{"gene":"SLC35A4","run_date":"2026-06-10T07:46:33","timeline":{"discoveries":[{"year":2017,"finding":"SLC35A4 (reference protein) localizes to the Golgi apparatus as determined by immunofluorescence of endogenous protein, and has an even number of transmembrane domains with both N- and C-termini facing the cytosol (contradicting in silico topology predictions). FLIM-FRET analysis showed SLC35A4 does not form homomers and does not associate with other SLC35A subfamily members except SLC35A5, but is within 10–40 nm of SLC35A2 and SLC35A3. CRISPR-Cas9 knockout of SLC35A4 altered subcellular distribution of SLC35A2/SLC35A3 complexes, and overexpression of SLC35A4-BFP with SLC35A3 and SLC35A2-Golgi splice variant negatively affected the SLC35A2/SLC35A3 interaction, indicating a modulatory role in intracellular trafficking of these complexes.","method":"Immunofluorescence, experimental topology assay, FLIM-FRET interaction analysis, CRISPR-Cas9 knockout with glycosylation and localization readouts","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal FLIM-FRET, CRISPR-KO with defined phenotype, topology assay; single lab with multiple orthogonal methods","pmids":["28167211"],"is_preprint":false},{"year":2019,"finding":"SLC35A4 forms higher-order assemblies with SLC35A2 and SLC35A3 in Golgi membranes in vivo, as detected by high-throughput FRET- and BiFC-based interaction screens. Novel ternary complexes between NSTs were also identified.","method":"High-throughput FRET and BiFC-based in vivo interaction screens","journal":"Cellular and molecular life sciences : CMLS","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — BiFC-FRET in vivo interaction screens, single lab but two orthogonal methods confirming complex formation","pmids":["30737517"],"is_preprint":false},{"year":2021,"finding":"SLC35A4 acts as a redundant transporter of CDP-ribitol into the Golgi apparatus alongside SLC35A1. Mutagenesis of the predicted binding pocket of SLC35A1 to introduce bulky residues present in SLC35A4 abolished sialylation but preserved ribitol phosphorylation, demonstrating that the size of the binding pocket restricts SLC35A4 to smaller cytosine nucleotide conjugates such as CDP-ribitol but not bulkier CMP-sialic acid.","method":"Site-directed mutagenesis of binding pocket residues, functional cell-based assay in SLC35A1 KO cells measuring sialylation and ribitol phosphorylation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis of active-site residues with two orthogonal functional readouts (sialylation abolished, ribitol phosphorylation preserved), mechanistic interpretation well-supported","pmids":["34015330"],"is_preprint":false},{"year":2021,"finding":"Pulldown experiments identified novel interaction partners of SLC35A4, including two ATPases (ATP2A2, ATP2C1), Golgi pH regulator B (GPR89B), calcium channel (TMCO1), and basigin (BSG); selected interactions were confirmed in vitro using the NanoBiT split-luciferase complementation assay.","method":"Co-immunoprecipitation/pull-down followed by mass spectrometry; NanoBiT split-luciferase confirmation","journal":"Journal of proteomics","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — pulldown-MS with NanoBiT confirmation, single lab, two orthogonal methods but no deeper mechanistic characterization of interaction consequences","pmids":["34242836"],"is_preprint":false},{"year":2015,"finding":"The SLC35A4 mRNA contains an upstream ORF (uORF) in its 5'-UTR that represses translation of the main coding ORF under normal conditions; phylogenetic analysis suggests this uORF encodes a functional protein product. Ribosome profiling during sodium arsenite-induced stress showed the SLC35A4 main ORF is among those resistant to eIF2-mediated translational repression.","method":"Ribosome profiling in human cells under sodium arsenite stress; phylogenetic analysis","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ribosome profiling with genome-wide coverage and phylogenetic support, but functional protein product from uORF not directly validated in this paper","pmids":["25621764"],"is_preprint":false},{"year":2024,"finding":"The uORF of SLC35A4 encodes a 103-amino acid microprotein (SLC35A4-MP/AltSLC35A4) that localizes to the inner mitochondrial membrane (IMM) as a single-pass transmembrane protein. Loss-of-function studies showed SLC35A4-MP KO significantly diminishes maximal cellular respiration, establishing a role for this microprotein in cellular metabolism.","method":"Biochemical fractionation, microscopy, loss-of-function (KO) with Seahorse respirometry assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — IMM localization by fractionation, loss-of-function with quantitative metabolic readout, replicated by independent study (PMID:40545711)","pmids":["38580077"],"is_preprint":false},{"year":2024,"finding":"SLC35A4 uORF-encoded microprotein (SLC35A4-MP) was identified as a mitochondrial protein in primary living samples (dissociated