TMEM67

TMEM67 (meckelin) is a seven-transmembrane glycoprotein that functions as a ciliary transition zone component and Wnt signaling receptor, with its dual roles structurally separated by ADAMTS9-mediated extracellular cleavage: the cleaved C-terminal fragment localizes to the transition zone to regulate ciliogenesis and ciliary gate function controlling entry of ARL13B and INPP5E, while the uncleaved full-length form acts as a Wnt5a co-receptor that promotes ROR2 phosphorylation and non-canonical Wnt signaling (PMID:40436881, PMID:26035863, PMID:36334440). Loss of TMEM67 causes Meckel syndrome (MKS3) and related ciliopathies, with mutant tissues exhibiting aberrant canonical Wnt/β-catenin signaling driven by HOXB5, hyperactive mTOR signaling, cystogenesis, and disrupted Hedgehog responsiveness in the cerebellum (PMID:16415887, PMID:30931988, PMID:33574160, PMID:23393159). TMEM67 protein stability at the transition zone depends on FUT8-mediated core fucosylation, which protects it from autophagic degradation (PMID:40728580). TMEM67 also localizes to the endoplasmic reticulum, where its lumenal domain bridges misfolded substrates to the cytosolic p97 AAA-ATPase for ERAD-mediated degradation (PMID:19815549).

1. 2006 High

   Positional cloning identified TMEM67 as the MKS3 gene, establishing that mutations in a novel seven-transmembrane protein cause Meckel syndrome and linking it to ciliopathy pathogenesis.

   Evidence Positional cloning and mutation identification in MKS families with expression analysis in fetal tissues

   PMID:16415887

   Open questions at the time

   • Subcellular localization not determined
   • No functional data on cilia or signaling at this stage
   • Ligand or pathway involvement unknown
2. 2009 High

   Studies in rat, mouse, and C. elegans models demonstrated that TMEM67/MKS-3 is required for ciliary structure and function, localizes to the cilium base/transition zone, and operates in a genetic module with other MKS proteins parallel to the NPHP pathway.

   Evidence wpk rat phenotyping (eye, kidney, sperm), IMCD3 shRNA knockdown, C. elegans genetic epistasis and localization

   PMID:19515853 PMID:20150540

   Open questions at the time

   • Mechanism by which TMEM67 controls cilia length and centrosome number not resolved
   • Whether TMEM67 is a structural or regulatory component of the TZ unclear
3. 2009 High

   TMEM67 was found to localize predominantly to the ER, where it bridges misfolded lumenal cargo (mutant SP-C) to cytosolic p97, establishing an unexpected role in ERAD distinct from its ciliary function.

   Evidence Co-immunoprecipitation with domain deletions, subcellular fractionation, siRNA knockdown blocking ERAD of mutant SP-C in cell culture

   PMID:19815549

   Open questions at the time

   • Whether the ER pool and TZ pool represent the same biosynthetic route or distinct functional populations is unknown
   • Range of ERAD substrates beyond SP-C not defined
4. 2013 High

   Tmem67-null mice revealed that loss of TMEM67 upregulates canonical Wnt/β-catenin signaling in cystic tissues and activates JNK/ERK pathways, while zebrafish morphants showed convergent extension defects, placing TMEM67 as a regulator of Wnt and MAPK signaling rather than solely a structural ciliary protein.

   Evidence Tmem67 bpck mouse Wnt reporter assays, HEK293 overexpression with MAPK inhibitors, zebrafish morpholino knockdown

   PMID:23393159 PMID:23456819

   Open questions at the time

   • Direct molecular mechanism linking TMEM67 to canonical Wnt suppression unresolved
   • JNK/ERK activation mechanism (direct vs. indirect through cilia loss) unclear
5. 2015 High

   TMEM67 was identified as a receptor for Wnt5a that physically interacts with ROR2 to enable non-canonical Wnt signaling and epithelial branching morphogenesis, resolving how TMEM67 engages the Wnt pathway at the molecular level.

