{"gene":"C2CD2L","run_date":"2026-06-09T22:02:45","timeline":{"discoveries":[{"year":2017,"finding":"TMEM24 is an ER-anchored membrane protein that localizes to ER-plasma membrane contact sites; its reversible localization is governed by phosphorylation and dephosphorylation in response to oscillations in cytosolic calcium. A lipid-binding SMP domain module in TMEM24 transports phosphatidylinositol (PI) between bilayers, allowing replenishment of PI(4,5)P2 hydrolyzed during signaling. Loss of TMEM24 abolishes calcium oscillations and impairs triggered insulin release in β-cells.","method":"Reconstitution of lipid transport activity; mutagenesis; live-cell imaging of ER-PM contacts; phosphorylation mapping; loss-of-function in β-cells with calcium oscillation and insulin secretion readouts","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro lipid transport reconstitution, mutagenesis, and structural characterization combined with loss-of-function cellular phenotype in a single rigorous study","pmids":["28209843"],"is_preprint":false},{"year":2019,"finding":"TMEM24 populates ER-PM contacts in resting neurons but elevations of cytosolic Ca2+ trigger its transient redistribution throughout the entire ER. This dissociation is mediated by phosphorylation of an array of sites in the C-terminal region. The paralogue C2CD2 has only partially conserved phosphorylation sites and correspondingly shows much lower Ca2+ sensitivity. TMEM24 is enriched in neurons versus glia and increases with neuronal differentiation.","method":"Live-cell fluorescence imaging; phosphorylation site mapping; comparative analysis of TMEM24 vs C2CD2 Ca2+ sensitivity; subcellular fractionation/localization","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct live imaging of Ca2+-dependent redistribution combined with phosphorylation mapping; replicates and extends 2017 Science findings in neurons","pmids":["30819882"],"is_preprint":false},{"year":2013,"finding":"TMEM24 was identified as a component of the insulin biosynthetic interaction network (insulin BIN) in pancreatic β-cells via conformation-based proteomics and mass spectrometry. TMEM24 manages glucose-stimulated insulin secretion from a reserve pool of granules.","method":"Conformation-based pulldown combined with mass spectrometry (insulin BIN proteomics); functional assay of glucose-stimulated insulin secretion","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mass spectrometry-based interactome with functional insulin secretion readout, single lab","pmids":["24012759"],"is_preprint":false},{"year":2021,"finding":"TMEM24 dissociates from the PM in response to both diacylglycerol and nanomolar elevations of cytosolic Ca2+. Loss of TMEM24 in β-cells results in hyper-accumulation of Ca2+ in the ER and excess Ca2+ entry into mitochondria, with resulting impairment in glucose-stimulated ATP production, explaining suppressed Ca2+ oscillations and insulin secretion.","method":"Live-cell Ca2+ imaging; mitochondrial and ER Ca2+ measurements; TMEM24 knockout β-cells; ATP production assays","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with multiple orthogonal readouts (ER Ca2+, mitochondrial Ca2+, ATP), single lab","pmids":["34821358"],"is_preprint":false},{"year":2021,"finding":"Alt-RPL36, a small protein co-encoded with RPL36, interacts with TMEM24 at the ER. Knockout of alt-RPL36 increases plasma membrane PI(4,5)P2 levels, upregulates PI3K-AKT-mTOR signaling, and increases cell size. Four phosphoserine residues in alt-RPL36 are required for its interaction with TMEM24; their mutation abolishes the interaction and eliminates alt-RPL36 effects on PI3K signaling and cell size.","method":"Co-immunoprecipitation; PI(4,5)P2 biosensor imaging; knockout cell lines; phosphoserine point mutations; PI3K-AKT-mTOR pathway activity assays","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with mutagenesis and functional PI(4,5)P2/signaling readouts, single lab","pmids":["33479206"],"is_preprint":false},{"year":2024,"finding":"TMEM24 and its paralog C2CD2 form a complex