mouse tissues, primary human T cells) using bioorthogonal photocatalytic proximity labeling (CAT-S), confirming its presence in the native mitochondrial proteome.","method":"Bioorthogonal photocatalytic proximity labeling (CAT-S) followed by proteomics in primary tissues","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel proximity labeling method in primary tissues confirms mitochondrial localization, single study","pmids":["38548729"],"is_preprint":false},{"year":2025,"finding":"AltSLC35A4 (SLC35A4-MP) localizes to the inner mitochondrial membrane, confirmed by microscopy and biochemical analyses. Knockout of the reference SLC35A4 or AltSLC35A4 enhanced sensitivity to oxidative stress in a rescuable manner. During oxidative stress (sodium arsenite), translational upregulation of SLC35A4 reference protein occurs via the uORF in an upstream ORF-dependent manner, while AltSLC35A4 expression remains unchanged under stress.","method":"Microscopy, biochemical fractionation, knockout cell lines with oxidative stress challenge (rescue experiment), ribosome/translation analysis","journal":"Protein science : a publication of the Protein Society","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with rescuable phenotype, IMM localization confirmed by two methods, single lab","pmids":["40545711"],"is_preprint":false},{"year":2025,"finding":"SLC35A4-MP (STREMI) knockout in mice disrupts mitochondrial lipid composition in brown adipose tissue, decreasing cardiolipins and phosphatidylethanolamine under high-fat diet conditions. KO mice show impaired mitochondrial activity, altered mitochondrial number and morphology, increased inflammation, and accumulation of acylcarnitines during cold exposure indicating defective fatty acid oxidation.","method":"Knockout mouse model, lipidomics, electron microscopy of mitochondria, metabolomics (acylcarnitines), cold exposure and HFD physiological challenge","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo KO mouse model with multiple orthogonal readouts (lipidomics, EM, metabolomics, physiological challenges), independent replication of mitochondrial role","pmids":["40880489"],"is_preprint":false},{"year":2026,"finding":"The SLC35A4 5'UTR-encoded microprotein (named STREMI) localizes to the inner mitochondrial membrane, shares topology and motifs with the MICOS core subunit MIC10, and regulates mitochondrial cristae morphogenesis in mice and human cells. The STREMI-encoding uORF also mediates stress-responsive translation of the downstream SLC35A4 Golgi transporter ORF during the integrated stress response.","method":"Bioinformatic pipeline (four-layered), functional cell and mouse studies of cristae morphology, evolutionary analysis, translation regulation assay","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional cristae morphogenesis phenotype in cells and mice, structural homology to MIC10, single lab with multiple methods but abstract lacks full methodological detail","pmids":["42069946"],"is_preprint":false}],"current_model":"SLC35A4 is a dual-coding gene: the reference protein is a Golgi-localized nucleotide sugar transporter that redundantly transports CDP-ribitol into the Golgi (alongside SLC35A1, with binding-pocket size determining substrate selectivity) and modulates SLC35A2/SLC35A3 transporter complex distribution; the uORF in its 5'UTR encodes a 103-amino acid single-pass inner mitochondrial membrane microprotein (SLC35A4-MP/AltSLC35A4/STREMI) that regulates mitochondrial cristae morphogenesis, lipid composition, fatty acid oxidation, and maximal cellular respiration, while the uORF also mediates stress-responsive upregulation of the downstream SLC35A4 reference protein during the integrated stress response."},"narrative":{"mechanistic_narrative":"SLC35A4 is a dual-coding locus whose two products act in distinct organelles: a Golgi-resident reference transporter and a mitochondrial microprotein encoded by a 5'UTR upstream ORF [PMID:28167211, PMID:38580077, PMID:42069946]. The reference protein localizes to the Golgi apparatus with an even number of transmembrane domains and cytosol-facing N- and C-termini, and functions as a redundant nucleotide-sugar transporter that imports CDP-ribitol alongside SLC35A1; the size of its substrate-binding pocket restricts it to small cytosine nucleotide conjugates and excludes bulkier CMP-sialic acid [PMID:28167211, PMID:34015330]. Beyond transport, it modulates the subcellular distribution and assembly of SLC35A2/SLC35A3 transporter complexes, forming higher-order assemblies with these partners in Golgi membranes [PMID:28167211, PMID:30737517]. The uORF encodes a 103-amino acid single-pass inner mitochondrial membrane microprotein (SLC35A4-MP/AltSLC35A4/STREMI) that shares topology and motifs with the MICOS subunit MIC10 and regulates cristae morphogenesis, mitochondrial lipid composition, fatty acid oxidation, and maximal respiration [PMID:38580077, PMID:40880489, PMID:42069946]. The