{"gene":"GRIK5","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":1992,"finding":"KA-2 (GRIK5) does not form functional homomeric ion channels when expressed alone, but coexpression with GluR5 or GluR6 subunits produces agonist-gated currents. GluR5(R)/KA-2 heteromers show more rapid desensitization and different current-voltage relations compared to GluR5(Q) homomers; GluR6/KA-2 heteromers are gated by AMPA, which fails to gate homomeric GluR6 channels.","method":"Heterologous expression in Xenopus oocytes or HEK cells with electrophysiological recording","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct electrophysiological reconstitution of heteromeric channels; foundational paper replicated by multiple subsequent studies","pmids":["1373632"],"is_preprint":false},{"year":1994,"finding":"KA2 and GluR6 subunits co-assemble into heteromeric complexes in both transfected cells and native rat brain, as demonstrated by co-immunoprecipitation with subunit-specific antibodies. Each antibody selectively immunoprecipitated [3H]kainate binding activity but not [3H]AMPA binding activity. GluR1 and GluR2 (AMPA subunits) also co-immunoprecipitated with GluR6 in co-transfected cells, but such mixed complexes appeared limited in brain.","method":"Co-immunoprecipitation from transfected HEK cells and detergent-solubilized rat brain membranes; immunoblot; deglycosylation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP in both heterologous cells and native brain with multiple orthogonal biochemical methods","pmids":["8288598"],"is_preprint":false},{"year":1994,"finding":"KA2 protein is localized postsynaptically in the rat nervous system, with immunostaining concentrated in postsynaptic densities apposed by unstained presynaptic terminals, and in associated dendrites.","method":"Immunohistochemistry and electron microscopic ultrastructural localization using affinity-purified anti-KA2 C-terminus antibody","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ultrastructural localization by EM immunostaining, single lab","pmids":["7852627"],"is_preprint":false},{"year":1995,"finding":"KA2 is expressed in hippocampal neurons, is enriched in dendritic spines after ~14 days in culture, and co-localizes with synaptophysin indicating synaptic localization. Co-immunoprecipitation showed no direct interaction between KA2 and AMPA subunit GluR1, confirming AMPA and kainate subunits do not co-assemble into mixed receptor complexes despite co-localizing at synapses.","method":"Immunocytochemistry in cultured hippocampal neurons; co-immunoprecipitation","journal":"Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct subcellular localization with functional inference; negative co-IP result also mechanistically informative; single lab","pmids":["8552236"],"is_preprint":false},{"year":1996,"finding":"Homomeric GluR6(R) channels have a very small unitary conductance (~231–264 fS by noise analysis). Heteromeric GluR6(R)/KA2 channels have a two- to threefold larger unitary conductance (~572 fS), different KA affinity (EC50 ~1.62 µM vs 0.47 µM for GluR6 homomers), and can be activated by AMPA unlike GluR6 homomers.","method":"Patch-clamp electrophysiology and noise analysis in transfected HEK293 cells","journal":"Journal of neurophysiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct in vitro electrophysiological characterization with noise analysis; rigorous single-lab study with multiple measures","pmids":["8836240"],"is_preprint":false},{"year":1997,"finding":"GluR5 and KA-2 subunits are co-expressed at high levels in trigeminal ganglion (TG) neurons and form functional heteromeric channels in vivo. Native kainate receptor channels in TG neurons closely resemble recombinant GluR5(R)/KA-2 heteromers in pharmacological properties, desensitization, rectification, ion permeability, and mean channel conductance.","method":"RT-PCR, whole-cell patch-clamp electrophysiology, pharmacological characterization of acutely dissociated TG neurons","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal methods (molecular + electrophysiological + pharmacological) in native neurons; single lab","pmids":["9254673"],"is_preprint":false},{"year":1997,"finding":"The rat grik5 gene has 20 exons spanning >54 kb. The first intron contains a negative regulatory element (within 500 bp of the 3'-end of intron 1) that inhibits transcription in an orientation- and distance-independent manner; a 24-nucleotide sequence within this region binds nuclear proteins from neural and non-neural cells.","method":"Reporter gene (CAT) assays, footprinting, gel shift assays, genomic library screening","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional reporter assays combined with DNA-protein binding assays; single lab","pmids":["9079693"],"is_preprint":false},{"year":2001,"finding":"The GRIK5 promoter is TATA-less and GC-rich with multiple consensus initiator sequences. Neural-cell-specific promoter activity maps to a 1200-bp 5'-flanking region; transcriptional activity involves a TFIID-containing complex on an initiator sequence 1100 bp upstream of the first intron. A 77-bp intragenic sequence within the first exon acts as a silencer in non-neural cells via an SP1-binding site and a neuron-restrictive silencer element-like sequence.","method":"Transgenic mouse lacZ reporter assays, transfection reporter assays in neural (CG-4) and non-neural cells, EMSA","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo transgenic reporter combined with cell-based assays and protein-DNA binding; single lab","pmids":["11533047"],"is_preprint":false},{"year":2003,"finding":"KA2 homomeric receptors fail to reach the plasma membrane and are retained in the endoplasmic reticulum (ER). This retention is mediated by discrete trafficking signals: an arginine-rich ER retention/retrieval motif and a di-leucine endocytic sequence in the C-terminus. Disruption of both motifs allows ER exit and surface expression of KA2 homomers that remain non-functional. The ER retention signal is sterically shielded upon heteromeric assembly, allowing delivery of functional heteromeric receptors to the plasma membrane.","method":"Surface biotinylation, immunofluorescence, mutagenesis, electrophysiology in heterologous expression systems and primary neurons","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (biotinylation, mutagenesis, IF, electrophysiology) in both heterologous and neuronal systems; replicated in principle by subsequent studies","pmids":["12878702"],"is_preprint":false},{"year":2003,"finding":"KA2 is retained in the ER when expressed alone; co-expression with GluR5-7 (but not with GluR1 or NR1) dramatically increases surface expression of KA2. Synaptic KARs from neocortex have a relatively high KA2 content compared to microsomal fractions, indicating preferential synaptic targeting of heteromeric KA2-containing receptors.","method":"Surface biotinylation, cobalt uptake assay, subcellular fractionation of neocortex in HEK293 cells","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical methods; single lab; confirms and extends findings of PMID 12878702","pmids":["12950450"],"is_preprint":false},{"year":2003,"finding":"In KA2-/- (GluK5 knockout) mice, presynaptic mossy-fiber kainate receptor facilitation by heterosynaptic spillover of glutamate from CA3 collaterals is absent, while homosynaptic facilitation is normal. Postsynaptic kainate-mediated EPSCs show shorter half-decay times. These results identify KA2 as a determinant of both presynaptic affinity for glutamate and postsynaptic EPSC kinetics at mossy-fiber-CA3 synapses.","method":"Electrophysiological recordings in hippocampal slices from KA2-/- knockout mice","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean genetic knockout with defined synaptic phenotypes at pre- and postsynaptic sites; foundational loss-of-function study","pmids":["12533602"],"is_preprint":false},{"year":2003,"finding":"A chimeric reporter approach identified an ER-retention motif within the KA2 cytosolic C-terminal domain (RRRRR stretch). However, mutation of this arginine motif alone (with alternating glutamic acid residues) was