with band 4.1 family proteins, which bind PM proteins including the cell adhesion molecule SynCAM 1. This complex enriches TMEM24/C2CD2-containing ER-PM junctions at sites of cell-cell contact. When band 4.1 is part of the junction, TMEM24 at cell-adjacent ER-PM junctions is not shed from the PM upon Ca2+ rise, unlike TMEM24 at non-cell-adjacent junctions. TMEM24 tethers the ER to the PM via a polybasic C-terminus motif.","method":"Proximity ligation analysis; Co-immunoprecipitation; live-cell imaging of Ca2+-dependent dynamics at cell-contact vs. non-contact junctions","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proximity ligation and Co-IP with functional dynamic imaging, single lab, peer-reviewed","pmids":["39158698"],"is_preprint":false},{"year":2025,"finding":"TLR4 mediates lipotoxicity in β-cells by directly binding to TMEM24 and downregulating its protein expression, thereby suppressing PI3K/AKT signaling and impairing insulin secretion. TLR4 knockout ameliorates islet function impairment through restoration of TMEM24/PI3K/AKT signaling in high-fat diet obese rats.","method":"In vitro lipotoxic β-cell model; TLR4 knockout rats; co-immunoprecipitation (TLR4-TMEM24 interaction); PI3K/AKT pathway activity measurements; insulin secretion assays","journal":"Acta biochimica et biophysica Sinica","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single Co-IP for TLR4-TMEM24 binding with downstream pathway inference; limited methodological detail in abstract","pmids":["40170616"],"is_preprint":false}],"current_model":"TMEM24 (C2CD2L) is an ER-anchored lipid transport protein that localizes to ER-plasma membrane contact sites, where it uses its SMP domain to transfer phosphatidylinositol from the ER to the PM to replenish PI(4,5)P2 consumed during signaling; its dynamic association with the PM is regulated by Ca2+-triggered phosphorylation of C-terminal sites, and it forms complexes with band 4.1 proteins at cell-cell contacts and with alt-RPL36 to modulate PI3K-AKT-mTOR signaling, collectively coordinating calcium oscillations and pulsatile insulin secretion in β-cells."},"narrative":{"mechanistic_narrative":"C2CD2L (TMEM24) is an ER-anchored lipid transport protein that operates at ER–plasma membrane (ER-PM) contact sites to sustain phosphoinositide homeostasis and calcium signaling, most prominently in β-cells and neurons [PMID:28209843, PMID:30819882]. Its SMP domain transfers phosphatidylinositol between bilayers, replenishing PM PI(4,5)P2 consumed during signaling, while a polybasic C-terminal motif tethers the ER to the PM [PMID:28209843, PMID:39158698]. Its residence at ER-PM junctions is dynamic: nanomolar elevations of cytosolic Ca2+ and diacylglycerol trigger C-terminal phosphorylation that redistributes TMEM24 away from the PM into the broader ER, coupling lipid delivery to the oscillatory state of the cell [PMID:28209843, PMID:30819882, PMID:34821358]. Loss of TMEM24 abolishes Ca2+ oscillations and triggered insulin release, causing aberrant ER and mitochondrial Ca2+ accumulation and impaired glucose-stimulated ATP production [PMID:28209843, PMID:34821358]. TMEM24 forms a complex with band 4.1 family proteins that enriches and stabilizes ER-PM junctions at cell-cell contacts, exempting these junctions from Ca2+-triggered shedding [PMID:39158698], and interacts with the microprotein alt-RPL36 through phosphoserine-dependent binding to restrain PM PI(4,5)P2 levels and PI3K-AKT-mTOR signaling and cell size [PMID:33479206].","teleology":[{"year":2013,"claim":"Before any mechanistic role was known, TMEM24 was placed in the β-cell insulin secretory machinery, establishing it as a regulator of glucose-stimulated insulin release from a reserve granule pool.","evidence":"Conformation-based pulldown plus mass spectrometry (insulin BIN proteomics) with glucose-stimulated insulin secretion assays","pmids":["24012759"],"confidence":"Medium","gaps":["Did not define a molecular activity for TMEM24","Interaction network from a single lab without orthogonal validation","No link to lipid