{"gene":"GRIA4","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2025,"finding":"Cryo-EM structures of GluA4:TARP-γ2 complex trapped in active, resting, and desensitized states reveal that GluA4 AMPARs adopt a canonical Y-shaped conformation, open the ion channel via asymmetric hinging of all channel helices upon glutamate binding, and that TARP-γ2 has a regulatory site in the ligand-binding domain that modulates gating kinetics.","method":"Cryo-electron microscopy (cryo-EM) structures with functional validation","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures covering full gating cycle with identification of TARP regulatory site","pmids":["40954371"],"is_preprint":false},{"year":2026,"finding":"Cryo-EM of GluA4 AMPARs shows a canonical Y-shaped architecture with domain-swapped NTD/LBD dimer pairs; all four LBDs bind glutamate yet ion channel opening occurs by asymmetric hinging of all four channel helices, and LBD conformational plasticity under saturating glutamate tunes subconductance states.","method":"Cryo-electron microscopy (cryo-EM) combined with single-channel bilayer recordings","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures with bilayer electrophysiology, multiple orthogonal methods","pmids":["41656278"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM of GluA4 alone reveals classical Y-shaped conformation; in resting conditions GluA4:TARP-γ2 adopts two conformations, one resembling the desensitized state of other GluA subunits, indicating subunit-specific structural dynamics.","method":"Cryo-electron microscopy (cryo-EM)","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 1 method but preprint; findings largely corroborated by published peer-reviewed paper (PMID 41656278)","pmids":["40667226"],"is_preprint":true},{"year":2003,"finding":"GluA4 (GluR-D) surface expression requires a 14-residue cytoplasmic C-terminal segment that mediates interaction with 4.1 family proteins; point mutations within this segment abolish both 4.1 binding and surface expression of homomeric GluA4 receptors.","method":"C-terminal deletion analysis, GST pull-down, co-immunoprecipitation from HEK293 cells and rat brain, surface ELISA","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus GST pulldown plus mutagenesis in two cell systems","pmids":["12574408"],"is_preprint":false},{"year":1999,"finding":"The N-terminal extracellular domain (X domain) of GluA4 mediates dimerization of the receptor ectodomain, whereas the S1S2 ligand-binding domain is monomeric; the X domain does not itself bind AMPA or glutamate.","method":"Hydrodynamic analysis (gel filtration, sedimentation) of recombinant soluble ectodomain fragments expressed in insect cells; radiolabeled ligand binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical reconstitution with multiple orthogonal methods","pmids":["10506139"],"is_preprint":false},{"year":1996,"finding":"The ligand-binding domain of GluA4 (GluR-D) expressed as a soluble S1-S2 fusion protein in E. coli binds [3H]AMPA with high affinity (Kd ~60 nM) in a pharmacology typical of native AMPA receptors; N-glycosylation is not required for formation or maintenance of the ligand-binding site; deletion of the C-terminal one-third of S2 abolishes binding.","method":"Bacterial expression of S1-S2 fusion protein, [3H]AMPA radioligand binding assay, deletion mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with mutagenesis","pmids":["8663017"],"is_preprint":false},{"year":1998,"finding":"A disulfide bond between conserved cysteines C260 and C315 exists in the ligand-binding domain of GluA4 (GluR-D); this disulfide is inaccessible to DTT in the intact receptor, explaining insensitivity to redox modulation. Single C260S and C315S mutants show 2-3-fold higher ligand affinity, and mutants lacking the disulfide show non-native oligomerization and dramatically reduced specific activity, indicating the disulfide stabilizes the ligand-binding domain.","method":"Biochemical disulfide detection, site-directed mutagenesis, ligand binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with biochemical assays","pmids":["9737972"],"is_preprint":false},{"year":2002,"finding":"Site-directed mutagenesis of GluA4 ligand-binding domain identifies Leu-672 and Thr-677 in helix F (lobe 2) as critical for binding all agonists; mutations at Asp-673, Ser-674, Gly-675, Ser-676, and Lys-678 selectively affect specific agonists. In contrast, antagonist ([3H]Ro 48-8587, DNQX) binding is unaffected by any of these mutations, demonstrating selective engagement of helix F side chains in agonist binding and suggesting conformational changes in this region underlie receptor activation.","method":"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand competition binding assays, ligand docking","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with radioligand binding and computational docking","pmids":["12167621"],"is_preprint":false},{"year":2002,"finding":"Mutagenesis of GluA4 ligand-binding domain shows that R507 is essential for both agonist and antagonist binding (even conservative R507K abolishes binding), while E727 is essential for agonist binding but not for antagonist binding, revealing differential ionic interactions in agonist vs. antagonist recognition.","method":"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays, ligand docking","journal":"European journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with radioligand assays","pmids":["12473122"],"is_preprint":false},{"year":2010,"finding":"Native GluA4's C-terminal PDZ motif is blocked by a conserved proline residue; deletion of this proline confers avid binding to SAP97. Mass spectrometric analysis of native brain GluA4 confirms the C-terminus is intact (proline not cleaved), so GluA4 does not engage canonical PDZ interactions and its association with SAP97 in vivo is indirect.","method":"Co-immunoprecipitation, mass spectrometry, generation of proline-deleted mutant and antibody against cleaved C-terminus","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1-2 — Co-IP plus mass spectrometry confirming intact C-terminus","pmids":["20090852"],"is_preprint":false},{"year":2009,"finding":"Ethanol concentration-dependently accelerates desensitization of GluA4 (GluR-D) homomeric receptors; co-expression of TARPs (stargazin/γ4) slows desensitization onset and increases steady-state current, and potentiates the ethanol-induced increase in desensitization rate. γ4 also slows recovery from desensitization but ethanol does not affect this step.","method":"Whole-cell electrophysiology in HEK293 cells expressing recombinant GluA4 ± TARPs, with ethanol application","journal":"Alcohol","confidence":"Medium","confidence_rationale":"Tier 2 — electrophysiological assay in heterologous system, single lab","pmids":["19560629"],"is_preprint":false},{"year":2019,"finding":"Both stargazin (γ-2) and γ-4 TARPs slow GluA4 channel opening (kop) and closing (kcl) rates ~3-4 fold without changing channel-opening