TTYH2

TTYH2 is a five-transmembrane domain protein with an extracellular N-terminus and cytoplasmic C-terminus that localizes to the plasma membrane and endosomal compartments and acts as a facilitator of lipid transfer at endosomal membranes (PMID:40562935, PMID:18260827). Cryo-EM structures revealed an extended extracellular domain bearing a partially solvent-exposed hydrophobic pocket; in the presence of Ca2+ TTYH2 assembles into cis-dimers bridged by extracellularly coordinated Ca2+, while in its absence it forms trans-dimers spanning opposing membranes across ~130 Å and a monomeric state, providing a Ca2+-regulated conformational basis for membrane bridging (PMID:34824283). The extended extracellular domain forms an epitope that faces the endosomal lumen and binds APOE-containing lipoprotein particles, and TTYH2 accelerates lipid extraction and insertion from these lipoproteins into membranes in reconstituted assays (PMID:40562935). TTYH2 surface and total abundance are controlled by post-translational and trafficking machinery: Nedd4-2 binds the TTYH2 PY motif and ubiquitinates it to regulate surface and cellular levels (PMID:18577513), N-glycosylation modulates expression and ubiquitination without being essential for plasma-membrane delivery (PMID:18260827), and β-COP binds the C-terminal region to negatively regulate TTYH2 surface trafficking (PMID:30670146). Although earlier electrophysiology proposed TTYH2 as a pore-forming subunit of volume-regulated anion channels (PMID:31138989, PMID:31181821), the cryo-EM structures showed no ion-conducting pathway and detected no TTYH2-dependent channel activity, arguing against a pore-forming role (PMID:34824283). TTYH2 also serves as a glycan-dependent binding partner of the SARS-CoV-2 spike receptor-binding domain that drives proinflammatory responses in myeloid cells without supporting viral replication (PMID:34048708).

1. 2007 Low

   Established an initial cellular phenotype for TTYH2 by asking whether it contributes to cancer cell behavior.

   Evidence siRNA knockdown with proliferation and aggregation assays in DLD-1 and Caco-2 colon cancer cells

   PMID:17569141

   Open questions at the time

   • No molecular mechanism identified
   • Single knockdown approach without rescue
   • Does not connect phenotype to a defined biochemical activity
2. 2008 High

   Defined how TTYH2 abundance and surface levels are post-translationally controlled, identifying its first regulatory partner.

   Evidence Endogenous co-IP, PY-motif mutagenesis, and ubiquitination/surface expression assays for Nedd4-2 binding; N-glycosylation site mutagenesis establishing five-TM topology

   PMID:18260827 PMID:18577513

   Open questions at the time

   • Did not establish the molecular function being regulated
   • Physiological context of ubiquitin turnover unclear
   • N-glycosylation role only partially defined
3. 2019 Medium

   Tested the long-standing hypothesis that TTYH2 is an ion channel and concurrently identified a trafficking regulator.

   Evidence Whole-cell patch-clamp with gene silencing and heterologous reconstitution in astrocytes and cancer cells; Y2H and co-IP for β-COP binding with surface/activity readouts

   PMID:30670146 PMID:31138989 PMID:31181821

   Open questions at the time

   • Channel assignment later contradicted by structural data
   • Pore residue mutagenesis not reconciled with absence of a conducting pathway
   • β-COP regulatory mechanism characterized only in heterologous and cancer-cell systems
4. 2019 Low

   Linked TTYH2 to migratory and EMT-related signaling, extending its cancer-cell role beyond proliferation.

   Evidence siRNA silencing with invasion/migration assays and western blot for Slug and ZEB1 in U2OS cells

   PMID:31230749

   Open questions at the time

   • Single knockdown approach in one cell line
   • Mechanism upstream of EMT factors undefined
   • No direct biochemical link to lipid-transfer function
5. 2021 High

   Resolved the structural fold and overturned the channel model, redefining TTYH2 as a Ca2+-regulated membrane-bridging protein.

   Evidence Cryo-EM in lipid nanodiscs capturing cis-dimer, trans-dimer, and monomeric states with cellular electrophysiology controls

   PMID:34824283

   Open questions at the time

   • Functional output of the conformational switch not defined at this stage
   • Endogenous ligand of the hydrophobic pocket unknown
   • Physiological trigger for cis/trans transition in cells unresolved
6. 2021 Medium

   Identified an unexpected role for TTYH2 as a glycan-dependent spike-binding partner that drives inflammation.

   Evidence Myeloid receptor-focused ectopic expression screen with binding, replication, and cytokine assays

   PMID:34048708

   Open questions at the time

   • Relationship to endogenous lipid-transfer function unclear
   • Glycan dependence not structurally mapped
   • Single study without orthogonal in vivo validation
7. 2025 High

   Assigned a concrete molecular function by identifying APOE as the endogenous partner and demonstrating lipid transfer.

   Evidence Endogenous pull-down, fractionation, immunocytochemistry, structural mapping of the lumen-facing epitope, and in vitro lipid transfer assays

   PMID:40562935

   Open questions at the time

   • Directionality and physiological cargo of lipid transfer in vivo not fully defined
   • Connection between Ca2+-driven cis/trans switch and lipid transfer mechanism not explicitly resolved
   • Downstream consequences of endosomal lipid handling uncharacterized

• How the Ca2+-regulated conformational switch, trafficking control, and APOE-dependent lipid transfer integrate into a defined physiological pathway remains open.
• No in vivo demonstration linking lipid-transfer activity to a tissue-level phenotype
• Mechanistic coupling of cis/trans transition to lipid extraction undefined
• Reconciliation of cancer-cell phenotypes with the lipid-transfer model unresolved