transport or Ca2+ signaling yet"]},{"year":2017,"claim":"This work defined the core biochemical function — that TMEM24 is an ER-anchored SMP-domain lipid transfer protein at ER-PM contacts whose phosphorylation tracks Ca2+ oscillations, mechanistically explaining how it supports insulin release.","evidence":"In vitro reconstitution of lipid transport, mutagenesis, phosphorylation mapping, live-cell ER-PM contact imaging, and β-cell loss-of-function with Ca2+ and insulin readouts","pmids":["28209843"],"confidence":"High","gaps":["Did not resolve the kinase/phosphatase pair driving the Ca2+-responsive cycle","Cargo specificity beyond PI not fully delineated","No structural model of the contact-site tether"]},{"year":2019,"claim":"Extended the Ca2+-regulated localization model to neurons and showed paralog divergence, establishing that the density of conserved C-terminal phosphosites sets Ca2+ sensitivity and distinguishes TMEM24 from C2CD2.","evidence":"Live-cell fluorescence imaging of Ca2+-triggered ER redistribution, phosphosite mapping, and comparative analysis of TMEM24 vs C2CD2","pmids":["30819882"],"confidence":"High","gaps":["Functional consequence of redistribution in neurons not defined","Identity of the responsible kinases unresolved","Physiological role of C2CD2's lower Ca2+ sensitivity unclear"]},{"year":2021,"claim":"Connected TMEM24 loss to downstream organellar Ca2+ handling, showing it prevents ER Ca2+ overload and excess mitochondrial Ca2+ entry that otherwise impairs ATP production and oscillations.","evidence":"TMEM24 knockout β-cells with ER and mitochondrial Ca2+ measurements, ATP assays, and DAG/Ca2+-triggered dissociation imaging","pmids":["34821358"],"confidence":"Medium","gaps":["Causal chain from PI(4,5)P2 replenishment to ER Ca2+ buffering not directly demonstrated","Single-lab knockout system","Whether DAG and Ca2+ act through the same phosphosites unresolved"]},{"year":2021,"claim":"Identified a regulatory partner, the microprotein alt-RPL36, revealing that phosphoserine-dependent binding to TMEM24 tunes PM PI(4,5)P2 levels, PI3K-AKT-mTOR signaling, and cell size.","evidence":"Reciprocal Co-IP, phosphoserine point mutations, PI(4,5)P2 biosensor imaging, and PI3K-AKT-mTOR pathway and cell-size assays in knockout cells","pmids":["33479206"],"confidence":"Medium","gaps":["How alt-RPL36 binding modulates TMEM24 transport activity mechanistically unknown","Single-lab finding","Relevance to β-cell physiology not tested"]},{"year":2024,"claim":"Revealed spatial regulation of ER-PM junctions by showing TMEM24/C2CD2 complex with band 4.1 proteins enriches junctions at cell-cell contacts and protects them from Ca2+-triggered shedding.","evidence":"Proximity ligation, Co-IP, and live-cell imaging comparing Ca2+ dynamics at cell-contact vs non-contact junctions","pmids":["39158698"],"confidence":"Medium","gaps":["Functional output of contact-localized junctions not defined","Role of SynCAM 1 link unclear","Single-lab evidence"]},{"year":2025,"claim":"Proposed a disease-relevant upstream regulator, with TLR4 binding TMEM24 and downregulating it to suppress PI3K/AKT signaling during β-cell lipotoxicity.","evidence":"Lipotoxic β-cell model, TLR4 knockout rats, TLR4-TMEM24 Co-IP, and PI3K/AKT and insulin secretion measurements","pmids":["40170616"],"confidence":"Low","gaps":["Single Co-IP without reciprocal validation for the TLR4-TMEM24 interaction","Mechanism of TMEM24 downregulation by TLR4 not defined","Direct binding versus indirect effect not distinguished"]},{"year":null,"claim":"The kinases and phosphatases that read cytosolic Ca2+ to drive TMEM24's reversible contact-site association, and how lipid transfer is mechanistically coupled to organellar Ca2+ buffering, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Identity of responsible kinase/phosphatase not established","No structural model of the SMP tether at contacts","Causal link between PI delivery and ER/mitochondrial Ca2+ handling not directly