probability; γ-4 more strongly slows desensitization while γ-2 produces a larger left-shift in glutamate dose-response relationship.","method":"Laser-pulse photolysis rapid-perfusion electrophysiology measuring channel-opening and closing rate constants","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinetic assay with defined rate constants, rigorous mechanistic resolution","pmids":["31267004"],"is_preprint":false},{"year":2014,"finding":"GluA4 expression in immature CA1 pyramidal neurons is sufficient to confer PKA-dependent LTP; PKA activation drives synaptic insertion of GluA4-containing AMPARs. In GluA4-deficient mice, neonatal PKA-dependent LTP is abolished. Lentiviral re-expression of GluA4 at any developmental stage restores PKA-dependent synaptic potentiation, establishing GluA4 as the molecular determinant of the developmental switch in LTP kinase dependency from PKA to CaMKII.","method":"GluA4-knockout mice, lentiviral GluA4 expression, whole-cell electrophysiology, pharmacological kinase manipulation","journal":"PNAS","confidence":"High","confidence_rationale":"Tier 2 — KO plus lentiviral rescue with defined electrophysiological phenotype","pmids":["24599589"],"is_preprint":false},{"year":2016,"finding":"PKA activation drives insertion of GluA4 to synaptic sites with weak or silent AMPAR-mediated transmission; this requires the extreme C-terminal end of GluA4 which interacts with the membrane-proximal region of its own C-terminal domain to control trafficking. GluA4-deficient mice show significantly delayed strengthening of AMPAR-mediated transmission during postnatal development.","method":"Electrophysiology at CA1 synapses, GluA4-KO mice, C-terminal domain deletion/interaction analysis, PKA pharmacology","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — KO mice with electrophysiology plus domain-interaction mapping","pmids":["27157711"],"is_preprint":false},{"year":2014,"finding":"SAP97 forms the molecular backbone for sequential delivery of GluA4-containing AMPARs to synapses during classical conditioning; conditioning induces formation of a SAP97-KSR1/PKC-GluA4 complex that delivers GluA4 to synapses via a SAP97-PSD95 interaction.","method":"Co-immunoprecipitation, in vitro classical conditioning model, pharmacological kinase inhibition","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP with conditioning-induced complex formation, single lab","pmids":["24567325"],"is_preprint":false},{"year":2008,"finding":"Gria4 knockout mice exhibit frequent spike-wave discharges (absence seizures). In Gria4 mutants, synaptic excitation of inhibitory reticular thalamic neurons is enhanced with increased duration of synaptic responses, consistent with loss of the kinetically faster AMPA receptor subunit encoded by Gria4. Gria3 loss in contrast lowers SWD, establishing genetic epistasis between Gria4 and Gria3 in thalamic circuit synchrony.","method":"Gria4 knockout mice, EEG recording, whole-cell electrophysiology, genetic epistasis (Gria3/Gria4 double mutant)","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — KO mice with electrophysiology and genetic epistasis, replicated with natural mutation","pmids":["18316356"],"is_preprint":false},{"year":2011,"finding":"Gria4-deficient mice show selective reduction in synaptic strength at the cortico-reticular thalamic (nRT) projection but not at the cortico-TC pathway; this reveals that cortico-thalamo-cortical oscillations can be initiated via a cortico-TC-nRT-TC bypass pathway when direct cortico-nRT excitation is weakened.","method":"Gria4-KO mice, in vivo optogenetics, whole-cell electrophysiology, EEG","journal":"Nature neuroscience","confidence":"High","confidence_rationale":"Tier 2 — KO with optogenetic circuit dissection and electrophysiology, high-impact replicated findings","pmids":["21857658"],"is_preprint":false},{"year":2013,"finding":"Repeated morphine treatment causes synaptic insertion of GluA4-containing (Ca2+-permeable) AMPARs in spinal cord dorsal horn laminae III-V; co-immunoprecipitation shows increased GluA4 homomers in the postsynaptic density; intracellular infusion of GluA4 antibody via patch pipette reverses increased EPSC rectification, directly implicating GluA4-containing AMPARs in morphine-induced mechanical hypersensitivity.","method":"Co-immunoprecipitation, western blotting of PSD fractions, whole-cell electrophysiology with Ca2+-permeable AMPAR blocker and intracellular antibody, immunohistochemistry","journal":"Neuropsychopharmacology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including intracellular antibody blockade confirming GluA4 role","pmids":["23403695"],"is_preprint":false},{"year":2021,"finding":"GluA4-knockout mice show ~80% reduction in mossy fiber-to-granule cell synaptic transmission in the cerebellum, with decreased fidelity of granule cell spike output despite compensatory increases in NMDA receptor-mediated transmission and reduced tonic inhibition; GluA4-KO mice fail eyeblink conditioning but retain locomotor coordination, demonstrating an essential role for GluA4 in cerebellar expansion coding and associative memory formation.","method":"GluA4-KO mice, whole-cell electrophysiology at mossy fiber-granule cell synapses, computational network modeling, behavioral eyeblink conditioning","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — KO with electrophysiology, modeling, and behavioral assay with defined phenotypic readout","pmids":["34219651"],"is_preprint":false},{"year":2015,"finding":"Virus-mediated knockdown of GluA4 in POm relay neurons almost abolishes EPSC amplitude at the L5B-POm giant corticothalamic synapse, strongly delaying onset of postsynaptic action potential generation, establishing GluA4 as the primary AMPAR subunit responsible for the large-amplitude driver EPSCs at this synapse.","method":"Virus-mediated genetic knockdown, direct electrical stimulation of single corticothalamic terminals, whole-cell recording","journal":"European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — virus KD with direct synapse electrophysiology","pmids":["26390982"],"is_preprint":false},{"year":2016,"finding":"Acoustic trauma (AT) decreases GluA4 mRNA and increases GluA1 mRNA in the lateral superior olive, slowing AMPAR-EPSC decay kinetics; this change in subunit composition (replacement of fast GluA4 by slow GluA1) compensates for hearing loss by prolonging EPSCs to maintain binaural function, as confirmed by computational modeling.","method":"Voltage-clamp electrophysiology, RT-PCR, auditory brainstem responses, computational modeling","journal":"The Journal of physiology","confidence":"Medium","confidence_rationale":"Tier 2 — electrophysiology with mRNA quantification and modeling, but KO not performed","pmids":["27104476"],"is_preprint":false},{"year":2017,"finding":"Quantitative freeze-fracture replica immunogold labeling shows GluA4 subunits are present at higher number and density at auditory nerve-fusiform cell synapses versus auditory