all PMPs use this route unresolved
7. 2015 High

   Mapped PEX16 into separable functional domains, distinguishing self-trafficking from PMP-recruitment activity and demonstrating conservation of recruitment in plants.

   Evidence Systematic deletion/point mutagenesis with imaging readouts and cross-species complementation

   PMID:25903784

   Open questions at the time

   • Structural basis of each domain unknown
   • Residue-level binding interfaces undefined
8. 2016 Medium

   Placed PEX16 in adipocyte metabolism as a PPARγ target whose loss impairs peroxisomal fatty-acid oxidation and differentiation.

   Evidence Stable shRNA silencing in 3T3-L1 cells with lipid mass spectrometry, oxygen consumption assays, and rosiglitazone rescue

   PMID:28017862

   Open questions at the time

   • Direct PPARγ binding to PEX16 promoter not shown here
   • Whether defects are secondary to reduced peroxisome number unresolved
9. 2021 Medium

   Showed PEX16 restrains peroxisome turnover, as its depletion lowers peroxisome abundance through ATG5-dependent pexophagy.

   Evidence siRNA knockdown, ATG5-KO epistasis, autophagy flux assays, and p62/ABCD3 co-localization in RPE-1 cells

   PMID:34360754

   Open questions at the time

   • Mechanism linking PEX16 loss to pexophagy signaling undefined
   • Single study, single cell type
10. 2022 Medium

    Refined the necessity of PEX16, demonstrating it accelerates but is not absolutely required for de novo peroxisome formation, while a patient mutant acts dominant-negatively.

    Evidence CRISPR/Cas9 knockout across three mammalian cell lines with re-expression rescue and dominant-negative mutant assays

    PMID:35437598

    Open questions at the time

    • Compensating factors enabling PEX16-independent biogenesis unidentified
    • Molecular basis of dominant-negative effect unresolved

• How PEX16 mechanistically couples to downstream signaling pathways and the structural basis of its cargo recruitment remain open.
• No structural model of PEX16 or its cargo complexes
• Direct biochemical PMP-binding interfaces undefined
• Connection to signaling pathways (e.g. Wnt/β-catenin) uncharacterized mechanistically