{"gene":"SLC6A20","run_date":"2026-06-10T07:46:34","timeline":{"discoveries":[{"year":2005,"finding":"SLC6A20 (SIT1) was identified as the molecular correlate of the classical System IMINO transporter. When expressed in Xenopus oocytes, rat SIT1 mediated Na+- and Cl--dependent, voltage-dependent electrogenic uptake of imino acids (proline, K0.5 ~0.2 mM; pipecolate) and N-methylated amino acids (MeAIB, sarcosine). Transport was pH-independent, insensitive to alanine or lysine inhibition, and Li+ (but not H+) could substitute for Na+. Human SIT1 also functioned as a Na+-dependent proline transporter.","method":"Functional expression in Xenopus laevis oocytes with electrophysiology and radiolabeled substrate uptake assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted transport activity in Xenopus oocytes with detailed kinetic and ion-dependence characterization; replicated for both rat and human orthologs in same study","pmids":["15632147"],"is_preprint":false},{"year":2005,"finding":"Mouse SLC6A20 has two homologs (XT3 and XT3s1). Expression of XT3s1 (but not XT3) in Xenopus oocytes induced electrogenic Na+- and Cl--dependent transport of proline, hydroxyproline, betaine, N-methylaminoisobutyric acid, and pipecolic acid, confirming XT3s1 as the IMINO system. XT3s1 was expressed in brain, kidney, small intestine, thymus, spleen and lung; XT3 was expressed mainly in kidney and lung.","method":"Functional expression in Xenopus laevis oocytes; electrophysiology; RT-PCR for tissue distribution","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — electrogenic transport reconstituted in Xenopus oocytes; negative control (XT3 non-functional) strengthens specificity; independent of PMID 15632147","pmids":["15689184"],"is_preprint":false},{"year":2005,"finding":"In opossum kidney (OK) cells, SLC6A20 ortholog (oSIT1) mediates not only imino acid (L-Pro) but also Na+-dependent neutral L-amino acid transport. Antisense RNA to oSIT1 (but not oXT2/SLC6A18) inhibited Na+-dependent neutral amino acid transport from OK cell mRNA injected into Xenopus oocytes; siRNA/shRNA knockdown of oSIT1 in OK cells selectively reduced Na+-dependent neutral L-amino acid uptake.","method":"Antisense RNA injection and siRNA/shRNA knockdown in opossum kidney cells and Xenopus oocytes; radiolabeled amino acid uptake assays","journal":"American journal of physiology. Renal physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal loss-of-function approaches (antisense RNA, siRNA) in a single lab; functional readout with defined substrate specificity","pmids":["16234310"],"is_preprint":false},{"year":2006,"finding":"SLC6A20 (IMINO transporter) is distinguished from SLC36A1 (PAT1/imino acid carrier) as two distinct intestinal transport systems: SLC6A20 corresponds to the betaine carrier and IMINO transporter (Na+-dependent, proline-specific, alanine-insensitive) identified in hamster/rabbit/guinea pig, while SLC36A1 is a proton-coupled imino acid symporter that cooperates with NHE3 to appear Na+-dependent in intact epithelia.","method":"Functional comparison and review of heterologous expression data from prior studies; integration of pharmacological and kinetic properties","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — mechanistic distinction based on integration of multiple prior functional expression studies; no new primary experiment but clear pathway placement","pmids":["17123464"],"is_preprint":false},{"year":2010,"finding":"Developmental iminoglycinuria in neonatal mice is caused by delayed post-natal expression and apical membrane targeting of Slc6a20a (along with Slc6a18, Slc6a19, and Slc36a2) in kidney proximal tubule. qPCR and Western blot showed Slc6a20a expression was ~2.4-fold lower at birth than in adult kidney; immunofluorescence confocal microscopy confirmed absence of apical Slc6a20a protein in neonatal kidney, correlating with iminoglycinuria during the first week of life.","method":"qPCR, Western blot, immunofluorescence confocal microscopy in developing mouse kidney; correlation with urinary amino acid measurements","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct subcellular localization by immunofluorescence tied to