nerve-bushy cell synapses, where GluA3 predominates; GluA4 immunogold is homogeneously distributed along both synapse types, while GluA3 is concentrated centrally at AN-BC synapses.","method":"Quantitative freeze-fracture replica immunogold labeling, GluA3-KO mice","journal":"Brain structure & function","confidence":"High","confidence_rationale":"Tier 2 — quantitative ultrastructural immunogold with KO controls","pmids":["28397107"],"is_preprint":false},{"year":2017,"finding":"De novo variants in the SYTANLAAF motif of GluA4 transmembrane domain M3 that face the pore center are predicted by molecular modeling to disturb gating mechanism; a fourth SYTANLAAF variant reduces permeability; an extracellular domain variant interferes with monomer-monomer binding, establishing structural mechanisms for pathogenic GluA4 gain-of-function variants.","method":"Whole-exome sequencing, molecular modeling of variant positions in transmembrane domain","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 3-4 — molecular modeling without functional electrophysiology validation","pmids":["29220673"],"is_preprint":false},{"year":2016,"finding":"GluA4-deficient mice lack both homeostatic upregulation of glutamatergic transmission in neonatal CA3 (triggered by 15-h TTX blockade) and Hebbian weakening of AMPAR transmission in CA1 following attenuation of correlated bursting, demonstrating that GluA4 mediates both homeostatic and Hebbian plasticity mechanisms at immature hippocampal synapses.","method":"GluA4-KO mice, whole-cell electrophysiology, TTX activity blockade paradigm","journal":"Journal of neurophysiology","confidence":"High","confidence_rationale":"Tier 2 — KO mice with electrophysiology and defined plasticity paradigms","pmids":["26961102"],"is_preprint":false}],"current_model":"GluA4 (GRIA4) is an AMPA-type ionotropic glutamate receptor subunit with a canonical Y-shaped extracellular architecture in which NTD/LBD dimer pairs are domain-swapped; its ion channel opens via asymmetric hinging of all four channel helices upon glutamate binding, with LBD conformational plasticity tuning subconductance states. Surface trafficking requires a 14-residue C-terminal segment that binds 4.1 family proteins, while its conserved C-terminal proline precludes canonical PDZ interactions with SAP97. TARPs (stargazin/γ4) slow both channel opening and closing rates and modulate desensitization kinetics via a site in the ligand-binding domain. Functionally, GluA4 is the predominant fast-kinetics AMPAR subunit in reticular thalamic neurons, cerebellar granule cells, auditory relay neurons, and immature hippocampal pyramidal neurons, where PKA-driven synaptic insertion of GluA4 defines the mechanism for neonatal LTP, silent synapse activation, and homeostatic plasticity during critical periods of circuit development."},"narrative":{"teleology":[{"year":1996,"claim":"Establishing that the GluA4 ligand-binding domain can be reconstituted as a soluble S1-S2 fusion with native-like AMPA pharmacology showed that ligand recognition is an intrinsic property of this domain, independent of the full receptor and N-glycosylation.","evidence":"Bacterial expression of S1-S2 fusion protein with [3H]AMPA radioligand binding and deletion mutagenesis","pmids":["8663017"],"confidence":"High","gaps":["No channel function measured from isolated domain","Agonist vs. antagonist binding determinants not yet mapped"]},{"year":1998,"claim":"Identification of a buried C260–C315 disulfide bond in the ligand-binding domain resolved why GluA4 is insensitive to redox modulation and showed the disulfide constrains domain conformation to maintain proper oligomerization and binding activity.","evidence":"Biochemical disulfide detection, site-directed mutagenesis, ligand binding assays","pmids":["9737972"],"confidence":"High","gaps":["No structural data at atomic resolution","Whether the disulfide functions identically in heteromeric assemblies was untested"]},{"year":1999,"claim":"Demonstrating that the N-terminal (X) domain mediates dimerization while the S1-S2 domain is monomeric established the modular architecture of GluA4 ectodomain assembly, separating oligomerization from ligand binding.","evidence":"Hydrodynamic analysis of recombinant ectodomain fragments; radiolabeled ligand binding","pmids":["10506139"],"confidence":"High","gaps":["Tetrameric assembly mechanism not yet resolved","No structural visualization of the full ectodomain"]},{"year":2002,"claim":"Systematic mutagenesis of helix F (lobe 2) and interdomain ionic residues delineated which side chains in the LBD selectively engage agonists versus antagonists, providing a mechanistic basis for activation-specific conformational changes.","evidence":"Site-directed mutagenesis with [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays and computational docking","pmids":["12167621","12473122"],"confidence":"High","gaps":["No electrophysiological correlate of mutant function","No crystal structure of LBD with bound ligand at this time"]},{"year":2003,"claim":"Identifying a 14-residue C-terminal segment that binds protein 4.1 family members and is required for surface expression revealed the primary trafficking determinant for GluA4-containing AMPARs, distinct from PDZ-based mechanisms used by other subunits.","evidence":"C-terminal deletion analysis, GST pull-down, co-IP from HEK293 cells and rat brain, surface ELISA","pmids":["12574408"],"confidence":"High","gaps":["Which 4.1 family member is most relevant in vivo unknown","Whether this segment also controls synaptic versus extrasynaptic localization was untested"]},{"year":2008,"claim":"The finding that Gria4-knockout mice exhibit absence seizures with enhanced reticular thalamic neuron excitation established GluA4 as essential for thalamocortical circuit synchrony and linked it to epilepsy pathogenesis.","evidence":"Gria4-KO mice with EEG, electrophysiology, and genetic epistasis with Gria3","pmids":["18316356"],"confidence":"High","gaps":["Cell-type-specific rescue not performed","Human genetic confirmation not yet available at the time"]},{"year":2010,"claim":"Demonstrating that a conserved C-terminal proline blocks canonical PDZ binding to SAP97, with mass spectrometry confirming the proline is not cleaved in vivo, resolved conflicting reports of GluA4–SAP97 association and showed this interaction is necessarily indirect.","evidence":"Co-IP, mass spectrometry, proline-deletion mutant","pmids":["20090852"],"confidence":"High","gaps":["The bridging molecule mediating indirect SAP97 association not identified"]},{"year":2011,"claim":"Optogenetic circuit dissection in Gria4-KO mice revealed that weakened cortico-nRT excitation enables a bypass cortico-TC-nRT-TC pathway to initiate oscillations, explaining how selective loss of GluA4 at one synapse type reorganizes thalamocortical dynamics.","evidence":"In vivo optogenetics, whole-cell electrophysiology, EEG in Gria4-KO mice","pmids":["21857658"],"confidence":"High","gaps":["Whether