same uORF couples the two products to cellular stress: it represses reference-protein translation under normal conditions but mediates its translational upregulation during the integrated stress response, and loss of either product sensitizes cells to oxidative stress [PMID:25621764, PMID:40545711, PMID:42069946].","teleology":[{"year":2015,"claim":"Established that SLC35A4 is not a simple single-protein gene but carries a 5'UTR uORF that represses main-ORF translation and is itself translated, while the main ORF escapes eIF2-mediated repression during stress.","evidence":"Ribosome profiling under sodium arsenite stress plus phylogenetic conservation analysis in human cells","pmids":["25621764"],"confidence":"Medium","gaps":["uORF protein product not directly validated in this study","subcellular location and function of any uORF product unknown","mechanism of stress-resistant translation not resolved"]},{"year":2017,"claim":"Defined the reference protein's Golgi localization, experimental membrane topology, and a modulatory role in trafficking of SLC35A2/SLC35A3 complexes, recasting SLC35A4 from an orphan transporter to a regulator of other NSTs.","evidence":"Immunofluorescence, experimental topology assay, FLIM-FRET interaction analysis, and CRISPR-Cas9 knockout with localization/glycosylation readouts","pmids":["28167211"],"confidence":"Medium","gaps":["transported substrate not identified at this stage","molecular basis of complex redistribution not defined","interaction with SLC35A5 functionally uncharacterized"]},{"year":2019,"claim":"Showed that SLC35A4 participates in higher-order ternary assemblies with SLC35A2 and SLC35A3, extending the binary-modulation model to multi-NST complexes.","evidence":"High-throughput FRET- and BiFC-based in vivo interaction screens in Golgi membranes","pmids":["30737517"],"confidence":"Medium","gaps":["stoichiometry and architecture of complexes unknown","functional consequence of ternary assembly not tested","no structural model"]},{"year":2021,"claim":"Identified the reference protein's transport substrate and the structural determinant of its selectivity, answering what the Golgi transporter actually moves.","evidence":"Site-directed mutagenesis of binding-pocket residues with sialylation and ribitol-phosphorylation readouts in SLC35A1 KO cells","pmids":["34015330"],"confidence":"High","gaps":["physiological contribution of SLC35A4 versus SLC35A1 redundancy in vivo not quantified","direct transport assay with purified protein not reported","other potential small-conjugate substrates not surveyed"]},{"year":2021,"claim":"Expanded the reference protein interactome to ATPases, a Golgi pH regulator, a calcium channel, and basigin, hinting at links to Golgi ion/pH homeostasis.","evidence":"Co-IP/pulldown mass spectrometry with NanoBiT split-luciferase confirmation","pmids":["34242836"],"confidence":"Medium","gaps":["functional consequences of these interactions not characterized","no reciprocal validation of all hits","interactions of reference protein vs microprotein not distinguished"]},{"year":2024,"claim":"Validated that the uORF product is a real 103-aa inner mitochondrial membrane microprotein required for maximal respiration, establishing the gene's second functional product.","evidence":"Biochemical fractionation and microscopy for IMM localization; KO with Seahorse respirometry; independent proximity-labeling proteomics in primary tissues","pmids":["38580077","38548729"],"confidence":"High","gaps":["molecular mechanism linking the microprotein to respiration not yet defined","interaction partners within mitochondria not identified at this stage"]},{"year":2025,"claim":"Tied both gene products to oxidative-stress resilience and clarified that stress upregulates the reference protein via the uORF while microprotein levels stay constant, separating the regulation of the two products.","evidence":"Microscopy and fractionation for IMM localization; KO cell lines with rescuable oxidative-stress phenotype; translation analysis under sodium arsenite","pmids":["40545711"],"confidence":"Medium","gaps":["mechanism by which each product protects against oxidative stress unknown","single-lab phenotype","downstream stress effectors not identified"]},{"year":2025,"claim":"Demonstrated in vivo that the microprotein governs mitochondrial lipid composition and fatty acid oxidation, providing organismal evidence for its metabolic role.","evidence":"Knockout mouse model with lipidomics, electron microscopy, acylcarnitine metabolomics, and cold/high-fat-diet physiological challenges in brown adipose tissue","pmids":["40880489"],"confidence":"High","gaps":["direct molecular target controlling cardiolipin/PE levels not identified","tissue-specificity of the phenotype beyond BAT not mapped"]},{"year":2026,"claim":"Unified the model by assigning the microprotein (STREMI) a cristae-morphogenesis function through MIC10-like topology/motifs and confirming the uORF