insufficient to disrupt ER-retention of KA2, suggesting a unique mechanism distinct from the NR1 RRR motif.","method":"Chimeric reporter protein, mutagenesis, immunofluorescence in heterologous cells","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — single lab, single primary method with chimeric reporter; partially superseded by more complete analysis in PMID 12878702 and 16807331","pmids":["14511640"],"is_preprint":false},{"year":2006,"finding":"An additional ER retention motif in the intracellular loop region of KA2 was identified. Mutation of this loop motif together with C-terminal motifs significantly increases KA2 surface expression; however, surface-expressed KA2 homomers still do not form functional ion channels. In GluR6 knockout mice, native KA2 surface expression is dramatically reduced, whereas it is unaffected in GluR5 knockout mice, demonstrating that GluR6 oligomerization is specifically required for KA2 ER egress and cell-surface transport.","method":"Site-directed mutagenesis, immunofluorescence, surface biotinylation, electrophysiology, knockout mouse analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods including in vivo knockout validation; identifies intracellular loop motif and in vivo GluR6 dependence","pmids":["16807331"],"is_preprint":false},{"year":2006,"finding":"COPI vesicle coat subunits co-immunoprecipitate with KA2 subunits from cerebellum and COS-7 cells; β-COP protein interacts directly with KA2 peptides containing the arginine-rich retention/retrieval determinant. Alanine substitution of this signal reduces COPI–KA2 association. Assembly of heteromeric GluR6a/KA2 receptors reduces COPI binding and concomitantly increases association with 14-3-3 proteins, which mediate forward trafficking, representing a checkpoint for functional KAR biosynthesis.","method":"Co-immunoprecipitation from cerebellum and COS-7 cells, GST peptide pulldown, temperature-sensitive COPI degradation, surface localization assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP in both native tissue and cells, direct peptide binding, multiple orthogonal methods identifying COPI and 14-3-3 as sequential trafficking checkpoints","pmids":["16595684"],"is_preprint":false},{"year":2010,"finding":"Crystal structures of the GluK5 amino-terminal domain (ATD) were determined at high resolution. GluK5 ATD crystallizes as a dimer with a strikingly different dimer assembly at the R1 interface compared to GluK3 ATD, while the R2 domain dimer assembly is similar to other non-NMDA iGluRs. The aberrant R1 dimer interface is consistent with GluK5 being unable to form functional homomeric channels and requiring obligate co-assembly with GluK1-3 subunits.","method":"X-ray crystallography of isolated ATD domain","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — high-resolution crystal structure with structural interpretation of assembly incompatibility; single lab but rigorous structural method","pmids":["20951142"],"is_preprint":false},{"year":2012,"finding":"GluK2 and GluK5 subunits assemble into heterotetrameric receptors with a fixed 2:2 stoichiometry, as determined by direct single-molecule counting of each subunit type in the plasma membrane of live cells.","method":"Single-molecule imaging (single-molecule bleaching step counting) in live cell plasma membranes","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — single-molecule reconstitution approach with direct subunit counting; rigorous quantitative method; single lab","pmids":["22509486"],"is_preprint":false},{"year":2013,"finding":"CaMKII phosphorylates three residues in the C-terminal domain of GluK5, markedly increasing lateral mobility of kainate receptors (KARs), likely by decreasing GluK5 binding to PSD-95. CaMKII activation promotes surface expression of KARs at extrasynaptic sites but decreases synaptic KAR content. This CaMKII-dependent phosphorylation of GluK5 is necessary for long-term depression of KAR-mediated responses at hippocampal mossy fiber synapses (KAR-LTD), established using a molecular replacement strategy.","method":"Molecular replacement, lateral mobility (single-particle tracking/FRAP), phosphorylation assays, electrophysiology at mossy fiber synapses in hippocampal slices","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct phosphorylation identified with molecular replacement validation and multiple orthogonal methods (biochemistry, live imaging, electrophysiology)","pmids":["23288040"],"is_preprint":false},{"year":2013,"finding":"Ligand binding to the GluK5 subunit (not GluK2) is both necessary and sufficient for surface trafficking of heteromeric GluK2/GluK5 receptors. Mutations reducing agonist affinity in GluK5 prevent functional surface expression and cannot be rescued by wild-type GluK2 or antagonist, whereas GluK2 binding-site mutations can be rescued by co-assembly with wild-type GluK5.","method":"Site-directed mutagenesis of ligand-binding domain, surface expression assays, electrophysiology in heterologous cells","journal":"Cellular and molecular neurobiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis with functional readout (surface expression + electrophysiology); single lab, two orthogonal methods","pmids":["23975096"],"is_preprint":false},{"year":2014,"finding":"Domoate causes long-lasting inhibition of the GluK5 subunit in heteromeric GluK2/K5 receptors: brief domoate exposure prevents GluK5 activation by other agonists for several minutes. A mutation in GluK5 that reduces agonist binding affinity prevents this inhibition. Domoate-bound GluK2/K5 heteromers can still be fully activated through the GluK2 subunit, demonstrating that the two subunits within the tetramer can function independently to open the ion channel.","method":"Electrophysiology (whole-cell recordings) in heterologous cells expressing wild-type or mutant recombinant kainate receptors","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — direct electrophysiological characterization with mutagenesis; single lab, single primary method","pmids":["24859608"],"is_preprint":false},{"year":2017,"finding":"GluK5 (GRIK5) is localized presynaptically to the ribbon synapses of rod photoreceptor terminals in the mouse retina. In GluK5-deficient mice, the structural integrity of synaptic ribbons is severely altered, indicating a novel role for GluK5 in organizing presynaptic ribbon structure.","method":"Double-immunofluorescence, electron microscopy, GluK5 knockout mice analysis","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ultrastructural localization combined with knockout phenotyping; single lab, two complementary methods","pmids":["28235022"],"is_preprint":false},{"year":2018,"finding":"Two ER retention signals in the GluK5 C-terminus (arginine-based signal and di-leucine motif) are physically masked by SAP97 in the presence of CASK: SAP97 adopts an extended conformation making its SH3 and GuK domains available to bind and sterically block both ER retention signals. SAP97 and CASK are necessary for sorting GluK2/GluK5 receptor complexes into the local dendritic secretory pathway in neurons, rather than the somatic Golgi pathway.","method":"Co-immunoprecipitation, conformational analysis, mutagenesis, neuronal trafficking assays","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with domain mapping and neuronal trafficking assays; single lab, multiple methods but abstract-level detail only","pmids":["30339823"],"is_preprint":false},{"year":2019,"finding":"smFRET studies established that GluK2 and GluK5 subunits arrange within the heterotetrameric receptor in a defined 'dimer of dimers' configuration. The heteromeric GluK2/GluK5 receptor shows distinct conformational dynamics at the amino-terminal and agonist-binding domain interfaces compared to homomeric GluK2, particularly in resting versus desensitized states.","method":"Single-molecule FRET (smFRET) on full-length receptors","journal":"Biochimica et biophysica acta. Biomembranes","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — single-molecule structural/dynamic characterization of full-length receptor; single lab","pmids":["31194959"],"is_preprint":false},{"year":2007,"finding":"KA2 