compensatory changes in other receptor types contribute was not excluded"]},{"year":2014,"claim":"Showing that GluA4 is both necessary and sufficient for neonatal PKA-dependent LTP in CA1—and that lentiviral re-expression at any age restores it—established GluA4 as the molecular determinant of the developmental switch in LTP kinase dependency from PKA to CaMKII.","evidence":"GluA4-KO mice, lentiviral rescue, electrophysiology, PKA pharmacology","pmids":["24599589","27157711"],"confidence":"High","gaps":["Phosphorylation site(s) on GluA4 mediating PKA-driven insertion not identified","Whether GluA4 is sufficient in non-hippocampal circuits unknown"]},{"year":2016,"claim":"GluA4-KO mice lacking both homeostatic upregulation and Hebbian weakening at neonatal hippocampal synapses demonstrated that GluA4 is the shared effector for both forms of early-life plasticity, not just Hebbian LTP.","evidence":"GluA4-KO mice with TTX activity-blockade paradigm, whole-cell electrophysiology","pmids":["26961102"],"confidence":"High","gaps":["Downstream signaling cascade linking activity blockade to GluA4 insertion not mapped"]},{"year":2019,"claim":"Kinetic measurements showing TARPs γ-2 and γ-4 slow GluA4 opening and closing rates 3–4-fold without changing open probability quantified how auxiliary subunits tune GluA4 channel kinetics, with γ-4 preferentially slowing desensitization.","evidence":"Laser-pulse photolysis rapid-perfusion electrophysiology measuring kop and kcl","pmids":["31267004"],"confidence":"High","gaps":["Structural basis of differential TARP effects unknown at the time"]},{"year":2021,"claim":"~80% loss of mossy fiber-to-granule cell transmission in GluA4-KO mice with failure of eyeblink conditioning established GluA4 as the essential AMPAR subunit for cerebellar expansion coding and associative motor learning.","evidence":"GluA4-KO mice, electrophysiology at mossy fiber–granule cell synapses, computational modeling, eyeblink conditioning","pmids":["34219651"],"confidence":"High","gaps":["Whether GluA4 loss affects Purkinje cell-dependent plasticity not tested","Contribution of heteromeric GluA4 assemblies versus homomers not resolved"]},{"year":2025,"claim":"Cryo-EM structures of GluA4 alone and in complex with TARP-γ2 across resting, active, and desensitized states revealed that ion channel opening occurs via asymmetric hinging of all four channel helices, that LBD conformational plasticity tunes subconductance states, and that TARP-γ2 engages a specific regulatory site in the LBD.","evidence":"Cryo-EM structures with functional validation by single-channel bilayer recordings","pmids":["40954371","41656278"],"confidence":"High","gaps":["Structures of GluA4 heteromeric assemblies (e.g. GluA2/4) not yet solved","How TARP regulatory site engagement translates to kinetic rate changes at atomic level not fully modeled"]},{"year":null,"claim":"Key unresolved questions include the specific PKA phosphorylation site(s) on GluA4 that drive synaptic insertion, the identity of the bridging partner enabling indirect GluA4–SAP97 association in vivo, and whether GluA4 heteromeric complexes adopt distinct structural conformations compared to homomers.","evidence":"","pmids":[],"confidence":"Low","gaps":["PKA phosphorylation site on GluA4 not mapped","Bridging molecule for SAP97 association unknown","No cryo-EM of GluA4-containing heteromeric AMPARs"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,11,15,18,19]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[10,11]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,13,17]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[15,16,18,19,20]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[12,13,23]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,1,5]}],"complexes":["AMPA receptor (GluA4 homomer)","GluA4:TARP-γ2 complex"],"partners":["TARP-Γ2 (STARGAZIN/CACNG2)","TARP-Γ4 (CACNG4)","EPB41 (PROTEIN 4.1)","SAP97 (DLG1)","KSR1"],"other_free_text":[]},"mechanistic_narrative":"GRIA4 encodes GluA4, an AMPA-type ionotropic glutamate receptor subunit that serves as the principal fast-kinetics AMPAR at multiple central synapses—including reticular thalamic neurons, cerebellar granule cells, auditory relay nuclei, and immature hippocampal pyramidal cells—where it is essential for high-fidelity synaptic transmission, circuit synchrony, and developmental plasticity [PMID:18316356, PMID:34219651, PMID:26390982, PMID:24599589]. Structurally, GluA4 assembles into a Y-shaped tetramer with domain-swapped NTD/LBD dimer pairs; the N-terminal domain mediates dimerization while the S1-S2 ligand-binding domain binds glutamate with high affinity, stabilized by a buried C260–C315 disulfide bond, and ion channel opening proceeds through asymmetric hinging of all four channel helices with LBD conformational plasticity tuning subconductance states [PMID:41656278, PMID:10506139, PMID:9737972, PMID:8663017]. TARP auxiliary subunits (stargazin/γ-4) engage a regulatory site within the ligand-binding domain to slow both opening and closing rates and modulate desensitization kinetics; surface trafficking requires a C-terminal 14-residue segment that binds protein 4.1 family members, while a conserved C-terminal proline precludes canonical PDZ interactions with SAP97 [PMID:40954371, PMID:31267004, PMID:12574408, PMID:20090852]. In neonatal hippocampus, PKA-driven synaptic insertion of GluA4 defines the mechanism for early-life LTP and homeostatic plasticity, and Gria4 loss causes absence seizures linked to altered thalamocortical synchrony [PMID:24599589, PMID:26961102, PMID:18316356]."},"prefetch_data":{"uniprot":{"accession":"P48058","full_name":"Glutamate receptor 4","aliases":["AMPA-selective glutamate receptor 4","GluR-D","Glutamate receptor ionotropic, AMPA 4"],"length_aa":902,"mass_kda":100.9,"function":"Ionotropic glutamate receptor that functions as a ligand-gated cation channel, gated by L-glutamate and glutamatergic agonists such as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid, and kainic acid (By similarity). L-glutamate acts as an excitatory neurotransmitter at many synapses in the central nervous system and plays an important role in fast excitatory synaptic transmission (By similarity). Binding of the excitatory neurotransmitter L-glutamate induces a conformation change, leading to the opening of the cation channel, and thereby converts the chemical signal to an electrical impulse upon entry of monovalent and divalent cations such as sodium and calcium. The receptor then desensitizes rapidly and enters a transient inactive state, characterized by the presence of bound agonist (By similarity). In the presence of CACNG8, shows resensitization which is characterized by a delayed accumulation of current flux upon continued application of L-glutamate (PubMed:21172611)","subcellular_location":"Cell membrane; Postsynaptic cell membrane; Cell projection, dendrite; Postsynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/P48058/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GRIA4","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GRIA4","total_profiled":1310},"omim":[{"mim_id":"617864","title":"NEURODEVELOPMENTAL DISORDER WITH OR WITHOUT SEIZURES AND GAIT ABNORMALITIES; NEDSGA","url":"https://www.omim.org/entry/617864"},{"mim_id":"138248","title":"GLUTAMATE RECEPTOR, IONOTROPIC, AMPA 1; 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all four LBDs bind glutamate yet ion channel opening occurs by asymmetric hinging of all four channel helices, and LBD conformational plasticity under saturating glutamate tunes subconductance states.