drives integrated-stress-response translation of the downstream Golgi transporter.","evidence":"Bioinformatic pipeline, evolutionary analysis, cristae-morphology studies in cells and mice, and translation-regulation assays","pmids":["42069946"],"confidence":"Medium","gaps":["whether STREMI is a bona fide MICOS subunit not biochemically established","abstract-level methodological detail","interplay between Golgi and mitochondrial functions of the locus unresolved"]},{"year":null,"claim":"It remains unknown how the two physically and functionally distinct products of this single locus are coordinated, and what the in vivo physiological consequence is of coupling Golgi nucleotide-sugar transport to mitochondrial cristae/metabolism via a shared stress-responsive uORF.","evidence":"","pmids":[],"confidence":"Medium","gaps":["no integrated model linking Golgi transport and mitochondrial function","human disease association not established in the corpus","mechanism of microprotein action within MICOS/IMM not biochemically resolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[9]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[5,6,8]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[5,8]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[4,7,9]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2]}],"complexes":["MICOS (STREMI/MIC10-like)","SLC35A2/SLC35A3 NST complex"],"partners":["SLC35A2","SLC35A3","SLC35A1","SLC35A5","ATP2A2","ATP2C1","GPR89B","BSG"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96G79","full_name":"Probable UDP-sugar transporter protein SLC35A4","aliases":["Solute carrier family 35 member A4"],"length_aa":324,"mass_kda":34.6,"function":"Mediates the transport of CDP-ribitol (PubMed:34015330). Does not exhibit CMP-sialic acid, UDP-galactose and UDP-N-acetylglucosamine transport activity (PubMed:28167211, PubMed:34015330)","subcellular_location":"Golgi apparatus membrane","url":"https://www.uniprot.org/uniprotkb/Q96G79/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC35A4","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SLC35A4","total_profiled":1310},"omim":[{"mim_id":"620297","title":"SOLUTE CARRIER FAMILY 35, MEMBER A4; SLC35A4","url":"https://www.omim.org/entry/620297"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Endoplasmic reticulum","reliability":"Uncertain"},{"location":"Centrosome","reliability":"Uncertain"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SLC35A4"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"Q96G79","domains":[{"cath_id":"-","chopping":"51-72_82-162_182-241_249-323","consensus_level":"high","plddt":91.5834,"start":51,"end":323}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96G79","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96G79-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96G79-F1-predicted_aligned_error_v6.png","plddt_mean":84.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC35A4","jax_strain_url":"https://www.jax.org/strain/search?query=SLC35A4"},"sequence":{"accession":"Q96G79","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96G79.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96G79/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96G79"}},"corpus_meta":[{"pmid":"25621764","id":"PMC_25621764","title":"Translation of 5' leaders is pervasive in genes resistant to eIF2 repression.","date":"2015","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/25621764","citation_count":282,"is_preprint":false},{"pmid":"38548729","id":"PMC_38548729","title":"Bioorthogonal photocatalytic proximity labeling in primary living samples.","date":"2024","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/38548729","citation_count":47,"is_preprint":false},{"pmid":"30737517","id":"PMC_30737517","title":"N-acetylglucosaminyltransferases and nucleotide sugar transporters form multi-enzyme-multi-transporter assemblies in golgi membranes in vivo.","date":"2019","source":"Cellular and molecular life sciences : CMLS","url":"https://pubmed.ncbi.nlm.nih.gov/30737517","citation_count":46,"is_preprint":false},{"pmid":"28167211","id":"PMC_28167211","title":"An insight into the orphan nucleotide sugar transporter SLC35A4.","date":"2017","source":"Biochimica et biophysica acta. Molecular cell research","url":"https://pubmed.ncbi.nlm.nih.gov/28167211","citation_count":21,"is_preprint":false},{"pmid":"34015330","id":"PMC_34015330","title":"The promiscuous binding pocket of SLC35A1 ensures redundant transport of CDP-ribitol to the Golgi.","date":"2021","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/34015330","citation_count":19,"is_preprint":false},{"pmid":"38580077","id":"PMC_38580077","title":"An Inner Mitochondrial Membrane Microprotein from the SLC35A4 Upstream ORF Regulates Cellular Metabolism.","date":"2024","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/38580077","citation_count":18,"is_preprint":false},{"pmid":"34242836","id":"PMC_34242836","title":"Identification