ESYT3

ESYT3 (E-Syt3) is an ER-resident transmembrane protein that uses three C-terminal C2 domains to build and regulate ER–plasma membrane (ER-PM) contact sites and to transfer lipids across them (PMID:17360437, PMID:23791178, PMID:40425857). Its C2A domain binds Ca2+ and phospholipids at micromolar Ca2+, while the C2C domain functions as a targeting motif that directs the protein toward the plasma membrane independently of the ER-anchoring transmembrane region (PMID:17360437). E-Syt3 tethers the ER to the PM through PI(4,5)P2-dependent C2-domain interactions and forms heteromeric complexes with E-Syt1 and E-Syt2 to confer cytosolic Ca2+ regulation onto contact formation, a process placed downstream of a RASSF4–ARF6–PIP5K axis that controls PM PI(4,5)P2 levels (PMID:23791178, PMID:28600435). At these junctions E-Syt3 transfers phosphatidylserine, and its removal of PtdSer dissociates a cAMP-signaling complex to restrain CFTR and IRBIT-dependent NBCe1-B activation, with E-Syt3 depletion enhancing chloride flux and fluid secretion in salivary glands and pancreatic ducts in mice (PMID:40425857). Through its ER-PM coupling role E-Syt3 supports plasma-membrane channel function, including ANO1 chloride current density (PMID:29154949). Beyond contact-site lipid handling, E-Syt3 has tissue- and pathway-specific roles: it promotes diet-induced obesity by limiting POMC processing to α-MSH and modulating PKC/AP-1 signaling in hypothalamic POMC neurons (PMID:32747560), undergoes proteasome-dependent C2C cleavage during adipocyte differentiation to seed a specialized ER cisterna for lipid-droplet biogenesis (PMID:34693607), and directly interacts with STING to activate cGAS-STING type I interferon signaling (PMID:39103908).

1. 2007 High

   Established the domain logic of E-Syt3 — how an ER protein engages membranes and lipids — by defining a Ca2+/phospholipid-binding C2A domain and a C2C domain that targets the protein toward the plasma membrane.

   Evidence In vitro Ca2+-dependent phospholipid binding with recombinant fragments plus domain-swap/deletion analysis in transfected cells

   PMID:17360437

   Open questions at the time

   • Did not establish a physiological ligand or contact partner at the PM
   • C2C targeting mechanism (the PM lipid recognized) left undefined
2. 2013 High

   Resolved how E-Syt3 functions in cells by showing it tethers ER to PM via PI(4,5)P2-dependent C2 interactions and forms Ca2+-regulated heteromers with E-Syt1/2, while excluding a requirement for these contacts in SOCE.

   Evidence Live-cell imaging of ER-PM contacts, reciprocal co-IP, PI(4,5)P2 manipulation, and SOCE measurements

   PMID:23791178

   Open questions at the time

   • Did not identify what is transported or signaled at the contacts
   • Stoichiometry of E-Syt1/2/3 heteromers not defined
3. 2014 Medium

   Provided the first organismal phenotype for the E-Syt2/E-Syt3 pair, linking them to cell migration and survival under oxidative/stringent stress.

   Evidence esyt2/esyt3 double-knockout mice and MEF migration and oxidative-stress survival assays

   PMID:25486202

   Open questions at the time

   • Did not separate E-Syt3-specific from E-Syt2-specific contributions
   • Molecular basis connecting contact sites to migration/stress unresolved
4. 2015 Medium

   Connected E-Syt3 to receptor signaling by showing it (with E-Syt2) selectively interacts with activated FGFR1 in a conformation-dependent, autophosphorylation-independent manner via TM-adjacent sequences that also mediate E-Syt dimerization.

   Evidence Co-IP in transfected and embryo cells with kinase-dead and conformation-specific receptor mutants and deletion constructs

   PMID:25922075

   Open questions at the time

   • Functional consequence of the E-Syt3–FGFR1 interaction for signaling not established
   • Single-lab Co-IP without orthogonal interaction method
5. 2017 Medium

   Placed E-Syt3 within an upstream regulatory circuit, showing RASSF4 controls its ER-PM localization through ARF6/PIP5K-dependent PM PI(4,5)P2 levels.