   Evidence In vitro Wnt5a binding to TMEM67 N-terminal domain, Co-IP with ROR2, Tmem67 KO embryonic lung culture rescued by RhoA activation

   PMID:26035863

   Open questions at the time

   • Structural basis of Wnt5a–TMEM67 interaction unresolved
   • Whether TMEM67 acts as a classical co-receptor or independent receptor not distinguished
6. 2019 High

   In the cerebellum, loss of TMEM67 was shown to activate canonical Wnt signaling through increased HOXB5 occupancy at the β-catenin promoter, providing a transcription-factor-level mechanism for Wnt dysregulation and linking it to disrupted Shh responsiveness.

   Evidence Tmem67 KO mouse cerebellum transcriptomics and ChIP for HOXB5 at β-catenin promoter

   PMID:30931988

   Open questions at the time

   • How TMEM67 loss leads to HOXB5 upregulation is not defined
   • Whether this HOXB5 mechanism operates outside the cerebellum is unknown
7. 2021 Medium

   Hyperactive mTOR signaling was identified as a pathogenic effector downstream of TMEM67 loss, with genetic and pharmacological mTOR inhibition rescuing cystogenesis and ciliary defects in adult zebrafish, opening a potential therapeutic axis.

   Evidence TALEN-generated tmem67 zebrafish mutants, rapamycin treatment and mtor hypomorphic epistasis, 3D kidney imaging

   PMID:33574160

   Open questions at the time

   • Whether mTOR hyperactivation is cilia-dependent or a direct TMEM67 effect is unresolved
   • Not validated in mammalian models
8. 2022 High

   CRISPR knockout in human cells established that TMEM67 is required for transition zone gating of ARL13B and INPP5E into cilia but is dispensable for recruitment of other TZ components (CEP290, RPGRIP1L, NPHP5), and revealed that ciliopathy-associated mutants can restore gating but not cilia length control, separating these functions.

   Evidence CRISPR/Cas9 KO in hTERT-RPE1 cells with immunofluorescence and mutant rescue

   PMID:36334440

   Open questions at the time

   • Molecular basis by which TMEM67 enforces the diffusion barrier is unknown
   • Structural arrangement of TMEM67 within the TZ Y-link not resolved
9. 2025 High

   ADAMTS9-mediated extracellular cleavage of TMEM67 was shown to generate two structurally and functionally separable forms — a cleaved TZ-localized fragment for ciliogenesis and a full-length form for Wnt signaling — resolving how one protein executes dual functions; patient variants in the cleavage motif disrupt only ciliary function.

   Evidence Biochemical cleavage assay, non-cleavable TMEM67 knock-in mouse phenocopying ciliopathy but with normal Wnt signaling, patient variant analysis in mammalian cells and C. elegans

   PMID:40436881

   Open questions at the time

   • Identity of the TMEM67 cleavage fragment released into the extracellular space and whether it has signaling activity is unknown
   • Regulation of ADAMTS9-mediated cleavage timing and tissue specificity not defined
10. 2025 High

    FUT8-mediated core fucosylation was identified as a post-translational mechanism that stabilizes TMEM67 by preventing its autophagic degradation, ensuring proper TZ localization and ciliogenesis across multiple organs.

    Evidence Mass spectrometry identification of core fucosylation, Co-IP of FUT8 with TMEM67, Fut8 KO mouse ciliary defects in kidney/brain/trachea, autophagy inhibition rescue

    PMID:40728580

    Open questions at the time

    • Which specific glycosylation sites on TMEM67 are critical is not fully mapped
    • Whether fucosylation status affects ADAMTS9 cleavage efficiency is untested

• Key unresolved questions include the structural basis of TMEM67 within the transition zone diffusion barrier, how its ER-localized ERAD function relates to its ciliary role, whether mTOR hyperactivation upon TMEM67 loss is a direct or cilia-dependent effect, and the identity and fate of the released N-terminal cleavage product.
• No high-resolution structural model of TMEM67 at the TZ
• Relationship between ER pool and TZ pool mechanistically undefined
• Full spectrum of ERAD substrates unknown