antisense knockdown in vivo suppresses the assembly of the GluR6/KA2-PSD95-MLK3 signaling module, inhibits JNK activation and c-jun phosphorylation, and increases neuronal survival in hippocampal CA1 after ischemia/reperfusion. Combined KA2 and GluR6 knockdown has greater neuroprotective effect than either alone, indicating functional cooperation between KA2 and GluR6 in mediating ischemic neuronal death through the PSD95-MLK3-MKK4/7-JNK3 pathway.","method":"Intracerebroventricular antisense oligodeoxynucleotide injection in rat ischemia model; Western blot for signaling components; neuronal survival counting","journal":"Journal of neuroscience research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo knockdown with defined molecular pathway readouts and neuronal survival; single lab, multiple biochemical endpoints","pmids":["17639597"],"is_preprint":false},{"year":2019,"finding":"Knockdown of the GRIK5 ortholog in zebrafish caused reduced blood vessel numbers, reduced vascular integrity in the eye, and increased vascular permeability, indicating a role for GRIK5 in eye vascular biology beyond classical glutamate receptor function.","method":"Morpholino knockdown in zebrafish, vascular imaging","journal":"American journal of human genetics","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single-organism knockdown in zebrafish with vascular phenotype but limited mechanistic follow-up; single lab","pmids":["30827500"],"is_preprint":false},{"year":2023,"finding":"In heteromeric GluK2/GluK5 receptors, partial agonism by AMPA is mediated primarily through the GluK2 subunit: AMPA induces intermediate cleft closure states at the GluK2 agonist-binding domain (between apo and full-agonist glutamate-bound conformations), while the GluK5 agonist-binding domain shows no significant difference in cleft closure between AMPA and glutamate conditions. Additionally, the agonist-binding domain dimer interface is not decoupled in the AMPA-bound state, unlike the glutamate-bound state.","method":"Single-molecule FRET (smFRET) on full-length heteromeric GluK2/GluK5 receptors","journal":"Proteins","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — smFRET with defined conformational states for each subunit; single lab","pmids":["37526035"],"is_preprint":false}],"current_model":"GRIK5 (GluK5/KA2) is a high-affinity kainate receptor subunit that cannot form functional homomeric ion channels because arginine-rich and di-leucine ER retention signals in its C-terminus and intracellular loop retain it in the ER; co-assembly with GluK2 (GluR6)—mediated by COPI release and 14-3-3/SAP97-CASK masking of retention signals—is required for ER egress and surface delivery of functional 2:2 GluK2/GluK5 heterotetramers, in which GluK5 confers high glutamate affinity, controls presynaptic facilitation kinetics and postsynaptic EPSC decay at mossy-fiber synapses, undergoes CaMKII phosphorylation on its C-terminal domain to drive lateral diffusion away from PSD-95 and produce KAR-LTD, and in rod photoreceptors occupies presynaptic ribbon terminals where its loss disrupts ribbon structural integrity."},"narrative":{"mechanistic_narrative":"GRIK5 (GluK5/KA2) is a high-affinity kainate receptor subunit that cannot form functional homomeric ion channels and instead functions as an obligate partner subunit shaping the properties of heteromeric kainate receptors that mediate excitatory neurotransmission [PMID:1373632, PMID:20951142]. Expressed alone, KA2 is retained in the endoplasmic reticulum through discrete trafficking signals—an arginine-rich ER retention/retrieval motif and a di-leucine motif in the C-terminus plus an additional motif in the intracellular loop—so that even forced surface expression yields non-functional homomers [PMID:12878702, PMID:16807331]; the structural basis lies in an aberrant R1 dimer interface of its amino-terminal domain incompatible with self-assembly [PMID:20951142]. ER egress requires obligate co-assembly with GluK2/GluR6 (not GluK1 or AMPA/NMDA subunits) into a fixed 2:2 'dimer of dimers' heterotetramer, which sterically masks the retention signals [PMID:12878702, PMID:16807331, PMID:22509486, PMID:31194959]. This trafficking is gated by a sequential checkpoint: β-COP/COPI binds the arginine-rich signal to retain unassembled subunits, and heteromeric assembly displaces COPI in favor of 14-3-3-mediated forward trafficking, with SAP97/CASK additionally masking the retention signals to route receptors into the dendritic secretory pathway [PMID:16595684, PMID:30339823]. Within the assembled receptor GluK5 confers high glutamate affinity and larger unitary conductance, and ligand binding through the GluK5 site is necessary and sufficient for surface delivery [PMID:8836240, PMID:23975096]. Functionally, KA2 determines presynaptic mossy-fiber facilitation and postsynaptic EPSC decay kinetics, and CaMKII phosphorylation of three C-terminal GluK5 residues reduces PSD-95 binding to increase receptor lateral mobility and drive kainate receptor long-term depression [PMID:12533602, PMID:23288040]. Beyond canonical signaling, GluK5 localizes to rod photoreceptor ribbon synapses where its loss disrupts ribbon structural integrity [PMID:28235022], and KA2 participates with GluR6 in a PSD95-MLK3-JNK signaling module mediating ischemic neuronal death [PMID:17639597].","teleology":[{"year":1992,"claim":"Established that KA2 is not an independent channel-forming subunit but modifies the gating and pharmacology of kainate receptors only when co-assembled, defining it as an accessory subunit.","evidence":"Heterologous expression in oocytes/HEK cells with electrophysiology comparing homomers and heteromers","pmids":["1373632"],"confidence":"High","gaps":["Stoichiometry and assembly mechanism unknown","Native receptor composition not yet established"]},{"year":1994,"claim":"Showed that KA2/GluR6 heteromeric complexes exist in native brain and that AMPA and kainate subunits do not appreciably co-assemble, defining the molecular boundary of native kainate receptor composition.","evidence":"Reciprocal co-immunoprecipitation from transfected cells and rat brain with radioligand binding","pmids":["8288598","8552236"],"confidence":"High","gaps":["Subunit stoichiometry not quantified","Trafficking determinants of assembly not addressed"]},{"year":1994,"claim":"Placed KA2 at the postsynaptic density and dendritic spines, linking the subunit to synaptic signaling sites.","evidence":"EM immunostaining and immunocytochemistry in rat nervous system and cultured hippocampal neurons","pmids":["7852627","8552236"],"confidence":"Medium","gaps":["Does not address later-described presynaptic localizations","Anchoring partners not identified at this stage"]},{"year":1996,"claim":"Quantified how KA2 incorporation alters single-channel conductance and agonist affinity, defining its contribution to receptor biophysics.","evidence":"Patch-clamp and noise analysis in transfected HEK293 cells","pmids":["8836240"],"confidence":"High","gaps":["Conductance measured in heterologous, not native, channels","Structural basis of conductance change unknown"]},{"year":1997,"claim":"Confirmed that recombinant GluR5/KA2 heteromers recapitulate native kainate channels in vivo, validating the heteromeric model in real neurons.","evidence":"RT-PCR, whole-cell patch-clamp and pharmacology in dissociated trigeminal ganglion neurons","pmids":["9254673"],"confidence":"High","gaps":["Does not establish trafficking control of native assembly","Limited to one neuronal population"]},{"year":1997,"claim":"Mapped transcriptional control elements of grik5, including intronic negative regulatory and neural-specific promoter/silencer elements governing its expression.","evidence":"Reporter assays, footprinting, EMSA and transgenic lacZ reporters","pmids":["9079693","11533047"],"confidence":"Medium","gaps":["Trans-acting factors only partially identified","Physiological regulation in vivo not demonstrated"]},{"year":2003,"claim":"Defined the molecular basis of obligate heteromerization: ER retention motifs in the C-terminus retain KA2 until heteromeric assembly masks