of novel potential interaction partners of UDP-galactose (SLC35A2), UDP-N-acetylglucosamine (SLC35A3) and an orphan (SLC35A4) nucleotide sugar transporters.","date":"2021","source":"Journal of proteomics","url":"https://pubmed.ncbi.nlm.nih.gov/34242836","citation_count":13,"is_preprint":false},{"pmid":"32066731","id":"PMC_32066731","title":"Expression quantitative trait loci-derived scores and white matter microstructure in UK Biobank: a novel approach to integrating genetics and neuroimaging.","date":"2020","source":"Translational psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/32066731","citation_count":10,"is_preprint":false},{"pmid":"40880489","id":"PMC_40880489","title":"Abnormal mitochondrial structure and function in brown adipose tissue of SLC35A4-MP knockout mice.","date":"2025","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/40880489","citation_count":3,"is_preprint":false},{"pmid":"40545711","id":"PMC_40545711","title":"The dual-coding gene SLC35A4 protects against oxidative stress.","date":"2025","source":"Protein science : a publication of the Protein Society","url":"https://pubmed.ncbi.nlm.nih.gov/40545711","citation_count":3,"is_preprint":false},{"pmid":"40058972","id":"PMC_40058972","title":"A Portrait of Three Mammalian Bicistronic mRNA Transcripts, Derived from the Genes ASNSD1, SLC35A4, and MIEF1.","date":"2025","source":"Biochemistry. Biokhimiia","url":"https://pubmed.ncbi.nlm.nih.gov/40058972","citation_count":0,"is_preprint":false},{"pmid":"42069946","id":"PMC_42069946","title":"STREMI: a dual-function upstream ORF-encoded regulator of mitochondrial cristae architecture.","date":"2026","source":"EMBO reports","url":"https://pubmed.ncbi.nlm.nih.gov/42069946","citation_count":0,"is_preprint":false},{"pmid":"40560747","id":"PMC_40560747","title":"Early Onset Memory Deficit of WMI Rats Compared to Their Nearly Isogenic WLIs Is Reversed by Enriched Environment in Females.","date":"2025","source":"Genes, brain, and behavior","url":"https://pubmed.ncbi.nlm.nih.gov/40560747","citation_count":0,"is_preprint":false},{"pmid":"40060636","id":"PMC_40060636","title":"Early onset memory deficit of WMI rats compared to their nearly isogenic WLIs is reversed by enriched environment in females.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40060636","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8965,"output_tokens":2810,"usd":0.034522,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10131,"output_tokens":3330,"usd":0.066952,"stage2_stop_reason":"end_turn"},"total_usd":0.101474,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2017,\n      \"finding\": \"SLC35A4 (reference protein) localizes to the Golgi apparatus as determined by immunofluorescence of endogenous protein, and has an even number of transmembrane domains with both N- and C-termini facing the cytosol (contradicting in silico topology predictions). FLIM-FRET analysis showed SLC35A4 does not form homomers and does not associate with other SLC35A subfamily members except SLC35A5, but is within 10–40 nm of SLC35A2 and SLC35A3. CRISPR-Cas9 knockout of SLC35A4 altered subcellular distribution of SLC35A2/SLC35A3 complexes, and overexpression of SLC35A4-BFP with SLC35A3 and SLC35A2-Golgi splice variant negatively affected the SLC35A2/SLC35A3 interaction, indicating a modulatory role in intracellular trafficking of these complexes.\",\n      \"method\": \"Immunofluorescence, experimental topology assay, FLIM-FRET interaction analysis, CRISPR-Cas9 knockout with glycosylation and localization readouts\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal FLIM-FRET, CRISPR-KO with defined phenotype, topology assay; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"28167211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SLC35A4 forms higher-order assemblies with SLC35A2 and SLC35A3 in Golgi membranes in vivo, as detected by high-throughput FRET- and BiFC-based interaction screens. Novel ternary complexes between NSTs were also identified.\",\n      \"method\": \"High-throughput FRET and BiFC-based in vivo interaction screens\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — BiFC-FRET in vivo interaction screens, single lab but two orthogonal methods confirming complex formation\",\n      \"pmids\": [\"30737517\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SLC35A4 acts as a redundant transporter of CDP-ribitol into the Golgi apparatus alongside SLC35A1. Mutagenesis of the predicted binding pocket of SLC35A1 to introduce bulky residues present in SLC35A4 abolished sialylation but preserved ribitol phosphorylation, demonstrating that the size of the binding pocket restricts SLC35A4 to smaller cytosine nucleotide conjugates such as CDP-ribitol but not bulkier CMP-sialic acid.