\",\n      \"method\": \"Cryo-electron microscopy (cryo-EM) combined with single-channel bilayer recordings\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures with bilayer electrophysiology, multiple orthogonal methods\",\n      \"pmids\": [\"41656278\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM of GluA4 alone reveals classical Y-shaped conformation; in resting conditions GluA4:TARP-γ2 adopts two conformations, one resembling the desensitized state of other GluA subunits, indicating subunit-specific structural dynamics.\",\n      \"method\": \"Cryo-electron microscopy (cryo-EM)\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 method but preprint; findings largely corroborated by published peer-reviewed paper (PMID 41656278)\",\n      \"pmids\": [\"40667226\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"GluA4 (GluR-D) surface expression requires a 14-residue cytoplasmic C-terminal segment that mediates interaction with 4.1 family proteins; point mutations within this segment abolish both 4.1 binding and surface expression of homomeric GluA4 receptors.\",\n      \"method\": \"C-terminal deletion analysis, GST pull-down, co-immunoprecipitation from HEK293 cells and rat brain, surface ELISA\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus GST pulldown plus mutagenesis in two cell systems\",\n      \"pmids\": [\"12574408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The N-terminal extracellular domain (X domain) of GluA4 mediates dimerization of the receptor ectodomain, whereas the S1S2 ligand-binding domain is monomeric; the X domain does not itself bind AMPA or glutamate.\",\n      \"method\": \"Hydrodynamic analysis (gel filtration, sedimentation) of recombinant soluble ectodomain fragments expressed in insect cells; radiolabeled ligand binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical reconstitution with multiple orthogonal methods\",\n      \"pmids\": [\"10506139\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The ligand-binding domain of GluA4 (GluR-D) expressed as a soluble S1-S2 fusion protein in E. coli binds [3H]AMPA with high affinity (Kd ~60 nM) in a pharmacology typical of native AMPA receptors; N-glycosylation is not required for formation or maintenance of the ligand-binding site; deletion of the C-terminal one-third of S2 abolishes binding.\",\n      \"method\": \"Bacterial expression of S1-S2 fusion protein, [3H]AMPA radioligand binding assay, deletion mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with mutagenesis\",\n      \"pmids\": [\"8663017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A disulfide bond between conserved cysteines C260 and C315 exists in the ligand-binding domain of GluA4 (GluR-D); this disulfide is inaccessible to DTT in the intact receptor, explaining insensitivity to redox modulation. Single C260S and C315S mutants show 2-3-fold higher ligand affinity, and mutants lacking the disulfide show non-native oligomerization and dramatically reduced specific activity, indicating the disulfide stabilizes the ligand-binding domain.\",\n      \"method\": \"Biochemical disulfide detection, site-directed mutagenesis, ligand binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with biochemical assays\",\n      \"pmids\": [\"9737972\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Site-directed mutagenesis of GluA4 ligand-binding domain identifies Leu-672 and Thr-677 in helix F (lobe 2) as critical for binding all agonists; mutations at Asp-673, Ser-674, Gly-675, Ser-676, and Lys-678 selectively affect specific agonists. In contrast, antagonist ([3H]Ro 48-8587, DNQX) binding is unaffected by any of these mutations, demonstrating selective engagement of helix F side chains in agonist binding and suggesting conformational changes in this region underlie receptor activation.\",\n      \"method\": \"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand competition binding assays, ligand docking\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with radioligand binding and computational docking\",\n      \"pmids\": [\"12167621\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Mutagenesis of GluA4 ligand-binding domain shows that R507 is essential for both agonist and antagonist binding (even conservative R507K abolishes binding), while E727 is essential for agonist binding but not for antagonist binding, revealing differential ionic interactions in agonist vs. antagonist recognition.\",\n      \"method\": \"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays, ligand docking\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with radioligand assays\",\n      \"pmids\": [\"12473122\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Native GluA4's C-terminal PDZ motif is blocked by a conserved proline residue; deletion of this proline confers avid binding to SAP97. Mass spectrometric analysis of native brain GluA4 confirms the C-terminus is intact (proline not cleaved), so GluA4 does not engage canonical PDZ interactions and its association with SAP97 in vivo is indirect.\",\n      \"method\": \"Co-immunoprecipitation, mass spectrometry, generation of proline-deleted mutant and antibody against cleaved C-terminus\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — Co-IP plus mass spectrometry confirming intact C-terminus\",\n      \"pmids\": [\"20090852\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Ethanol concentration-dependently accelerates desensitization of GluA4 (GluR-D) homomeric receptors; co-expression of TARPs (stargazin/γ4) slows desensitization onset and increases steady-state current, and potentiates the ethanol-induced increase in desensitization rate. γ4 also slows recovery from desensitization but ethanol does not affect this step.\",\n      \"method\": \"Whole-cell electrophysiology in HEK293 cells expressing recombinant GluA4 ± TARPs, with ethanol application\",\n      \"journal\": \"Alcohol\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — electrophysiological assay in heterologous system, single lab\",\n      \"pmids\": [\"19560629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Both stargazin (γ-2) and γ-4 TARPs slow GluA4 channel opening (kop) and closing (kcl) rates ~3-4 fold without changing channel-opening probability; γ-4 more strongly slows desensitization while γ-2 produces a larger left-shift in glutamate dose-response relationship.