them, explaining why KA2 cannot reach the surface alone.","evidence":"Surface biotinylation, mutagenesis, immunofluorescence and electrophysiology in heterologous cells and neurons","pmids":["12878702","12950450","14511640"],"confidence":"High","gaps":["Loop-region retention signal not yet found","Identity of proteins reading the retention signals unknown"]},{"year":2003,"claim":"Assigned physiological consequences to KA2 in vivo, showing it controls presynaptic mossy-fiber facilitation and postsynaptic EPSC decay kinetics.","evidence":"Slice electrophysiology in KA2-/- knockout mice","pmids":["12533602"],"confidence":"High","gaps":["Molecular mechanism of affinity contribution not resolved here","Behavioral consequences not addressed"]},{"year":2006,"claim":"Completed the trafficking model by identifying a second retention motif and the sequential COPI-to-14-3-3 checkpoint, and demonstrated genetically that GluR6 specifically drives KA2 surface delivery.","evidence":"Mutagenesis, biotinylation, peptide pulldown, COPI degradation, co-IP from cerebellum/COS-7 and knockout mouse analysis","pmids":["16807331","16595684"],"confidence":"High","gaps":["How heteromerization mechanically displaces COPI not fully resolved","14-3-3 binding site mapping incomplete"]},{"year":2007,"claim":"Linked the GluR6/KA2 module to a pathological signaling cascade, implicating it in ischemic neuronal death via PSD95-MLK3-JNK signaling.","evidence":"In vivo antisense knockdown in a rat ischemia model with Western blot and survival counting","pmids":["17639597"],"confidence":"Medium","gaps":["Antisense specificity not exhaustively controlled","Direct vs indirect role of KA2 in the module not dissected"]},{"year":2010,"claim":"Provided the structural explanation for assembly incompatibility, showing the GluK5 amino-terminal domain forms an aberrant R1 dimer interface.","evidence":"X-ray crystallography of the isolated GluK5 ATD","pmids":["20951142"],"confidence":"High","gaps":["Isolated domain may not reflect full-length assembly","Heteromeric interface not crystallized"]},{"year":2012,"claim":"Determined the fixed 2:2 GluK2:GluK5 stoichiometry of the heterotetramer, constraining all functional models.","evidence":"Single-molecule bleaching step counting in live-cell membranes","pmids":["22509486"],"confidence":"High","gaps":["Spatial arrangement of subunits not yet defined","Stoichiometry in native synapses not directly measured"]},{"year":2013,"claim":"Identified CaMKII phosphorylation of GluK5 as the molecular switch for kainate receptor LTD, coupling phosphorylation to reduced PSD-95 binding and increased lateral mobility.","evidence":"Molecular replacement, single-particle tracking/FRAP, phosphorylation assays and mossy-fiber electrophysiology","pmids":["23288040"],"confidence":"High","gaps":["Direct phospho-dependent PSD-95 affinity change not structurally resolved","Upstream activity triggers in vivo not defined"]},{"year":2013,"claim":"Established that agonist binding specifically through the GluK5 subunit is necessary and sufficient for heteromer surface trafficking, identifying a ligand-gated biosynthetic checkpoint.","evidence":"Ligand-binding domain mutagenesis with surface expression and electrophysiology in heterologous cells","pmids":["23975096"],"confidence":"Medium","gaps":["Single lab, two methods","Endogenous ligand source during biosynthesis unclear"]},{"year":2014,"claim":"Showed that GluK2 and GluK5 subunits can gate the channel independently and that domoate causes prolonged GluK5-specific inhibition, revealing functional autonomy within the tetramer.","evidence":"Whole-cell electrophysiology of wild-type and binding-affinity mutant recombinant receptors","pmids":["24859608"],"confidence":"Medium","gaps":["Single primary method","Structural basis of long-lasting inhibition unresolved"]},{"year":2017,"claim":"Revealed a non-canonical structural role for GluK5 at rod photoreceptor ribbon synapses, where it organizes presynaptic ribbon integrity.","evidence":"Double-immunofluorescence, EM and GluK5 knockout mouse retina analysis","pmids":["28235022"],"confidence":"Medium","gaps":["Molecular partners at the ribbon not identified","Whether channel function is required for the structural role unknown"]},{"year":2018,"claim":"Defined the SAP97/CASK mechanism that masks GluK5 retention signals and routes assembled receptors into the dendritic secretory pathway.","evidence":"Co-IP, conformational analysis, mutagenesis and neuronal trafficking assays","pmids":["30339823"],"confidence":"Medium","gaps":["Abstract-level mechanistic detail only","Interplay with COPI/14-3-3 checkpoint not integrated"]},{"year":2019,"claim":"Resolved the conformational arrangement and dynamics of GluK2/GluK5 heteromers, including a defined dimer-of-dimers geometry and subunit-specific responses to agonists.","evidence":"Single-molecule FRET on full-length receptors in resting, glutamate- and AMPA-bound states","pmids":["31194959","37526035"],"confidence":"Medium","gaps":["Single lab","Functional coupling of conformational states to gating not fully mapped"]},{"year":2019,"claim":"Implicated GRIK5 in eye vascular biology, expanding its potential roles beyond glutamatergic signaling.","evidence":"Morpholino knockdown in zebrafish with vascular imaging","pmids":["30827500"],"confidence":"Low","gaps":["Single-organism knockdown with limited mechanistic follow-up","Morpholino off-target effects not excluded","No mammalian validation"]},{"year":null,"claim":"How the channel-function, synaptic-organizing, and non-neural vascular roles of GRIK5 are mechanistically unified, and whether the ribbon-organizing and vascular roles require ion-channel activity, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the assembled heterotetramer","Channel-independent functions mechanistically undefined","Disease relevance of human GRIK5 variants not established in the corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,5,10]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,17]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[8,12,13]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[8,9,15]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[19]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,10,16]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[8,12,13,20]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[16,22]}],"complexes":["GluK2/GluK5 (GluR6/KA2) heterotetrameric kainate receptor","GluR6/KA2-PSD95-MLK3 signaling module"],"partners":["GRIK2","GRIK1","COPB1","14-3-3","DLG1","CASK","DLG4","CAMK2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q16478","full_name":"Glutamate receptor ionotropic, kainate 5","aliases":["Excitatory amino acid receptor 2","EAA2","Glutamate receptor KA-2","KA2"],"length_aa":980,"mass_kda":109.3,"function":"Ionotropic glutamate receptor that functions as a cation-permeable ligand-gated ion channel, gated by L-glutamate and the glutamatergic agonist kainic acid. Cannot form functional channels on its own and produces channel activity only in heteromeric assembly with GRIK1 and GRIK2 subunits (PubMed:1321949, PubMed:14511640, PubMed:8730589). Can form functional heteromeric receptors with GRIK3 (By similarity)","subcellular_location":"Cell membrane; Postsynaptic cell membrane; Presynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/Q16478/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GRIK5","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GRIK5","total_profiled":1310},"omim":[{"mim_id":"600283","title":"GLUTAMATE RECEPTOR, IONOTROPIC, KAINATE 5; GRIK5","url":"https://www.omim.org/entry/600283"},{"mim_id":"600282","title":"GLUTAMATE RECEPTOR, IONOTROPIC, KAINATE 4; GRIK4","url":"https://www.omim.org/entry/600282"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in 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GluR5(R)/KA-2 heteromers show more rapid desensitization and different current-voltage relations compared to GluR5(Q) homomers; GluR6/KA-2 heteromers are gated by AMPA, which fails to gate homomeric GluR6 channels.