shown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1,4]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,4]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0]}],"complexes":[],"partners":["C2CD2","ALT-RPL36","BAND 4.1 PROTEINS","TLR4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O14523","full_name":"Phospholipid transfer protein C2CD2L","aliases":["C2 domain-containing protein 2-like","C2CD2-like","Transmembrane protein 24"],"length_aa":706,"mass_kda":76.2,"function":"Lipid-binding protein that transports phosphatidylinositol, the precursor of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), from its site of synthesis in the endoplasmic reticulum to the cell membrane (PubMed:28209843). It thereby maintains the pool of cell membrane phosphoinositides, which are degraded during phospholipase C (PLC) signaling (PubMed:28209843). Plays a key role in the coordination of Ca(2+) and phosphoinositide signaling: localizes to sites of contact between the endoplasmic reticulum and the cell membrane, where it tethers the two bilayers (PubMed:28209843). In response to elevation of cytosolic Ca(2+), it is phosphorylated at its C-terminus and dissociates from the cell membrane, abolishing phosphatidylinositol transport to the cell membrane (PubMed:28209843). Positively regulates insulin secretion in response to glucose: phosphatidylinositol transfer to the cell membrane allows replenishment of PI(4,5)P2 pools and calcium channel opening, priming a new population of insulin granules (PubMed:28209843)","subcellular_location":"Endoplasmic reticulum membrane; Cell membrane","url":"https://www.uniprot.org/uniprotkb/O14523/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C2CD2L","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":[{"gene":"VAPA","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/C2CD2L","total_profiled":1310},"omim":[{"mim_id":"617582","title":"C2 CALCIUM-DEPENDENT DOMAIN-CONTAINING PROTEIN 2-LIKE; C2CD2L","url":"https://www.omim.org/entry/617582"},{"mim_id":"617581","title":"C2 CALCIUM-DEPENDENT DOMAIN-CONTAINING PROTEIN 2; C2CD2","url":"https://www.omim.org/entry/617581"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/C2CD2L"},"hgnc":{"alias_symbol":["KIAA0285"],"prev_symbol":["TMEM24"]},"alphafold":{"accession":"O14523","domains":[{"cath_id":"2.60.40.150","chopping":"286-410","consensus_level":"high","plddt":82.3269,"start":286,"end":410},{"cath_id":"-","chopping":"430-457_534-541","consensus_level":"medium","plddt":68.8717,"start":430,"end":541},{"cath_id":"3.15.10","chopping":"79-225_233-256","consensus_level":"high","plddt":84.7198,"start":79,"end":256}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O14523","model_url":"https://alphafold.ebi.ac.uk/files/AF-O14523-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O14523-F1-predicted_aligned_error_v6.png","plddt_mean":63.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C2CD2L","jax_strain_url":"https://www.jax.org/strain/search?query=C2CD2L"},"sequence":{"accession":"O14523","fasta_url":"https://rest.uniprot.org/uniprotkb/O14523.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O14523/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O14523"}},"corpus_meta":[{"pmid":"28209843","id":"PMC_28209843","title":"Lipid transport by TMEM24 at ER-plasma membrane contacts regulates pulsatile insulin secretion.","date":"2017","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/28209843","citation_count":163,"is_preprint":false},{"pmid":"30819882","id":"PMC_30819882","title":"Lipid transporter TMEM24/C2CD2L is a Ca2+-regulated component of ER-plasma membrane contacts in mammalian neurons.","date":"2019","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/30819882","citation_count":56,"is_preprint":false},{"pmid":"33479206","id":"PMC_33479206","title":"Alt-RPL36 downregulates the PI3K-AKT-mTOR signaling pathway by interacting with TMEM24.