\",\n      \"method\": \"Laser-pulse photolysis rapid-perfusion electrophysiology measuring channel-opening and closing rate constants\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinetic assay with defined rate constants, rigorous mechanistic resolution\",\n      \"pmids\": [\"31267004\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GluA4 expression in immature CA1 pyramidal neurons is sufficient to confer PKA-dependent LTP; PKA activation drives synaptic insertion of GluA4-containing AMPARs. In GluA4-deficient mice, neonatal PKA-dependent LTP is abolished. Lentiviral re-expression of GluA4 at any developmental stage restores PKA-dependent synaptic potentiation, establishing GluA4 as the molecular determinant of the developmental switch in LTP kinase dependency from PKA to CaMKII.\",\n      \"method\": \"GluA4-knockout mice, lentiviral GluA4 expression, whole-cell electrophysiology, pharmacological kinase manipulation\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO plus lentiviral rescue with defined electrophysiological phenotype\",\n      \"pmids\": [\"24599589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PKA activation drives insertion of GluA4 to synaptic sites with weak or silent AMPAR-mediated transmission; this requires the extreme C-terminal end of GluA4 which interacts with the membrane-proximal region of its own C-terminal domain to control trafficking. GluA4-deficient mice show significantly delayed strengthening of AMPAR-mediated transmission during postnatal development.\",\n      \"method\": \"Electrophysiology at CA1 synapses, GluA4-KO mice, C-terminal domain deletion/interaction analysis, PKA pharmacology\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with electrophysiology plus domain-interaction mapping\",\n      \"pmids\": [\"27157711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SAP97 forms the molecular backbone for sequential delivery of GluA4-containing AMPARs to synapses during classical conditioning; conditioning induces formation of a SAP97-KSR1/PKC-GluA4 complex that delivers GluA4 to synapses via a SAP97-PSD95 interaction.\",\n      \"method\": \"Co-immunoprecipitation, in vitro classical conditioning model, pharmacological kinase inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP with conditioning-induced complex formation, single lab\",\n      \"pmids\": [\"24567325\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Gria4 knockout mice exhibit frequent spike-wave discharges (absence seizures). In Gria4 mutants, synaptic excitation of inhibitory reticular thalamic neurons is enhanced with increased duration of synaptic responses, consistent with loss of the kinetically faster AMPA receptor subunit encoded by Gria4. Gria3 loss in contrast lowers SWD, establishing genetic epistasis between Gria4 and Gria3 in thalamic circuit synchrony.\",\n      \"method\": \"Gria4 knockout mice, EEG recording, whole-cell electrophysiology, genetic epistasis (Gria3/Gria4 double mutant)\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with electrophysiology and genetic epistasis, replicated with natural mutation\",\n      \"pmids\": [\"18316356\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Gria4-deficient mice show selective reduction in synaptic strength at the cortico-reticular thalamic (nRT) projection but not at the cortico-TC pathway; this reveals that cortico-thalamo-cortical oscillations can be initiated via a cortico-TC-nRT-TC bypass pathway when direct cortico-nRT excitation is weakened.\",\n      \"method\": \"Gria4-KO mice, in vivo optogenetics, whole-cell electrophysiology, EEG\",\n      \"journal\": \"Nature neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with optogenetic circuit dissection and electrophysiology, high-impact replicated findings\",\n      \"pmids\": [\"21857658\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Repeated morphine treatment causes synaptic insertion of GluA4-containing (Ca2+-permeable) AMPARs in spinal cord dorsal horn laminae III-V; co-immunoprecipitation shows increased GluA4 homomers in the postsynaptic density; intracellular infusion of GluA4 antibody via patch pipette reverses increased EPSC rectification, directly implicating GluA4-containing AMPARs in morphine-induced mechanical hypersensitivity.\",\n      \"method\": \"Co-immunoprecipitation, western blotting of PSD fractions, whole-cell electrophysiology with Ca2+-permeable AMPAR blocker and intracellular antibody, immunohistochemistry\",\n      \"journal\": \"Neuropsychopharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including intracellular antibody blockade confirming GluA4 role\",\n      \"pmids\": [\"23403695\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GluA4-knockout mice show ~80% reduction in mossy fiber-to-granule cell synaptic transmission in the cerebellum, with decreased fidelity of granule cell spike output despite compensatory increases in NMDA receptor-mediated transmission and reduced tonic inhibition; GluA4-KO mice fail eyeblink conditioning but retain locomotor coordination, demonstrating an essential role for GluA4 in cerebellar expansion coding and associative memory formation.\",\n      \"method\": \"GluA4-KO mice, whole-cell electrophysiology at mossy fiber-granule cell synapses, computational network modeling, behavioral eyeblink conditioning\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with electrophysiology, modeling, and behavioral assay with defined phenotypic readout\",\n      \"pmids\": [\"34219651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Virus-mediated knockdown of GluA4 in POm relay neurons almost abolishes EPSC amplitude at the L5B-POm giant corticothalamic synapse, strongly delaying onset of postsynaptic action potential generation, establishing GluA4 as the primary AMPAR subunit responsible for the large-amplitude driver EPSCs at this synapse.\",\n      \"method\": \"Virus-mediated genetic knockdown, direct electrical stimulation of single corticothalamic terminals, whole-cell recording\",\n      \"journal\": \"European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — virus KD with direct synapse electrophysiology\",\n      \"pmids\": [\"26390982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Acoustic trauma (AT) decreases GluA4 mRNA and increases GluA1 mRNA in the lateral superior olive, slowing AMPAR-EPSC decay kinetics; this change in subunit composition (replacement of fast GluA4 by slow GluA1) compensates for hearing loss by prolonging EPSCs to maintain binaural function, as confirmed by computational modeling.\",\n      \"method\": \"Voltage-clamp electrophysiology, RT-PCR, auditory brainstem responses, computational modeling\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — electrophysiology with mRNA quantification and modeling, but KO not performed\",\n      \"pmids\": [\"27104476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Quantitative freeze-fracture replica immunogold labeling shows GluA4 subunits are present at higher number and density at auditory nerve-fusiform cell synapses versus auditory nerve-bushy cell synapses, where GluA3 predominates; GluA4 immunogold is homogeneously distributed along both synapse types, while GluA3 is concentrated centrally at AN-BC synapses.