\",\n      \"method\": \"Heterologous expression in Xenopus oocytes or HEK cells with electrophysiological recording\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct electrophysiological reconstitution of heteromeric channels; foundational paper replicated by multiple subsequent studies\",\n      \"pmids\": [\"1373632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"KA2 and GluR6 subunits co-assemble into heteromeric complexes in both transfected cells and native rat brain, as demonstrated by co-immunoprecipitation with subunit-specific antibodies. Each antibody selectively immunoprecipitated [3H]kainate binding activity but not [3H]AMPA binding activity. GluR1 and GluR2 (AMPA subunits) also co-immunoprecipitated with GluR6 in co-transfected cells, but such mixed complexes appeared limited in brain.\",\n      \"method\": \"Co-immunoprecipitation from transfected HEK cells and detergent-solubilized rat brain membranes; immunoblot; deglycosylation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP in both heterologous cells and native brain with multiple orthogonal biochemical methods\",\n      \"pmids\": [\"8288598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"KA2 protein is localized postsynaptically in the rat nervous system, with immunostaining concentrated in postsynaptic densities apposed by unstained presynaptic terminals, and in associated dendrites.\",\n      \"method\": \"Immunohistochemistry and electron microscopic ultrastructural localization using affinity-purified anti-KA2 C-terminus antibody\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ultrastructural localization by EM immunostaining, single lab\",\n      \"pmids\": [\"7852627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"KA2 is expressed in hippocampal neurons, is enriched in dendritic spines after ~14 days in culture, and co-localizes with synaptophysin indicating synaptic localization. Co-immunoprecipitation showed no direct interaction between KA2 and AMPA subunit GluR1, confirming AMPA and kainate subunits do not co-assemble into mixed receptor complexes despite co-localizing at synapses.\",\n      \"method\": \"Immunocytochemistry in cultured hippocampal neurons; co-immunoprecipitation\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct subcellular localization with functional inference; negative co-IP result also mechanistically informative; single lab\",\n      \"pmids\": [\"8552236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Homomeric GluR6(R) channels have a very small unitary conductance (~231–264 fS by noise analysis). Heteromeric GluR6(R)/KA2 channels have a two- to threefold larger unitary conductance (~572 fS), different KA affinity (EC50 ~1.62 µM vs 0.47 µM for GluR6 homomers), and can be activated by AMPA unlike GluR6 homomers.\",\n      \"method\": \"Patch-clamp electrophysiology and noise analysis in transfected HEK293 cells\",\n      \"journal\": \"Journal of neurophysiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct in vitro electrophysiological characterization with noise analysis; rigorous single-lab study with multiple measures\",\n      \"pmids\": [\"8836240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"GluR5 and KA-2 subunits are co-expressed at high levels in trigeminal ganglion (TG) neurons and form functional heteromeric channels in vivo. Native kainate receptor channels in TG neurons closely resemble recombinant GluR5(R)/KA-2 heteromers in pharmacological properties, desensitization, rectification, ion permeability, and mean channel conductance.\",\n      \"method\": \"RT-PCR, whole-cell patch-clamp electrophysiology, pharmacological characterization of acutely dissociated TG neurons\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal methods (molecular + electrophysiological + pharmacological) in native neurons; single lab\",\n      \"pmids\": [\"9254673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The rat grik5 gene has 20 exons spanning >54 kb. The first intron contains a negative regulatory element (within 500 bp of the 3'-end of intron 1) that inhibits transcription in an orientation- and distance-independent manner; a 24-nucleotide sequence within this region binds nuclear proteins from neural and non-neural cells.\",\n      \"method\": \"Reporter gene (CAT) assays, footprinting, gel shift assays, genomic library screening\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional reporter assays combined with DNA-protein binding assays; single lab\",\n      \"pmids\": [\"9079693\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The GRIK5 promoter is TATA-less and GC-rich with multiple consensus initiator sequences. Neural-cell-specific promoter activity maps to a 1200-bp 5'-flanking region; transcriptional activity involves a TFIID-containing complex on an initiator sequence 1100 bp upstream of the first intron. A 77-bp intragenic sequence within the first exon acts as a silencer in non-neural cells via an SP1-binding site and a neuron-restrictive silencer element-like sequence.\",\n      \"method\": \"Transgenic mouse lacZ reporter assays, transfection reporter assays in neural (CG-4) and non-neural cells, EMSA\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo transgenic reporter combined with cell-based assays and protein-DNA binding; single lab\",\n      \"pmids\": [\"11533047\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"KA2 homomeric receptors fail to reach the plasma membrane and are retained in the endoplasmic reticulum (ER). This retention is mediated by discrete trafficking signals: an arginine-rich ER retention/retrieval motif and a di-leucine endocytic sequence in the C-terminus. Disruption of both motifs allows ER exit and surface expression of KA2 homomers that remain non-functional. The ER retention signal is sterically shielded upon heteromeric assembly, allowing delivery of functional heteromeric receptors to the plasma membrane.\",\n      \"method\": \"Surface biotinylation, immunofluorescence, mutagenesis, electrophysiology in heterologous expression systems and primary neurons\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (biotinylation, mutagenesis, IF, electrophysiology) in both heterologous and neuronal systems; replicated in principle by subsequent studies\",\n      \"pmids\": [\"12878702\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"KA2 is retained in the ER when expressed alone; co-expression with GluR5-7 (but not with GluR1 or NR1) dramatically increases surface expression of KA2. Synaptic KARs from neocortex have a relatively high KA2 content compared to microsomal fractions, indicating preferential synaptic targeting of heteromeric KA2-containing receptors.\",\n      \"method\": \"Surface biotinylation, cobalt uptake assay, subcellular fractionation of neocortex in HEK293 cells\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical methods; single lab; confirms and extends findings of PMID 12878702\",\n      \"pmids\": [\"12950450\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In KA2-/- (GluK5 knockout) mice, presynaptic mossy-fiber kainate receptor facilitation by heterosynaptic spillover of glutamate from CA3 collaterals is absent, while homosynaptic facilitation is normal. Postsynaptic kainate-mediated EPSCs show shorter half-decay times. These results identify KA2 as a determinant of both presynaptic affinity for glutamate and postsynaptic EPSC kinetics at mossy-fiber-CA3 synapses.\",\n      \"method\": \"Electrophysiological recordings in hippocampal slices from KA2-/- knockout mice\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean genetic knockout with defined synaptic phenotypes at pre- and postsynaptic sites; foundational loss-of-function study\",\n      \"pmids\": [\"12533602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A chimeric reporter approach identified an ER-retention motif within the KA2 cytosolic C-terminal domain (RRRRR stretch). However, mutation of this arginine motif alone (with alternating glutamic acid residues) was insufficient to disrupt ER-retention of KA2, suggesting a unique mechanism distinct from the NR1 RRR motif.