\",\n      \"method\": \"Site-directed mutagenesis of binding pocket residues, functional cell-based assay in SLC35A1 KO cells measuring sialylation and ribitol phosphorylation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis of active-site residues with two orthogonal functional readouts (sialylation abolished, ribitol phosphorylation preserved), mechanistic interpretation well-supported\",\n      \"pmids\": [\"34015330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Pulldown experiments identified novel interaction partners of SLC35A4, including two ATPases (ATP2A2, ATP2C1), Golgi pH regulator B (GPR89B), calcium channel (TMCO1), and basigin (BSG); selected interactions were confirmed in vitro using the NanoBiT split-luciferase complementation assay.\",\n      \"method\": \"Co-immunoprecipitation/pull-down followed by mass spectrometry; NanoBiT split-luciferase confirmation\",\n      \"journal\": \"Journal of proteomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — pulldown-MS with NanoBiT confirmation, single lab, two orthogonal methods but no deeper mechanistic characterization of interaction consequences\",\n      \"pmids\": [\"34242836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The SLC35A4 mRNA contains an upstream ORF (uORF) in its 5'-UTR that represses translation of the main coding ORF under normal conditions; phylogenetic analysis suggests this uORF encodes a functional protein product. Ribosome profiling during sodium arsenite-induced stress showed the SLC35A4 main ORF is among those resistant to eIF2-mediated translational repression.\",\n      \"method\": \"Ribosome profiling in human cells under sodium arsenite stress; phylogenetic analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ribosome profiling with genome-wide coverage and phylogenetic support, but functional protein product from uORF not directly validated in this paper\",\n      \"pmids\": [\"25621764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The uORF of SLC35A4 encodes a 103-amino acid microprotein (SLC35A4-MP/AltSLC35A4) that localizes to the inner mitochondrial membrane (IMM) as a single-pass transmembrane protein. Loss-of-function studies showed SLC35A4-MP KO significantly diminishes maximal cellular respiration, establishing a role for this microprotein in cellular metabolism.\",\n      \"method\": \"Biochemical fractionation, microscopy, loss-of-function (KO) with Seahorse respirometry assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — IMM localization by fractionation, loss-of-function with quantitative metabolic readout, replicated by independent study (PMID:40545711)\",\n      \"pmids\": [\"38580077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SLC35A4 uORF-encoded microprotein (SLC35A4-MP) was identified as a mitochondrial protein in primary living samples (dissociated mouse tissues, primary human T cells) using bioorthogonal photocatalytic proximity labeling (CAT-S), confirming its presence in the native mitochondrial proteome.\",\n      \"method\": \"Bioorthogonal photocatalytic proximity labeling (CAT-S) followed by proteomics in primary tissues\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel proximity labeling method in primary tissues confirms mitochondrial localization, single study\",\n      \"pmids\": [\"38548729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"AltSLC35A4 (SLC35A4-MP) localizes to the inner mitochondrial membrane, confirmed by microscopy and biochemical analyses. Knockout of the reference SLC35A4 or AltSLC35A4 enhanced sensitivity to oxidative stress in a rescuable manner. During oxidative stress (sodium arsenite), translational upregulation of SLC35A4 reference protein occurs via the uORF in an upstream ORF-dependent manner, while AltSLC35A4 expression remains unchanged under stress.\",\n      \"method\": \"Microscopy, biochemical fractionation, knockout cell lines with oxidative stress challenge (rescue experiment), ribosome/translation analysis\",\n      \"journal\": \"Protein science : a publication of the Protein Society\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with rescuable phenotype, IMM localization confirmed by two methods, single lab\",\n      \"pmids\": [\"40545711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SLC35A4-MP (STREMI) knockout in mice disrupts mitochondrial lipid composition in brown adipose tissue, decreasing cardiolipins and phosphatidylethanolamine under high-fat diet conditions. KO mice show impaired mitochondrial activity, altered mitochondrial number and morphology, increased inflammation, and accumulation of acylcarnitines during cold exposure indicating defective fatty acid oxidation.\",\n      \"method\": \"Knockout mouse model, lipidomics, electron microscopy of mitochondria, metabolomics (acylcarnitines), cold exposure and HFD physiological challenge\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo KO mouse model with multiple orthogonal readouts (lipidomics, EM, metabolomics, physiological challenges), independent replication of mitochondrial role\",\n      \"pmids\": [\"40880489\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"The SLC35A4 5'UTR-encoded microprotein (named STREMI) localizes to the inner mitochondrial membrane, shares topology and motifs with the MICOS core subunit MIC10, and regulates mitochondrial cristae morphogenesis in mice and human cells. The STREMI-encoding uORF also mediates stress-responsive translation of the downstream SLC35A4 Golgi transporter ORF during the integrated stress response.