TMED5

TMED5 (p24γ2) is a p24-family cargo receptor of the early secretory pathway that mediates selective ER-to-Golgi transport of secretory and GPI-anchored proteins (PMID:24778190, PMID:21118709). Cargo selectivity for GPI-anchored proteins is conferred by its membrane-adjacent α-helical region rather than its luminal GOLD domain, which is required for incorporation of GPI-APs into COPII-coated vesicles (PMID:24778190). The GOLD domain itself adopts a β-sandwich fold and crystallizes as a dimer assisted by a short C-terminal α-helix, consistent with a role in p24 complex assembly (PMID:28066915). Within the p24γ subfamily TMED5 is functionally non-redundant, supporting distinct cargo-processing roles such as the transport, glycosylation, sulfation, and cleavage of POMC compared to p24γ3 (PMID:21118709). Beyond its core trafficking function, TMED5 interacts with WNT7B to activate canonical WNT/β-catenin signaling in cervical cancer cells (PMID:30394198), and it accumulates in lysosomes under APOE4 expression where it drives lysosomal alkalinization and impaired lysosomal function (PMID:41103078, PMID:37873080).

1. 2010 Medium

   Established that p24 proteins within a single subfamily are functionally distinct, addressing whether p24γ2 simply substitutes for related subunits in cargo handling.

   Evidence Melanotrope-specific transgene expression in Xenopus with biochemical analysis of POMC glycosylation, sulfation, and cleavage

   PMID:21118709

   Open questions at the time

   • Mechanistic basis for the distinct roles of p24γ2 vs p24γ3 not defined
   • Direct cargo-binding interface for POMC not mapped
   • Single physiological model and lab
2. 2014 High

   Identified the membrane-adjacent α-helical region as the determinant of GPI-anchored protein recognition, localizing cargo selectivity to a specific domain rather than the GOLD domain.

   Evidence siRNA knockdown plus chimeric p24γ2/p24γ1 domain-swap rescue with a GPI-AP transport reporter

   PMID:24778190

   Open questions at the time

   • Direct physical contact between the α-helical region and GPI-AP cargo not demonstrated
   • How the α-helix couples cargo to the COPII machinery not resolved
3. 2017 Medium

   Provided the first structural model of the TMED5 GOLD domain and proposed dimerization as a basis for p24 complex formation.

   Evidence X-ray crystallography of the mouse p24γ2 GOLD domain at 2.8 Å (sulfur SAD)

   PMID:28066915

   Open questions at the time

   • Functional consequence of GOLD dimerization not validated in cells
   • Full-length structure including the cargo-selective α-helical region absent
   • Partner subunit interfaces not structurally defined
4. 2018 Medium

   Connected TMED5 to oncogenic signaling by showing physical interaction with WNT7B and activation of canonical WNT/β-catenin signaling in cancer cells.

   Evidence Co-immunoprecipitation, western blot, and cell-based pathway readouts in cervical cancer cells

   PMID:30394198

   Open questions at the time

   • Single Co-IP without reciprocal validation
   • Whether the interaction reflects secretory trafficking of WNT7B or a direct signaling role is unclear
   • Interaction not confirmed in independent systems
5. 2023 Low

   Linked TMED5 to hepatocellular carcinoma cell behavior, implicating it in proliferation, migration, invasion, and apoptosis control.

   Evidence siRNA knockdown with CCK-8 proliferation, flow cytometry, and Transwell assays in HCC cell lines

   PMID:36530030

   Open questions at the time

   • No direct mechanistic validation that TMED5 acts through cell cycle, mTOR, or TGF-β pathways
   • Single lab and phenotypic-only readouts
   • In vivo relevance not tested
6. 2025 Medium

   Revealed a non-secretory disease-relevant role by showing TMED5 accumulates in lysosomes under APOE4 expression and drives lysosomal alkalinization and dysfunction.

   Evidence LysoIP-mass spectrometry lysosomal proteomics with siRNA/overexpression and lysosomal pH and function assays in neuronal cells

   PMID:37873080 PMID:41103078

   Open questions at the time

   • Mechanism of TMED5 lysosomal mistargeting under APOE4 not defined
   • Molecular cause of alkalinization downstream of TMED5 accumulation unknown
   • Relationship to its secretory-pathway function unresolved

• How TMED5's defined ER-to-Golgi cargo receptor activity mechanistically connects to its reported roles in WNT signaling, cancer cell phenotypes, and APOE4-driven lysosomal dysfunction remains unresolved.
• No unifying mechanism linking secretory trafficking to signaling and lysosomal roles
• Direct cargo and partner interfaces largely unmapped at atomic resolution