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33479206","citation_count":47,"is_preprint":false},{"pmid":"24012759","id":"PMC_24012759","title":"Insulin biosynthetic interaction network component, TMEM24, facilitates insulin reserve pool release.","date":"2013","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/24012759","citation_count":36,"is_preprint":false},{"pmid":"34821358","id":"PMC_34821358","title":"The endoplasmic reticulum-plasma membrane tethering protein TMEM24 is a regulator of cellular Ca2+ homeostasis.","date":"2021","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/34821358","citation_count":18,"is_preprint":false},{"pmid":"39158698","id":"PMC_39158698","title":"A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts.","date":"2024","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/39158698","citation_count":7,"is_preprint":false},{"pmid":"15289880","id":"PMC_15289880","title":"Identification and characterization of TMEM24 family genes in silico.","date":"2004","source":"International journal of oncology","url":"https://pubmed.ncbi.nlm.nih.gov/15289880","citation_count":7,"is_preprint":false},{"pmid":"39738500","id":"PMC_39738500","title":"A novel lncRNA AC112721.1 promotes the progression of triple-negative breast cancer by directly binding to THBS1 and regulating miR-491-5p/C2CD2L axis.","date":"2024","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/39738500","citation_count":3,"is_preprint":false},{"pmid":"40170616","id":"PMC_40170616","title":"TLR4 mediates lipotoxic β-cell dysfunction by inhibiting the TMEM24/PI3K/AKT pathway.","date":"2025","source":"Acta biochimica et biophysica Sinica","url":"https://pubmed.ncbi.nlm.nih.gov/40170616","citation_count":1,"is_preprint":false},{"pmid":"38106008","id":"PMC_38106008","title":"A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts.","date":"2023","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/38106008","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6590,"output_tokens":2065,"usd":0.025372,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9115,"output_tokens":3656,"usd":0.068488,"stage2_stop_reason":"end_turn"},"total_usd":0.09386,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2017,\n      \"finding\": \"TMEM24 is an ER-anchored membrane protein that localizes to ER-plasma membrane contact sites; its reversible localization is governed by phosphorylation and dephosphorylation in response to oscillations in cytosolic calcium. A lipid-binding SMP domain module in TMEM24 transports phosphatidylinositol (PI) between bilayers, allowing replenishment of PI(4,5)P2 hydrolyzed during signaling. Loss of TMEM24 abolishes calcium oscillations and impairs triggered insulin release in β-cells.\",\n      \"method\": \"Reconstitution of lipid transport activity; mutagenesis; live-cell imaging of ER-PM contacts; phosphorylation mapping; loss-of-function in β-cells with calcium oscillation and insulin secretion readouts\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro lipid transport reconstitution, mutagenesis, and structural characterization combined with loss-of-function cellular phenotype in a single rigorous study\",\n      \"pmids\": [\"28209843\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TMEM24 populates ER-PM contacts in resting neurons but elevations of cytosolic Ca2+ trigger its transient redistribution throughout the entire ER. This dissociation is mediated by phosphorylation of an array of sites in the C-terminal region. The paralogue C2CD2 has only partially conserved phosphorylation sites and correspondingly shows much lower Ca2+ sensitivity. TMEM24 is enriched in neurons versus glia and increases with neuronal differentiation.\",\n      \"method\": \"Live-cell fluorescence imaging; phosphorylation site mapping; comparative analysis of TMEM24 vs C2CD2 Ca2+ sensitivity; subcellular fractionation/localization\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct live imaging of Ca2+-dependent redistribution combined with phosphorylation mapping; replicates and extends 2017 Science findings in neurons\",\n      \"pmids\": [\"30819882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"TMEM24 was identified as a component of the insulin biosynthetic interaction network (insulin BIN) in pancreatic β-cells via conformation-based proteomics and mass spectrometry. TMEM24 manages glucose-stimulated insulin secretion from a reserve pool of granules.