\",\n      \"method\": \"Bioinformatic pipeline (four-layered), functional cell and mouse studies of cristae morphology, evolutionary analysis, translation regulation assay\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional cristae morphogenesis phenotype in cells and mice, structural homology to MIC10, single lab with multiple methods but abstract lacks full methodological detail\",\n      \"pmids\": [\"42069946\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC35A4 is a dual-coding gene: the reference protein is a Golgi-localized nucleotide sugar transporter that redundantly transports CDP-ribitol into the Golgi (alongside SLC35A1, with binding-pocket size determining substrate selectivity) and modulates SLC35A2/SLC35A3 transporter complex distribution; the uORF in its 5'UTR encodes a 103-amino acid single-pass inner mitochondrial membrane microprotein (SLC35A4-MP/AltSLC35A4/STREMI) that regulates mitochondrial cristae morphogenesis, lipid composition, fatty acid oxidation, and maximal cellular respiration, while the uORF also mediates stress-responsive upregulation of the downstream SLC35A4 reference protein during the integrated stress response.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC35A4 is a dual-coding locus whose two products act in distinct organelles: a Golgi-resident reference transporter and a mitochondrial microprotein encoded by a 5'UTR upstream ORF [#0, #5, #9]. The reference protein localizes to the Golgi apparatus with an even number of transmembrane domains and cytosol-facing N- and C-termini, and functions as a redundant nucleotide-sugar transporter that imports CDP-ribitol alongside SLC35A1; the size of its substrate-binding pocket restricts it to small cytosine nucleotide conjugates and excludes bulkier CMP-sialic acid [#0, #2]. Beyond transport, it modulates the subcellular distribution and assembly of SLC35A2/SLC35A3 transporter complexes, forming higher-order assemblies with these partners in Golgi membranes [#0, #1]. The uORF encodes a 103-amino acid single-pass inner mitochondrial membrane microprotein (SLC35A4-MP/AltSLC35A4/STREMI) that shares topology and motifs with the MICOS subunit MIC10 and regulates cristae morphogenesis, mitochondrial lipid composition, fatty acid oxidation, and maximal respiration [#5, #8, #9]. The same uORF couples the two products to cellular stress: it represses reference-protein translation under normal conditions but mediates its translational upregulation during the integrated stress response, and loss of either product sensitizes cells to oxidative stress [#4, #7, #9].\",\n  \"teleology\": [\n    {\n      \"year\": 2015,\n      \"claim\": \"Established that SLC35A4 is not a simple single-protein gene but carries a 5'UTR uORF that represses main-ORF translation and is itself translated, while the main ORF escapes eIF2-mediated repression during stress.\",\n      \"evidence\": \"Ribosome profiling under sodium arsenite stress plus phylogenetic conservation analysis in human cells\",\n      \"pmids\": [\"25621764\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"uORF protein product not directly validated in this study\", \"subcellular location and function of any uORF product unknown\", \"mechanism of stress-resistant translation not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined the reference protein's Golgi localization, experimental membrane topology, and a modulatory role in trafficking of SLC35A2/SLC35A3 complexes, recasting SLC35A4 from an orphan transporter to a regulator of other NSTs.\",\n      \"evidence\": \"Immunofluorescence, experimental topology assay, FLIM-FRET interaction analysis, and CRISPR-Cas9 knockout with localization/glycosylation readouts\",\n      \"pmids\": [\"28167211\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"transported substrate not identified at this stage\", \"molecular basis of complex redistribution not defined\", \"interaction with SLC35A5 functionally uncharacterized\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Showed that SLC35A4 participates in higher-order ternary assemblies with SLC35A2 and SLC35A3, extending the binary-modulation model to multi-NST complexes.\",\n      \"evidence\": \"High-throughput FRET- and BiFC-based in vivo interaction screens in Golgi membranes\",\n      \"pmids\": [\"30737517\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"stoichiometry and architecture of complexes unknown\", \"functional consequence of ternary assembly not tested\", \"no structural model\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified the reference protein's transport substrate and the structural determinant of its selectivity, answering what the Golgi transporter actually moves.