\",\n      \"method\": \"Chimeric reporter protein, mutagenesis, immunofluorescence in heterologous cells\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — single lab, single primary method with chimeric reporter; partially superseded by more complete analysis in PMID 12878702 and 16807331\",\n      \"pmids\": [\"14511640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"An additional ER retention motif in the intracellular loop region of KA2 was identified. Mutation of this loop motif together with C-terminal motifs significantly increases KA2 surface expression; however, surface-expressed KA2 homomers still do not form functional ion channels. In GluR6 knockout mice, native KA2 surface expression is dramatically reduced, whereas it is unaffected in GluR5 knockout mice, demonstrating that GluR6 oligomerization is specifically required for KA2 ER egress and cell-surface transport.\",\n      \"method\": \"Site-directed mutagenesis, immunofluorescence, surface biotinylation, electrophysiology, knockout mouse analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods including in vivo knockout validation; identifies intracellular loop motif and in vivo GluR6 dependence\",\n      \"pmids\": [\"16807331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"COPI vesicle coat subunits co-immunoprecipitate with KA2 subunits from cerebellum and COS-7 cells; β-COP protein interacts directly with KA2 peptides containing the arginine-rich retention/retrieval determinant. Alanine substitution of this signal reduces COPI–KA2 association. Assembly of heteromeric GluR6a/KA2 receptors reduces COPI binding and concomitantly increases association with 14-3-3 proteins, which mediate forward trafficking, representing a checkpoint for functional KAR biosynthesis.\",\n      \"method\": \"Co-immunoprecipitation from cerebellum and COS-7 cells, GST peptide pulldown, temperature-sensitive COPI degradation, surface localization assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP in both native tissue and cells, direct peptide binding, multiple orthogonal methods identifying COPI and 14-3-3 as sequential trafficking checkpoints\",\n      \"pmids\": [\"16595684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structures of the GluK5 amino-terminal domain (ATD) were determined at high resolution. GluK5 ATD crystallizes as a dimer with a strikingly different dimer assembly at the R1 interface compared to GluK3 ATD, while the R2 domain dimer assembly is similar to other non-NMDA iGluRs. The aberrant R1 dimer interface is consistent with GluK5 being unable to form functional homomeric channels and requiring obligate co-assembly with GluK1-3 subunits.\",\n      \"method\": \"X-ray crystallography of isolated ATD domain\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — high-resolution crystal structure with structural interpretation of assembly incompatibility; single lab but rigorous structural method\",\n      \"pmids\": [\"20951142\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"GluK2 and GluK5 subunits assemble into heterotetrameric receptors with a fixed 2:2 stoichiometry, as determined by direct single-molecule counting of each subunit type in the plasma membrane of live cells.\",\n      \"method\": \"Single-molecule imaging (single-molecule bleaching step counting) in live cell plasma membranes\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — single-molecule reconstitution approach with direct subunit counting; rigorous quantitative method; single lab\",\n      \"pmids\": [\"22509486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CaMKII phosphorylates three residues in the C-terminal domain of GluK5, markedly increasing lateral mobility of kainate receptors (KARs), likely by decreasing GluK5 binding to PSD-95. CaMKII activation promotes surface expression of KARs at extrasynaptic sites but decreases synaptic KAR content. This CaMKII-dependent phosphorylation of GluK5 is necessary for long-term depression of KAR-mediated responses at hippocampal mossy fiber synapses (KAR-LTD), established using a molecular replacement strategy.\",\n      \"method\": \"Molecular replacement, lateral mobility (single-particle tracking/FRAP), phosphorylation assays, electrophysiology at mossy fiber synapses in hippocampal slices\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct phosphorylation identified with molecular replacement validation and multiple orthogonal methods (biochemistry, live imaging, electrophysiology)\",\n      \"pmids\": [\"23288040\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Ligand binding to the GluK5 subunit (not GluK2) is both necessary and sufficient for surface trafficking of heteromeric GluK2/GluK5 receptors. Mutations reducing agonist affinity in GluK5 prevent functional surface expression and cannot be rescued by wild-type GluK2 or antagonist, whereas GluK2 binding-site mutations can be rescued by co-assembly with wild-type GluK5.\",\n      \"method\": \"Site-directed mutagenesis of ligand-binding domain, surface expression assays, electrophysiology in heterologous cells\",\n      \"journal\": \"Cellular and molecular neurobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis with functional readout (surface expression + electrophysiology); single lab, two orthogonal methods\",\n      \"pmids\": [\"23975096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Domoate causes long-lasting inhibition of the GluK5 subunit in heteromeric GluK2/K5 receptors: brief domoate exposure prevents GluK5 activation by other agonists for several minutes. A mutation in GluK5 that reduces agonist binding affinity prevents this inhibition. Domoate-bound GluK2/K5 heteromers can still be fully activated through the GluK2 subunit, demonstrating that the two subunits within the tetramer can function independently to open the ion channel.\",\n      \"method\": \"Electrophysiology (whole-cell recordings) in heterologous cells expressing wild-type or mutant recombinant kainate receptors\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct electrophysiological characterization with mutagenesis; single lab, single primary method\",\n      \"pmids\": [\"24859608\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GluK5 (GRIK5) is localized presynaptically to the ribbon synapses of rod photoreceptor terminals in the mouse retina. In GluK5-deficient mice, the structural integrity of synaptic ribbons is severely altered, indicating a novel role for GluK5 in organizing presynaptic ribbon structure.\",\n      \"method\": \"Double-immunofluorescence, electron microscopy, GluK5 knockout mice analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ultrastructural localization combined with knockout phenotyping; single lab, two complementary methods\",\n      \"pmids\": [\"28235022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Two ER retention signals in the GluK5 C-terminus (arginine-based signal and di-leucine motif) are physically masked by SAP97 in the presence of CASK: SAP97 adopts an extended conformation making its SH3 and GuK domains available to bind and sterically block both ER retention signals. SAP97 and CASK are necessary for sorting GluK2/GluK5 receptor complexes into the local dendritic secretory pathway in neurons, rather than the somatic Golgi pathway.\",\n      \"method\": \"Co-immunoprecipitation, conformational analysis, mutagenesis, neuronal trafficking assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with domain mapping and neuronal trafficking assays; single lab, multiple methods but abstract-level detail only\",\n      \"pmids\": [\"30339823\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"smFRET studies established that GluK2 and GluK5 subunits arrange within the heterotetrameric receptor in a defined 'dimer of dimers' configuration. The heteromeric GluK2/GluK5 receptor shows distinct conformational dynamics at the amino-terminal and agonist-binding domain interfaces compared to homomeric GluK2, particularly in resting versus desensitized states.