\",\n      \"method\": \"Conformation-based pulldown combined with mass spectrometry (insulin BIN proteomics); functional assay of glucose-stimulated insulin secretion\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mass spectrometry-based interactome with functional insulin secretion readout, single lab\",\n      \"pmids\": [\"24012759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TMEM24 dissociates from the PM in response to both diacylglycerol and nanomolar elevations of cytosolic Ca2+. Loss of TMEM24 in β-cells results in hyper-accumulation of Ca2+ in the ER and excess Ca2+ entry into mitochondria, with resulting impairment in glucose-stimulated ATP production, explaining suppressed Ca2+ oscillations and insulin secretion.\",\n      \"method\": \"Live-cell Ca2+ imaging; mitochondrial and ER Ca2+ measurements; TMEM24 knockout β-cells; ATP production assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with multiple orthogonal readouts (ER Ca2+, mitochondrial Ca2+, ATP), single lab\",\n      \"pmids\": [\"34821358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Alt-RPL36, a small protein co-encoded with RPL36, interacts with TMEM24 at the ER. Knockout of alt-RPL36 increases plasma membrane PI(4,5)P2 levels, upregulates PI3K-AKT-mTOR signaling, and increases cell size. Four phosphoserine residues in alt-RPL36 are required for its interaction with TMEM24; their mutation abolishes the interaction and eliminates alt-RPL36 effects on PI3K signaling and cell size.\",\n      \"method\": \"Co-immunoprecipitation; PI(4,5)P2 biosensor imaging; knockout cell lines; phosphoserine point mutations; PI3K-AKT-mTOR pathway activity assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with mutagenesis and functional PI(4,5)P2/signaling readouts, single lab\",\n      \"pmids\": [\"33479206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TMEM24 and its paralog C2CD2 form a complex with band 4.1 family proteins, which bind PM proteins including the cell adhesion molecule SynCAM 1. This complex enriches TMEM24/C2CD2-containing ER-PM junctions at sites of cell-cell contact. When band 4.1 is part of the junction, TMEM24 at cell-adjacent ER-PM junctions is not shed from the PM upon Ca2+ rise, unlike TMEM24 at non-cell-adjacent junctions. TMEM24 tethers the ER to the PM via a polybasic C-terminus motif.\",\n      \"method\": \"Proximity ligation analysis; Co-immunoprecipitation; live-cell imaging of Ca2+-dependent dynamics at cell-contact vs. non-contact junctions\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proximity ligation and Co-IP with functional dynamic imaging, single lab, peer-reviewed\",\n      \"pmids\": [\"39158698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"TLR4 mediates lipotoxicity in β-cells by directly binding to TMEM24 and downregulating its protein expression, thereby suppressing PI3K/AKT signaling and impairing insulin secretion. TLR4 knockout ameliorates islet function impairment through restoration of TMEM24/PI3K/AKT signaling in high-fat diet obese rats.\",\n      \"method\": \"In vitro lipotoxic β-cell model; TLR4 knockout rats; co-immunoprecipitation (TLR4-TMEM24 interaction); PI3K/AKT pathway activity measurements; insulin secretion assays\",\n      \"journal\": \"Acta biochimica et biophysica Sinica\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single Co-IP for TLR4-TMEM24 binding with downstream pathway inference; limited methodological detail in abstract\",\n      \"pmids\": [\"40170616\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TMEM24 (C2CD2L) is an ER-anchored lipid transport protein that localizes to ER-plasma membrane contact sites, where it uses its SMP domain to transfer phosphatidylinositol from the ER to the PM to replenish PI(4,5)P2 consumed during signaling; its dynamic association with the PM is regulated by Ca2+-triggered phosphorylation of C-terminal sites, and it forms complexes with band 4.1 proteins at cell-cell contacts and with alt-RPL36 to modulate PI3K-AKT-mTOR signaling, collectively coordinating calcium oscillations and pulsatile insulin secretion in β-cells.