\",\n      \"evidence\": \"Site-directed mutagenesis of binding-pocket residues with sialylation and ribitol-phosphorylation readouts in SLC35A1 KO cells\",\n      \"pmids\": [\"34015330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"physiological contribution of SLC35A4 versus SLC35A1 redundancy in vivo not quantified\", \"direct transport assay with purified protein not reported\", \"other potential small-conjugate substrates not surveyed\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Expanded the reference protein interactome to ATPases, a Golgi pH regulator, a calcium channel, and basigin, hinting at links to Golgi ion/pH homeostasis.\",\n      \"evidence\": \"Co-IP/pulldown mass spectrometry with NanoBiT split-luciferase confirmation\",\n      \"pmids\": [\"34242836\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"functional consequences of these interactions not characterized\", \"no reciprocal validation of all hits\", \"interactions of reference protein vs microprotein not distinguished\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Validated that the uORF product is a real 103-aa inner mitochondrial membrane microprotein required for maximal respiration, establishing the gene's second functional product.\",\n      \"evidence\": \"Biochemical fractionation and microscopy for IMM localization; KO with Seahorse respirometry; independent proximity-labeling proteomics in primary tissues\",\n      \"pmids\": [\"38580077\", \"38548729\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"molecular mechanism linking the microprotein to respiration not yet defined\", \"interaction partners within mitochondria not identified at this stage\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Tied both gene products to oxidative-stress resilience and clarified that stress upregulates the reference protein via the uORF while microprotein levels stay constant, separating the regulation of the two products.\",\n      \"evidence\": \"Microscopy and fractionation for IMM localization; KO cell lines with rescuable oxidative-stress phenotype; translation analysis under sodium arsenite\",\n      \"pmids\": [\"40545711\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"mechanism by which each product protects against oxidative stress unknown\", \"single-lab phenotype\", \"downstream stress effectors not identified\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrated in vivo that the microprotein governs mitochondrial lipid composition and fatty acid oxidation, providing organismal evidence for its metabolic role.\",\n      \"evidence\": \"Knockout mouse model with lipidomics, electron microscopy, acylcarnitine metabolomics, and cold/high-fat-diet physiological challenges in brown adipose tissue\",\n      \"pmids\": [\"40880489\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"direct molecular target controlling cardiolipin/PE levels not identified\", \"tissue-specificity of the phenotype beyond BAT not mapped\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Unified the model by assigning the microprotein (STREMI) a cristae-morphogenesis function through MIC10-like topology/motifs and confirming the uORF drives integrated-stress-response translation of the downstream Golgi transporter.\",\n      \"evidence\": \"Bioinformatic pipeline, evolutionary analysis, cristae-morphology studies in cells and mice, and translation-regulation assays\",\n      \"pmids\": [\"42069946\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"whether STREMI is a bona fide MICOS subunit not biochemically established\", \"abstract-level methodological detail\", \"interplay between Golgi and mitochondrial functions of the locus unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how the two physically and functionally distinct products of this single locus are coordinated, and what the in vivo physiological consequence is of coupling Golgi nucleotide-sugar transport to mitochondrial cristae/metabolism via a shared stress-responsive uORF.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"no integrated model linking Golgi transport and mitochondrial function\", \"human disease association not established in the corpus\", \"mechanism of microprotein action within MICOS/IMM not biochemically resolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005743\", \"supporting_discovery_ids\": [5, 6, 7, 8, 9]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [5, 6, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5, 8]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [4, 7, 9]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [\"MICOS (STREMI/MIC10-like)\", \"SLC35A2/SLC35A3 NST complex\"],\n    \"partners\": [\"SLC35A2\", \"SLC35A3\", \"SLC35A1\", \"SLC35A5\", \"ATP2A2\", \"ATP2C1\", \"GPR89B\", \"BSG\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}