   Evidence RASSF4 siRNA, live imaging of E-Syt3 at contacts, PM PI(4,5)P2 measurement, and ARF6 activity assays

   PMID:28600435

   Open questions at the time

   • Whether RASSF4 acts directly on E-Syt3 versus only on lipid environment unresolved
   • Single lab
6. 2017 Medium

   Demonstrated a downstream output of E-Syt3 contact function — supporting plasma-membrane localization and current of the Ca2+-activated chloride channel ANO1.

   Evidence siRNA knockdown with ANO1 trafficking microscopy and electrophysiology

   PMID:29154949

   Open questions at the time

   • Mechanism coupling ER-PM contacts to ANO1 surface delivery not defined
   • Redundancy with E-Syt1/2 not dissected
7. 2017 Medium

   Implicated E-Syt3 in host defense as a negative regulator of HSV-1 membrane fusion, release, entry, spread, and syncytia formation.

   Evidence Knockdown/overexpression in HSV-1-infected cells with viral titer, plaque, entry, and syncytia readouts

   PMID:29046455

   Open questions at the time

   • Molecular target through which E-Syt3 restrains fusion unidentified
   • Contribution of contact-site lipids versus protein interactions unclear
8. 2020 High

   Revealed a neuroendocrine role: hypothalamic E-Syt3 promotes diet-induced obesity by limiting POMC processing to α-MSH and modulating PKC/AP-1 signaling.

   Evidence Whole-body and POMC-neuron conditional knockout mice with α-MSH, PKC, AP-1 reporter, prohormone convertase, and metabolic readouts

   PMID:32747560

   Open questions at the time

   • How an ER-PM contact protein controls prohormone convertase expression mechanistically unresolved
   • Link between contact-site lipid function and PKC/AP-1 activity not defined
9. 2021 Medium

   Showed E-Syt3 undergoes proteasome-dependent C2C cleavage in differentiating adipocytes, generating a truncated form that localizes to a specialized ER cisterna nucleating lipid-droplet biogenesis.

   Evidence Confocal and live time-lapse imaging, proteasome inhibitors, ΔC2C constructs, electron tomography, and siRNA with LD biogenesis readout

   PMID:34693607

   Open questions at the time

   • Identity of the protease/E3 ligase driving C2C cleavage unknown
   • How E-Syt3ΔC2C shapes the primordial cisterna mechanistically unresolved
10. 2024 Medium

    Linked E-Syt3 to innate immune signaling, showing it directly interacts with STING and activates cGAS-STING type I IFN/chemokine output and radiosensitizes lung adenocarcinoma.

    Evidence Co-IP and co-immunofluorescence with STING, pathway and cytokine assays, overexpression/KD, and in vivo tumor radiotherapy models

    PMID:39103908

    Open questions at the time

    • Whether the interaction is direct versus contact-site-mediated not confirmed by reciprocal/structural methods
    • How an ER-PM tether activates STING mechanistically unresolved
11. 2025 High

    Defined E-Syt3 as a phosphatidylserine transfer protein at ER/PM junctions whose PtdSer removal dissociates a cAMP complex to restrain CFTR and NBCe1-B, with depletion improving epithelial chloride and fluid secretion in vivo.

    Evidence Lipid transfer assays, C2C deletion, signaling-complex co-IP, PtdSer sensors, CFTR/NBCe1-B electrophysiology, and salivary/pancreatic secretion in E-Syt3-depleted mice

    PMID:40425857

    Open questions at the time

    • Directionality and net flux of PtdSer transfer in vivo not quantified
    • How PtdSer levels mechanistically assemble/disassemble the cAMP complex unresolved

• It remains unresolved how a single ER-PM tethering and lipid-transfer protein produces such divergent outputs — channel trafficking, prohormone processing, lipid-droplet biogenesis, antiviral restriction, and STING activation — and whether these reflect distinct lipid cargoes, tissue-specific partners, or proteolytic isoforms.
• No unifying mechanism connecting contact-site lipid transfer to the tissue-specific signaling phenotypes
• Structural basis of C2-domain lipid selectivity not determined
• E-Syt1/2/3 redundancy across these functions not systematically dissected