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"C2CD2L (TMEM24) is an ER-anchored lipid transport protein that operates at ER–plasma membrane (ER-PM) contact sites to sustain phosphoinositide homeostasis and calcium signaling, most prominently in β-cells and neurons [#0, #1]. Its SMP domain transfers phosphatidylinositol between bilayers, replenishing PM PI(4,5)P2 consumed during signaling, while a polybasic C-terminal motif tethers the ER to the PM [#0, #5]. Its residence at ER-PM junctions is dynamic: nanomolar elevations of cytosolic Ca2+ and diacylglycerol trigger C-terminal phosphorylation that redistributes TMEM24 away from the PM into the broader ER, coupling lipid delivery to the oscillatory state of the cell [#0, #1, #3]. Loss of TMEM24 abolishes Ca2+ oscillations and triggered insulin release, causing aberrant ER and mitochondrial Ca2+ accumulation and impaired glucose-stimulated ATP production [#0, #3]. TMEM24 forms a complex with band 4.1 family proteins that enriches and stabilizes ER-PM junctions at cell-cell contacts, exempting these junctions from Ca2+-triggered shedding [#5], and interacts with the microprotein alt-RPL36 through phosphoserine-dependent binding to restrain PM PI(4,5)P2 levels and PI3K-AKT-mTOR signaling and cell size [#4].\"\n}\n\nWait — I must return the full JSON object. Here it is:\n\n```json\n{\n  \"mechanistic_narrative\": \"C2CD2L (TMEM24) is an ER-anchored lipid transport protein that operates at ER–plasma membrane (ER-PM) contact sites to sustain phosphoinositide homeostasis and calcium signaling, most prominently in β-cells and neurons [#0, #1]. Its SMP domain transfers phosphatidylinositol between bilayers, replenishing PM PI(4,5)P2 consumed during signaling, while a polybasic C-terminal motif tethers the ER to the PM [#0, #5]. Its residence at ER-PM junctions is dynamic: nanomolar elevations of cytosolic Ca2+ and diacylglycerol trigger C-terminal phosphorylation that redistributes TMEM24 away from the PM into the broader ER, coupling lipid delivery to the oscillatory state of the cell [#0, #1, #3]. Loss of TMEM24 abolishes Ca2+ oscillations and triggered insulin release, causing aberrant ER and mitochondrial Ca2+ accumulation and impaired glucose-stimulated ATP production [#0, #3]. TMEM24 forms a complex with band 4.1 family proteins that enriches and stabilizes ER-PM junctions at cell-cell contacts, exempting these junctions from Ca2+-triggered shedding [#5], and interacts with the microprotein alt-RPL36 through phosphoserine-dependent binding to restrain PM PI(4,5)P2 levels and PI3K-AKT-mTOR signaling and cell size [#4].\",\n  \"teleology\": [\n    {\n      \"year\": 2013,\n      \"claim\": \"Before any mechanistic role was known, TMEM24 was placed in the β-cell insulin secretory machinery, establishing it as a regulator of glucose-stimulated insulin release from a reserve granule pool.\",\n      \"evidence\": \"Conformation-based pulldown plus mass spectrometry (insulin BIN proteomics) with glucose-stimulated insulin secretion assays\",\n      \"pmids\": [\"24012759\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not define a molecular activity for TMEM24\", \"Interaction network from a single lab without orthogonal validation\", \"No link to lipid transport or Ca2+ signaling yet\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"This work defined the core biochemical function — that TMEM24 is an ER-anchored SMP-domain lipid transfer protein at ER-PM contacts whose phosphorylation tracks Ca2+ oscillations, mechanistically explaining how it supports insulin release.