ESYT2

ESYT2 is an ER-anchored membrane tethering and lipid transfer protein that bridges the endoplasmic reticulum to the plasma membrane—and to lipid droplet–mitochondria interfaces—to coordinate phospholipid and diacylglycerol homeostasis, signaling lipid turnover, and inter-organelle fatty acid trafficking. Its SMP domain dimerizes to form a hydrophobic channel that transports glycerophospholipids between apposed bilayers, while its C2C domain targets the plasma membrane through PI(4,5)P2 binding and its C2A domain confers Ca²⁺-dependent phospholipid interaction (PMID:17360437, PMID:24847877, PMID:23791178). ESYT2-mediated ER–PM contacts position the Sac1 phosphatase to limit PI(4)P levels and regulate phosphoinositide pools that control store-operated Ca²⁺ entry, ANO1 channel activity, and DAG-dependent TCR signaling in T cells (PMID:27044890, PMID:32879390, PMID:38177911, PMID:40204782). ESYT2 also functions as an endocytic adaptor for activated FGFR1 through interactions with Adaptin-2 and PAK1, and participates—together with ESYT1 and VAPB—in a multimeric complex at lipid droplet–mitochondria–ER contacts that channels fatty acids toward mitochondrial β-oxidation (PMID:20833364, PMID:40032835).

1. 2007 High

   Establishing that ESYT2 is a multi-C2-domain protein whose C2A domain provides Ca²⁺-dependent phospholipid binding and whose C2C domain independently targets the plasma membrane resolved the basic molecular architecture underlying its membrane association.

   Evidence Recombinant Ca²⁺/phospholipid binding assays combined with domain deletion and immunofluorescence in transfected cells

   PMID:17360437

   Open questions at the time

   • Mechanism by which C2C recognizes specific PM lipids was not defined
   • No information on lipid transfer activity
2. 2008 Medium

   Demonstration that Ca²⁺ triggers reversible multimerization of ESYT2 (~100 µM Kd) and that the three C2 domains adopt an interdependent architecture suggested oligomerization as a regulatory mechanism.

   Evidence SAXS of recombinant ESYT2 with Ca²⁺ titration

   PMID:18977228

   Open questions at the time

   • Functional consequences of multimerization on membrane tethering were untested
   • In vivo relevance of oligomerization not addressed
3. 2010 High

   Identification of ESYT2 as an endocytic adaptor for activated FGFR1—interacting with Adaptin-2 and required for receptor internalization, ERK activation, and mesoderm induction—revealed an unexpected signaling-adaptor function beyond lipid biology.

   Evidence Morpholino knockdown in Xenopus; co-IP of ESYT2 with FGFR1 and Adaptin-2; epistasis upstream of Ras/ERK; replicated in human cells

   PMID:20833364

   Open questions at the time

   • Structural basis of ESYT2–FGFR1 interaction unknown
   • Whether lipid transfer activity contributes to FGFR signaling untested
4. 2012 Medium

   Showing that ESYT2 binds PAK1 via its C2C domain to suppress Cdc42/Rac-dependent actin polymerization linked the FGFR adaptor function to cytoskeletal regulation.

   Evidence Co-IP with domain mapping; actin polymerization and PAK1 activation assays

   PMID:23213466

   Open questions at the time

   • No in vivo validation of ESYT2–PAK1 axis
   • Whether PAK1 inhibition requires ESYT2 lipid binding or transfer is unclear
5. 2013 High

   Demonstrating that ESYT2 constitutively tethers the ER to the PM via PI(4,5)P2-dependent C2 domain interactions—without requiring elevated Ca²⁺, unlike ESYT1—defined the core ER–PM contact-site function and established the PI(4,5)P2 dependence that distinguishes ESYT2 from ESYT1.

   Evidence Live-cell imaging of ER–PM contacts; PI(4,5)P2 depletion; Ca²⁺ manipulation; co-IP for heteromeric complex formation

   PMID:23791178

   Open questions at the time

   • Whether tethering per se is sufficient for lipid transfer was unresolved
   • Identity of transferred lipid species unknown
6. 2013 High

   Crystal structures of the C2A–C2B tandem revealed a rigid V-shaped architecture with up to four Ca²⁺ ions bound to C2A but not C2B, providing the first atomic-level view of the C2 domain module.

   Evidence X-ray crystallography ± Ca²⁺; NMR validation of Ca²⁺ binding

   PMID:24373768

   Open questions at the time

   • No structure of the full-length protein or of C2 domains engaged with membranes
7. 2014 High

   Solving the SMP domain crystal structure as a homodimer with an ~90 Å hydrophobic channel containing glycerophospholipids directly established ESYT2 as a lipid-transfer protein.

   Evidence 2.44 Å crystal structure; mass spectrometry identification of bound glycerophospholipids

   PMID:24847877

   Open questions at the time

   • Directionality and selectivity of lipid transfer in membranes not determined
   • No reconstituted transfer assay between two bilayers
8. 2016 High

   Showing that ESYT2 positions the Sac1 phosphatase at ER–PM junctions to limit PM PI(4)P—and that GPCR-driven PI(4,5)P2 depletion disassembles these junctions, relieving PI(4)P control—placed ESYT2 at the center of phosphoinositide homeostasis.