\",\n      \"method\": \"Single-molecule FRET (smFRET) on full-length receptors\",\n      \"journal\": \"Biochimica et biophysica acta. Biomembranes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — single-molecule structural/dynamic characterization of full-length receptor; single lab\",\n      \"pmids\": [\"31194959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"KA2 antisense knockdown in vivo suppresses the assembly of the GluR6/KA2-PSD95-MLK3 signaling module, inhibits JNK activation and c-jun phosphorylation, and increases neuronal survival in hippocampal CA1 after ischemia/reperfusion. Combined KA2 and GluR6 knockdown has greater neuroprotective effect than either alone, indicating functional cooperation between KA2 and GluR6 in mediating ischemic neuronal death through the PSD95-MLK3-MKK4/7-JNK3 pathway.\",\n      \"method\": \"Intracerebroventricular antisense oligodeoxynucleotide injection in rat ischemia model; Western blot for signaling components; neuronal survival counting\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo knockdown with defined molecular pathway readouts and neuronal survival; single lab, multiple biochemical endpoints\",\n      \"pmids\": [\"17639597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Knockdown of the GRIK5 ortholog in zebrafish caused reduced blood vessel numbers, reduced vascular integrity in the eye, and increased vascular permeability, indicating a role for GRIK5 in eye vascular biology beyond classical glutamate receptor function.\",\n      \"method\": \"Morpholino knockdown in zebrafish, vascular imaging\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single-organism knockdown in zebrafish with vascular phenotype but limited mechanistic follow-up; single lab\",\n      \"pmids\": [\"30827500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In heteromeric GluK2/GluK5 receptors, partial agonism by AMPA is mediated primarily through the GluK2 subunit: AMPA induces intermediate cleft closure states at the GluK2 agonist-binding domain (between apo and full-agonist glutamate-bound conformations), while the GluK5 agonist-binding domain shows no significant difference in cleft closure between AMPA and glutamate conditions. Additionally, the agonist-binding domain dimer interface is not decoupled in the AMPA-bound state, unlike the glutamate-bound state.\",\n      \"method\": \"Single-molecule FRET (smFRET) on full-length heteromeric GluK2/GluK5 receptors\",\n      \"journal\": \"Proteins\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — smFRET with defined conformational states for each subunit; single lab\",\n      \"pmids\": [\"37526035\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GRIK5 (GluK5/KA2) is a high-affinity kainate receptor subunit that cannot form functional homomeric ion channels because arginine-rich and di-leucine ER retention signals in its C-terminus and intracellular loop retain it in the ER; co-assembly with GluK2 (GluR6)—mediated by COPI release and 14-3-3/SAP97-CASK masking of retention signals—is required for ER egress and surface delivery of functional 2:2 GluK2/GluK5 heterotetramers, in which GluK5 confers high glutamate affinity, controls presynaptic facilitation kinetics and postsynaptic EPSC decay at mossy-fiber synapses, undergoes CaMKII phosphorylation on its C-terminal domain to drive lateral diffusion away from PSD-95 and produce KAR-LTD, and in rod photoreceptors occupies presynaptic ribbon terminals where its loss disrupts ribbon structural integrity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GRIK5 (GluK5/KA2) is a high-affinity kainate receptor subunit that cannot form functional homomeric ion channels and instead functions as an obligate partner subunit shaping the properties of heteromeric kainate receptors that mediate excitatory neurotransmission [#0, #14]. Expressed alone, KA2 is retained in the endoplasmic reticulum through discrete trafficking signals\\u2014an arginine-rich ER retention/retrieval motif and a di-leucine motif in the C-terminus plus an additional motif in the intracellular loop\\u2014so that even forced surface expression yields non-functional homomers [#8, #12]; the structural basis lies in an aberrant R1 dimer interface of its amino-terminal domain incompatible with self-assembly [#14]. ER egress requires obligate co-assembly with GluK2/GluR6 (not GluK1 or AMPA/NMDA subunits) into a fixed 2:2 'dimer of dimers' heterotetramer, which sterically masks the retention signals [#8, #12, #15, #21]. This trafficking is gated by a sequential checkpoint: \\u03b2-COP/COPI binds the arginine-rich signal to retain unassembled subunits, and heteromeric assembly displaces COPI in favor of 14-3-3-mediated forward trafficking, with SAP97/CASK additionally masking the retention signals to route receptors into the dendritic secretory pathway [#13, #20]. Within the assembled receptor GluK5 confers high glutamate affinity and larger unitary conductance, and ligand binding through the GluK5 site is necessary and sufficient for surface delivery [#4, #17]. Functionally, KA2 determines presynaptic mossy-fiber facilitation and postsynaptic EPSC decay kinetics, and CaMKII phosphorylation of three C-terminal GluK5 residues reduces PSD-95 binding to increase receptor lateral mobility and drive kainate receptor long-term depression [#10, #16]. Beyond canonical signaling, GluK5 localizes to rod photoreceptor ribbon synapses where its loss disrupts ribbon structural integrity [#19], and KA2 participates with GluR6 in a PSD95-MLK3-JNK signaling module mediating ischemic neuronal death [#22].\",\n  \"teleology\": [\n    {\n      \"year\": 1992,\n      \"claim\": \"Established that KA2 is not an independent channel-forming subunit but modifies the gating and pharmacology of kainate receptors only when co-assembled, defining it as an accessory subunit.\",\n      \"evidence\": \"Heterologous expression in oocytes/HEK cells with electrophysiology comparing homomers and heteromers\",\n      \"pmids\": [\"1373632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and assembly mechanism unknown\", \"Native receptor composition not yet established\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Showed that KA2/GluR6 heteromeric complexes exist in native brain and that AMPA and kainate subunits do not appreciably co-assemble, defining the molecular boundary of native kainate receptor composition.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation from transfected cells and rat brain with radioligand binding\",\n      \"pmids\": [\"8288598\", \"8552236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Subunit stoichiometry not quantified\", \"Trafficking determinants of assembly not addressed\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Placed KA2 at the postsynaptic density and dendritic spines, linking the subunit to synaptic signaling sites.\",\n      \"evidence\": \"EM immunostaining and immunocytochemistry in rat nervous system and cultured hippocampal neurons\",\n      \"pmids\": [\"7852627\", \"8552236\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not address later-described presynaptic localizations\", \"Anchoring partners not identified at this stage\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Quantified how KA2 incorporation alters single-channel conductance and agonist affinity, defining its contribution to receptor biophysics.\",\n      \"evidence\": \"Patch-clamp and noise analysis in transfected HEK293 cells\",\n      \"pmids\": [\"8836240\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conductance measured in heterologous, not native, channels\", \"Structural basis of conductance change unknown\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Confirmed that recombinant GluR5/KA2 heteromers recapitulate native kainate channels in vivo, validating the heteromeric model in real neurons.\",\n      \"evidence\": \"RT-PCR, whole-cell patch-clamp and pharmacology in dissociated trigeminal ganglion neurons\",\n      \"pmids\": [\"9254673\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not establish trafficking control of native assembly\", \"Limited to one neuronal population\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Mapped transcriptional control elements of grik5, including intronic negative regulatory and neural-specific promoter/silencer elements governing its expression.