\",\n      \"evidence\": \"In vitro reconstitution of lipid transport, mutagenesis, phosphorylation mapping, live-cell ER-PM contact imaging, and β-cell loss-of-function with Ca2+ and insulin readouts\",\n      \"pmids\": [\"28209843\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve the kinase/phosphatase pair driving the Ca2+-responsive cycle\", \"Cargo specificity beyond PI not fully delineated\", \"No structural model of the contact-site tether\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Extended the Ca2+-regulated localization model to neurons and showed paralog divergence, establishing that the density of conserved C-terminal phosphosites sets Ca2+ sensitivity and distinguishes TMEM24 from C2CD2.\",\n      \"evidence\": \"Live-cell fluorescence imaging of Ca2+-triggered ER redistribution, phosphosite mapping, and comparative analysis of TMEM24 vs C2CD2\",\n      \"pmids\": [\"30819882\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional consequence of redistribution in neurons not defined\", \"Identity of the responsible kinases unresolved\", \"Physiological role of C2CD2's lower Ca2+ sensitivity unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Connected TMEM24 loss to downstream organellar Ca2+ handling, showing it prevents ER Ca2+ overload and excess mitochondrial Ca2+ entry that otherwise impairs ATP production and oscillations.\",\n      \"evidence\": \"TMEM24 knockout β-cells with ER and mitochondrial Ca2+ measurements, ATP assays, and DAG/Ca2+-triggered dissociation imaging\",\n      \"pmids\": [\"34821358\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Causal chain from PI(4,5)P2 replenishment to ER Ca2+ buffering not directly demonstrated\", \"Single-lab knockout system\", \"Whether DAG and Ca2+ act through the same phosphosites unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified a regulatory partner, the microprotein alt-RPL36, revealing that phosphoserine-dependent binding to TMEM24 tunes PM PI(4,5)P2 levels, PI3K-AKT-mTOR signaling, and cell size.\",\n      \"evidence\": \"Reciprocal Co-IP, phosphoserine point mutations, PI(4,5)P2 biosensor imaging, and PI3K-AKT-mTOR pathway and cell-size assays in knockout cells\",\n      \"pmids\": [\"33479206\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"How alt-RPL36 binding modulates TMEM24 transport activity mechanistically unknown\", \"Single-lab finding\", \"Relevance to β-cell physiology not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Revealed spatial regulation of ER-PM junctions by showing TMEM24/C2CD2 complex with band 4.1 proteins enriches junctions at cell-cell contacts and protects them from Ca2+-triggered shedding.\",\n      \"evidence\": \"Proximity ligation, Co-IP, and live-cell imaging comparing Ca2+ dynamics at cell-contact vs non-contact junctions\",\n      \"pmids\": [\"39158698\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional output of contact-localized junctions not defined\", \"Role of SynCAM 1 link unclear\", \"Single-lab evidence\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Proposed a disease-relevant upstream regulator, with TLR4 binding TMEM24 and downregulating it to suppress PI3K/AKT signaling during β-cell lipotoxicity.\",\n      \"evidence\": \"Lipotoxic β-cell model, TLR4 knockout rats, TLR4-TMEM24 Co-IP, and PI3K/AKT and insulin secretion measurements\",\n      \"pmids\": [\"40170616\"],\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single Co-IP without reciprocal validation for the TLR4-TMEM24 interaction\", \"Mechanism of TMEM24 downregulation by TLR4 not defined\", \"Direct binding versus indirect effect not distinguished\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The kinases and phosphatases that read cytosolic Ca2+ to drive TMEM24's reversible contact-site association, and how lipid transfer is mechanistically coupled to organellar Ca2+ buffering, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Identity of responsible kinase/phosphatase not established\", \"No structural model of the SMP tether at contacts\", \"Causal link between PI delivery and ER/mitochondrial Ca2+ handling not directly shown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1, 4]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"C2CD2\", \"alt-RPL36\", \"band 4.1 proteins\", \"TLR4\"],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}