   Evidence TIRF/confocal imaging; siRNA of ESYT2; PI(4,5)P2 biosensors; GPCR stimulation; Sac1 overexpression/depletion

   PMID:27044890

   Open questions at the time

   • Whether lipid transfer through the SMP channel is required for Sac1 positioning unclear
   • In vivo physiological consequence of Sac1 mis-localization not tested
9. 2016 High

   Triple-KO mice lacking all three ESyt isoforms are viable and fertile, indicating that ESyt-mediated ER–PM contacts are dispensable for basal mammalian physiology under laboratory conditions, though cellular phenotypes include impaired migration and oxidative-stress sensitivity.

   Evidence Constitutive triple KO and C2A Ca²⁺-binding knock-in mice; MEF migration and stress assays

   PMID:25486202 PMID:27348751 PMID:27399837

   Open questions at the time

   • Stress or disease contexts that unmask essential functions remain unknown
   • Compensation by other ER–PM tethering systems not excluded
10. 2017 High

    Single-molecule force spectroscopy quantified individual C2 domain–membrane binding at 2–7 pN and 4–14 kBT, providing the biophysical parameters governing ESYT2 tethering strength.

    Evidence Optical-tweezer force spectroscopy with membrane-coated beads; systematic lipid and Ca²⁺ variation

    PMID:29083305

    Open questions at the time

    • How these forces translate to ER–PM gap regulation in cells not modeled
    • Cooperativity between C2 domains in full-length protein not measured
11. 2020 Medium

    Identification of a short ESYT2 isoform that directly interacts with STIM1 to recruit it to ER–PM junctions for ORAI1 clustering and SOCE established ESYT2 as a modulator of store-operated Ca²⁺ entry, particularly in T cells.

    Evidence siRNA/CRISPR KO in Jurkat and primary T cells; co-IP of ESYT2S–STIM1; Ca²⁺ imaging; cytokine assays

    PMID:32879390

    Open questions at the time

    • Structural basis of ESYT2S–STIM1 interaction undefined
    • Relative contribution versus general tethering not fully delineated
12. 2023 Medium

    Demonstrating that ESYT2 reduces PM DAG levels in resting T cells and that its loss enhances DAG-dependent TCR signaling, cytotoxicity, and cytokine production identified DAG as a major ESYT2 lipid-transfer substrate with immunological significance.

    Evidence siRNA knockdown in primary T cells; DAG biosensor imaging; cytotoxicity, degranulation, and cytokine assays

    PMID:38177911

    Open questions at the time

    • Whether DAG is directly transported through the SMP channel or removed indirectly via phosphoinositide remodeling not distinguished
    • In vivo immune consequences of ESYT2 loss not tested
13. 2025 High

    Discovery that ESYT2 forms a multimeric complex with ESYT1 and VAPB at lipid-droplet–mitochondria–ER tripartite contacts and that its deletion limits fatty acid β-oxidation, depletes TCA metabolites, and induces lipotoxic stress expanded ESYT2's role from ER–PM tethering to inter-organelle fatty acid trafficking.

    Evidence BioID proximity proteomics; high-resolution co-localization imaging; ESYT2 deletion in cells and mice; metabolomics and lipidomics; fatty acid oxidation assays

    PMID:40032835

    Open questions at the time

    • Whether the SMP domain directly transfers fatty acids or acts indirectly is unresolved
    • Tissue-specific metabolic consequences in vivo remain to be defined
14. 2025 Medium

    Elucidation of an ANO1-IRBIT-ESYT2-AC6-AKAP11-PKA complex at STIM1-containing ER–PM junctions—where ESYT2 displaces VAPA to enable PKA-dependent ANO1 S221 phosphorylation and channel inhibition—revealed ESYT2 as a scaffold that transduces junctional phosphoinositide changes into ion-channel regulation.

    Evidence Co-IP of complex components; site-directed mutagenesis of ANO1 S221; lipid biosensor and Ca²⁺ imaging; knockdown/overexpression in epithelial cells; IRBIT KO mice

    PMID:40204782

    Open questions at the time

    • Whether ESYT2 lipid transfer activity is required for the scaffolding function is untested
    • Generalizability beyond epithelial ANO1 regulation unknown

• Key open questions include the directionality and substrate selectivity of SMP-mediated lipid transfer in reconstituted systems, the structural basis of full-length ESYT2 spanning a membrane contact site, stress or disease contexts that reveal essential in vivo functions, and how the adaptor/scaffolding and lipid-transfer activities of ESYT2 are coordinated or segregated at distinct contact sites.
• No reconstituted two-bilayer lipid transfer assay for full-length ESYT2
• No full-length structure bridging two membranes
• Physiological contexts where ESYT2 is essential remain unidentified