\",\n      \"evidence\": \"Reporter assays, footprinting, EMSA and transgenic lacZ reporters\",\n      \"pmids\": [\"9079693\", \"11533047\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Trans-acting factors only partially identified\", \"Physiological regulation in vivo not demonstrated\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the molecular basis of obligate heteromerization: ER retention motifs in the C-terminus retain KA2 until heteromeric assembly masks them, explaining why KA2 cannot reach the surface alone.\",\n      \"evidence\": \"Surface biotinylation, mutagenesis, immunofluorescence and electrophysiology in heterologous cells and neurons\",\n      \"pmids\": [\"12878702\", \"12950450\", \"14511640\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Loop-region retention signal not yet found\", \"Identity of proteins reading the retention signals unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Assigned physiological consequences to KA2 in vivo, showing it controls presynaptic mossy-fiber facilitation and postsynaptic EPSC decay kinetics.\",\n      \"evidence\": \"Slice electrophysiology in KA2-/- knockout mice\",\n      \"pmids\": [\"12533602\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of affinity contribution not resolved here\", \"Behavioral consequences not addressed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Completed the trafficking model by identifying a second retention motif and the sequential COPI-to-14-3-3 checkpoint, and demonstrated genetically that GluR6 specifically drives KA2 surface delivery.\",\n      \"evidence\": \"Mutagenesis, biotinylation, peptide pulldown, COPI degradation, co-IP from cerebellum/COS-7 and knockout mouse analysis\",\n      \"pmids\": [\"16807331\", \"16595684\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How heteromerization mechanically displaces COPI not fully resolved\", \"14-3-3 binding site mapping incomplete\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Linked the GluR6/KA2 module to a pathological signaling cascade, implicating it in ischemic neuronal death via PSD95-MLK3-JNK signaling.\",\n      \"evidence\": \"In vivo antisense knockdown in a rat ischemia model with Western blot and survival counting\",\n      \"pmids\": [\"17639597\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Antisense specificity not exhaustively controlled\", \"Direct vs indirect role of KA2 in the module not dissected\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Provided the structural explanation for assembly incompatibility, showing the GluK5 amino-terminal domain forms an aberrant R1 dimer interface.\",\n      \"evidence\": \"X-ray crystallography of the isolated GluK5 ATD\",\n      \"pmids\": [\"20951142\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Isolated domain may not reflect full-length assembly\", \"Heteromeric interface not crystallized\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Determined the fixed 2:2 GluK2:GluK5 stoichiometry of the heterotetramer, constraining all functional models.\",\n      \"evidence\": \"Single-molecule bleaching step counting in live-cell membranes\",\n      \"pmids\": [\"22509486\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Spatial arrangement of subunits not yet defined\", \"Stoichiometry in native synapses not directly measured\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified CaMKII phosphorylation of GluK5 as the molecular switch for kainate receptor LTD, coupling phosphorylation to reduced PSD-95 binding and increased lateral mobility.\",\n      \"evidence\": \"Molecular replacement, single-particle tracking/FRAP, phosphorylation assays and mossy-fiber electrophysiology\",\n      \"pmids\": [\"23288040\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct phospho-dependent PSD-95 affinity change not structurally resolved\", \"Upstream activity triggers in vivo not defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Established that agonist binding specifically through the GluK5 subunit is necessary and sufficient for heteromer surface trafficking, identifying a ligand-gated biosynthetic checkpoint.\",\n      \"evidence\": \"Ligand-binding domain mutagenesis with surface expression and electrophysiology in heterologous cells\",\n      \"pmids\": [\"23975096\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, two methods\", \"Endogenous ligand source during biosynthesis unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showed that GluK2 and GluK5 subunits can gate the channel independently and that domoate causes prolonged GluK5-specific inhibition, revealing functional autonomy within the tetramer.\",\n      \"evidence\": \"Whole-cell electrophysiology of wild-type and binding-affinity mutant recombinant receptors\",\n      \"pmids\": [\"24859608\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single primary method\", \"Structural basis of long-lasting inhibition unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Revealed a non-canonical structural role for GluK5 at rod photoreceptor ribbon synapses, where it organizes presynaptic ribbon integrity.\",\n      \"evidence\": \"Double-immunofluorescence, EM and GluK5 knockout mouse retina analysis\",\n      \"pmids\": [\"28235022\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular partners at the ribbon not identified\", \"Whether channel function is required for the structural role unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Defined the SAP97/CASK mechanism that masks GluK5 retention signals and routes assembled receptors into the dendritic secretory pathway.\",\n      \"evidence\": \"Co-IP, conformational analysis, mutagenesis and neuronal trafficking assays\",\n      \"pmids\": [\"30339823\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Abstract-level mechanistic detail only\", \"Interplay with COPI/14-3-3 checkpoint not integrated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Resolved the conformational arrangement and dynamics of GluK2/GluK5 heteromers, including a defined dimer-of-dimers geometry and subunit-specific responses to agonists.\",\n      \"evidence\": \"Single-molecule FRET on full-length receptors in resting, glutamate- and AMPA-bound states\",\n      \"pmids\": [\"31194959\", \"37526035\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Functional coupling of conformational states to gating not fully mapped\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Implicated GRIK5 in eye vascular biology, expanding its potential roles beyond glutamatergic signaling.\",\n      \"evidence\": \"Morpholino knockdown in zebrafish with vascular imaging\",\n      \"pmids\": [\"30827500\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single-organism knockdown with limited mechanistic follow-up\", \"Morpholino off-target effects not excluded\", \"No mammalian validation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the channel-function, synaptic-organizing, and non-neural vascular roles of GRIK5 are mechanistically unified, and whether the ribbon-organizing and vascular roles require ion-channel activity, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the assembled heterotetramer\", \"Channel-independent functions mechanistically undefined\", \"Disease relevance of human GRIK5 variants not established in the corpus\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 5, 10]},\n      {\"term_id\": \"GO:0005216\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [8, 12, 13]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [8, 9, 15]},\n      {\"term_id\": \"GO:0014069\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 10, 16]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [8, 12, 13, 20]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [16, 22]}\n    ],\n    \"complexes\": [\n      \"GluK2/GluK5 (GluR6/KA2) heterotetrameric kainate receptor\",\n      \"GluR6/KA2-PSD95-MLK3 signaling module\"\n    ],\n    \"partners\": [\n      \"GRIK2\",\n      \"GRIK1\",\n      \"COPB1\",\n      \"14-3-3\",\n      \"DLG1\",\n      \"CASK\",\n      \"DLG4\",\n      \"CAMK2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}