\",\n      \"method\": \"Quantitative freeze-fracture replica immunogold labeling, GluA3-KO mice\",\n      \"journal\": \"Brain structure & function\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — quantitative ultrastructural immunogold with KO controls\",\n      \"pmids\": [\"28397107\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"De novo variants in the SYTANLAAF motif of GluA4 transmembrane domain M3 that face the pore center are predicted by molecular modeling to disturb gating mechanism; a fourth SYTANLAAF variant reduces permeability; an extracellular domain variant interferes with monomer-monomer binding, establishing structural mechanisms for pathogenic GluA4 gain-of-function variants.\",\n      \"method\": \"Whole-exome sequencing, molecular modeling of variant positions in transmembrane domain\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3-4 — molecular modeling without functional electrophysiology validation\",\n      \"pmids\": [\"29220673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GluA4-deficient mice lack both homeostatic upregulation of glutamatergic transmission in neonatal CA3 (triggered by 15-h TTX blockade) and Hebbian weakening of AMPAR transmission in CA1 following attenuation of correlated bursting, demonstrating that GluA4 mediates both homeostatic and Hebbian plasticity mechanisms at immature hippocampal synapses.\",\n      \"method\": \"GluA4-KO mice, whole-cell electrophysiology, TTX activity blockade paradigm\",\n      \"journal\": \"Journal of neurophysiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with electrophysiology and defined plasticity paradigms\",\n      \"pmids\": [\"26961102\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GluA4 (GRIA4) is an AMPA-type ionotropic glutamate receptor subunit with a canonical Y-shaped extracellular architecture in which NTD/LBD dimer pairs are domain-swapped; its ion channel opens via asymmetric hinging of all four channel helices upon glutamate binding, with LBD conformational plasticity tuning subconductance states. Surface trafficking requires a 14-residue C-terminal segment that binds 4.1 family proteins, while its conserved C-terminal proline precludes canonical PDZ interactions with SAP97. TARPs (stargazin/γ4) slow both channel opening and closing rates and modulate desensitization kinetics via a site in the ligand-binding domain. Functionally, GluA4 is the predominant fast-kinetics AMPAR subunit in reticular thalamic neurons, cerebellar granule cells, auditory relay neurons, and immature hippocampal pyramidal neurons, where PKA-driven synaptic insertion of GluA4 defines the mechanism for neonatal LTP, silent synapse activation, and homeostatic plasticity during critical periods of circuit development.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GRIA4 encodes GluA4, an AMPA-type ionotropic glutamate receptor subunit that serves as the principal fast-kinetics AMPAR at multiple central synapses—including reticular thalamic neurons, cerebellar granule cells, auditory relay nuclei, and immature hippocampal pyramidal cells—where it is essential for high-fidelity synaptic transmission, circuit synchrony, and developmental plasticity [PMID:18316356, PMID:34219651, PMID:26390982, PMID:24599589]. Structurally, GluA4 assembles into a Y-shaped tetramer with domain-swapped NTD/LBD dimer pairs; the N-terminal domain mediates dimerization while the S1-S2 ligand-binding domain binds glutamate with high affinity, stabilized by a buried C260–C315 disulfide bond, and ion channel opening proceeds through asymmetric hinging of all four channel helices with LBD conformational plasticity tuning subconductance states [PMID:41656278, PMID:10506139, PMID:9737972, PMID:8663017]. TARP auxiliary subunits (stargazin/γ-4) engage a regulatory site within the ligand-binding domain to slow both opening and closing rates and modulate desensitization kinetics; surface trafficking requires a C-terminal 14-residue segment that binds protein 4.1 family members, while a conserved C-terminal proline precludes canonical PDZ interactions with SAP97 [PMID:40954371, PMID:31267004, PMID:12574408, PMID:20090852]. In neonatal hippocampus, PKA-driven synaptic insertion of GluA4 defines the mechanism for early-life LTP and homeostatic plasticity, and Gria4 loss causes absence seizures linked to altered thalamocortical synchrony [PMID:24599589, PMID:26961102, PMID:18316356].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that the GluA4 ligand-binding domain can be reconstituted as a soluble S1-S2 fusion with native-like AMPA pharmacology showed that ligand recognition is an intrinsic property of this domain, independent of the full receptor and N-glycosylation.\",\n      \"evidence\": \"Bacterial expression of S1-S2 fusion protein with [3H]AMPA radioligand binding and deletion mutagenesis\",\n      \"pmids\": [\"8663017\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No channel function measured from isolated domain\", \"Agonist vs. antagonist binding determinants not yet mapped\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identification of a buried C260–C315 disulfide bond in the ligand-binding domain resolved why GluA4 is insensitive to redox modulation and showed the disulfide constrains domain conformation to maintain proper oligomerization and binding activity.\",\n      \"evidence\": \"Biochemical disulfide detection, site-directed mutagenesis, ligand binding assays\",\n      \"pmids\": [\"9737972\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural data at atomic resolution\", \"Whether the disulfide functions identically in heteromeric assemblies was untested\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrating that the N-terminal (X) domain mediates dimerization while the S1-S2 domain is monomeric established the modular architecture of GluA4 ectodomain assembly, separating oligomerization from ligand binding.\",\n      \"evidence\": \"Hydrodynamic analysis of recombinant ectodomain fragments; radiolabeled ligand binding\",\n      \"pmids\": [\"10506139\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tetrameric assembly mechanism not yet resolved\", \"No structural visualization of the full ectodomain\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Systematic mutagenesis of helix F (lobe 2) and interdomain ionic residues delineated which side chains in the LBD selectively engage agonists versus antagonists, providing a mechanistic basis for activation-specific conformational changes.\",\n      \"evidence\": \"Site-directed mutagenesis with [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays and computational docking\",\n      \"pmids\": [\"12167621\", \"12473122\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No electrophysiological correlate of mutant function\", \"No crystal structure of LBD with bound ligand at this time\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identifying a 14-residue C-terminal segment that binds protein 4.1 family members and is required for surface expression revealed the primary trafficking determinant for GluA4-containing AMPARs, distinct from PDZ-based mechanisms used by other subunits.\",\n      \"evidence\": \"C-terminal deletion analysis, GST pull-down, co-IP from HEK293 cells and rat brain, surface ELISA\",\n      \"pmids\": [\"12574408\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which 4.1 family member is most relevant in vivo unknown\", \"Whether this segment also controls synaptic versus extrasynaptic localization was untested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"The finding that Gria4-knockout mice exhibit absence seizures with enhanced reticular thalamic neuron excitation established GluA4 as essential for thalamocortical circuit synchrony and linked it to epilepsy pathogenesis.\",\n      \"evidence\": \"Gria4-KO mice with EEG, electrophysiology, and genetic epistasis with Gria3\",\n      \"pmids\": [\"18316356\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cell-type-specific rescue not performed\", \"Human genetic confirmation not yet available at the time\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrating that a conserved C-terminal proline blocks canonical PDZ binding to SAP97, with mass spectrometry confirming the proline is not cleaved in vivo, resolved conflicting reports of GluA4–SAP97 association and showed this interaction is necessarily indirect.\",\n      \"evidence\": \"Co-IP, mass spectrometry, proline-deletion mutant\",\n      \"pmids\": [\"20090852\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The bridging molecule mediating indirect SAP97 association not identified\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Optogenetic circuit dissection in Gria4-KO mice revealed that weakened cortico-nRT excitation enables a bypass cortico-TC-nRT-TC pathway to initiate oscillations, explaining how selective loss of GluA4 at one synapse type reorganizes thalamocortical dynamics.\",\n      \"evidence\": \"In vivo optogenetics, whole-cell electrophysiology, EEG in Gria4-KO mice\",\n      \"pmids\": [\"21857658\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether compensatory changes in other receptor types contribute was not excluded\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that GluA4 is both necessary and sufficient for neonatal PKA-dependent LTP in CA1—and that lentiviral re-expression at any age restores it—established GluA4 as the molecular determinant of the developmental switch in LTP kinase dependency from PKA to CaMKII.\",\n      \"evidence\": \"GluA4-KO mice, lentiviral rescue, electrophysiology, PKA pharmacology\",\n      \"pmids\": [\"24599589\", \"27157711\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Phosphorylation site(s) on GluA4 mediating PKA-driven insertion not identified\", \"Whether GluA4 is sufficient in non-hippocampal circuits unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"GluA4-KO mice lacking both homeostatic upregulation and Hebbian weakening at neonatal hippocampal synapses demonstrated that GluA4 is the shared effector for both forms of early-life plasticity, not just Hebbian LTP.\",\n      \"evidence\": \"GluA4-KO mice with TTX activity-blockade paradigm, whole-cell electrophysiology\",\n      \"pmids\": [\"26961102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling cascade linking activity blockade to GluA4 insertion not mapped\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Kinetic measurements showing TARPs γ-2 and γ-4 slow GluA4 opening and closing rates 3–4-fold without changing open probability quantified how auxiliary subunits tune GluA4 channel kinetics, with γ-4 preferentially slowing desensitization.\",\n      \"evidence\": \"Laser-pulse photolysis rapid-perfusion electrophysiology measuring kop and kcl\",\n      \"pmids\": [\"31267004\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of differential TARP effects unknown at the time\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"~80% loss of mossy fiber-to-granule cell transmission in GluA4-KO mice with failure of eyeblink conditioning established GluA4 as the essential AMPAR subunit for cerebellar expansion coding and associative motor learning.\",\n      \"evidence\": \"GluA4-KO mice, electrophysiology at mossy fiber–granule cell synapses, computational modeling, eyeblink conditioning\",\n      \"pmids\": [\"34219651\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether GluA4 loss affects Purkinje cell-dependent plasticity not tested\", \"Contribution of heteromeric GluA4 assemblies versus homomers not resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-EM structures of GluA4 alone and in complex with TARP-γ2 across resting, active, and desensitized states revealed that ion channel opening occurs via asymmetric hinging of all four channel helices, that LBD conformational plasticity tunes subconductance states, and that TARP-γ2 engages a specific regulatory site in the LBD.\",\n      \"evidence\": \"Cryo-EM structures with functional validation by single-channel bilayer recordings\",\n      \"pmids\": [\"40954371\", \"41656278\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures of GluA4 heteromeric assemblies (e.g. GluA2/4) not yet solved\", \"How TARP regulatory site engagement translates to kinetic rate changes at atomic level not fully modeled\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the specific PKA phosphorylation site(s) on GluA4 that drive synaptic insertion, the identity of the bridging partner enabling indirect GluA4–SAP97 association in vivo, and whether GluA4 heteromeric complexes adopt distinct structural conformations compared to homomers.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"PKA phosphorylation site on GluA4 not mapped\", \"Bridging molecule for SAP97 association unknown\", \"No cryo-EM of GluA4-containing heteromeric AMPARs\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 11, 15, 18, 19]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [10, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 13, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0112316\", \"supporting_discovery_ids\": [15, 16, 18, 19, 20]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [15, 16, 18, 19, 20]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [12, 13, 23]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 5]}\n    ],\n    \"complexes\": [\n      \"AMPA receptor (GluA4 homomer)\",\n      \"GluA4:TARP-γ2 complex\"\n    ],\n    \"partners\": [\n      \"TARP-γ2 (stargazin/CACNG2)\",\n      \"TARP-γ4 (CACNG4)\",\n      \"EPB41 (protein 4.1)\",\n      \"SAP97 (DLG1)\",\n      \"KSR1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}