functional consequence (iminoglycinuria), supported by qPCR and Western blot; single lab","pmids":["20377526"],"is_preprint":false},{"year":2021,"finding":"SLC6A20A regulates brain glycine and proline homeostasis and NMDAR function in vivo. Mice overexpressing SLC6A20A (via enhanced β-catenin binding to the Slc6a20a gene promoter through mutant PTEN lacking C-terminus) showed reduced extracellular brain proline and glycine and decreased NMDAR currents. Antisense oligonucleotide knockdown of SLC6A20 or competitive GlyT1 antagonist sarcosine normalized NMDAR currents and repetitive behavior. SLC6A20A knockout mice showed increased extracellular glycine and NMDAR currents. Both mouse and human SLC6A20 proteins mediated proline and glycine transport in heterologous expression.","method":"Genetic mouse models (overexpression via PTEN mutation, knockout); antisense oligonucleotide knockdown; in vivo microdialysis for extracellular amino acids; electrophysiology for NMDAR currents; heterologous expression for transport characterization","journal":"EMBO molecular medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal genetic approaches (gain-of-function, loss-of-function KO, ASO knockdown), in vivo neurochemical measurements, and electrophysiological readouts; rescue experiments performed","pmids":["33428810"],"is_preprint":false},{"year":2023,"finding":"SLC6A20 is a sodium-dependent symporter in the SLC6 family that requires the ancillary protein ACE2 to reach the cell membrane, mediates intestinal absorption and renal reabsorption of imino acids (proline, hydroxyproline) and glycine, and mutations in SLC6A20 are associated with iminoglycinuria and/or hyperglycinuria.","method":"Review integrating functional expression, genetic association, and biochemical studies from multiple prior publications","journal":"Pharmacological reviews","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — review paper summarizing replicated findings from multiple labs including ACE2 requirement for membrane trafficking; no new primary experiment","pmids":["37940347"],"is_preprint":false}],"current_model":"SLC6A20 (SIT1/IMINO transporter) is a Na+- and Cl--dependent electrogenic symporter in the SLC6 family that transports imino acids (proline, hydroxyproline, pipecolic acid) and N-methylated amino acids at the apical membrane of intestinal epithelia and kidney proximal tubule (requiring ACE2 as an ancillary subunit for membrane targeting); in the brain, SLC6A20A regulates extracellular proline and glycine levels to modulate NMDA receptor activity, and its delayed post-natal renal expression underlies developmental iminoglycinuria."},"narrative":{"mechanistic_narrative":"SLC6A20 (SIT1) is the molecular correlate of the classical System IMINO transporter, an Na+- and Cl--dependent, voltage-dependent electrogenic symporter that mediates uptake of imino acids (proline, hydroxyproline, pipecolate) and N-methylated amino acids (MeAIB, sarcosine, betaine) [PMID:15632147, PMID:15689184]. Transport is pH-independent and insensitive to alanine or lysine, distinguishing it from the proton-coupled imino acid carrier SLC36A1/PAT1 [PMID:15632147, PMID:17123464]. At the apical membrane of kidney proximal tubule, delayed post-natal expression and membrane targeting of Slc6a20a, together with related transporters, accounts for the transient iminoglycinuria seen in neonatal mice [PMID:20377526]. In the brain, SLC6A20A transports both proline and glycine and thereby sets extracellular glycine and proline levels that tune NMDA receptor activity: gain-of-function lowers extracellular glycine/proline and NMDAR currents while knockout raises them, and pharmacological or antisense manipulation normalizes NMDAR currents and associated behavior [PMID:33428810]. Beyond these reconstituted transport and homeostatic roles, the structural basis of substrate recognition has not been characterized in the available corpus.","teleology":[{"year":2005,"claim":"Established the molecular identity of the long-described System IMINO activity by showing that SLC6A20/SIT1 is itself an electrogenic Na+/Cl--dependent imino acid transporter.","evidence":"Functional expression of rat and human SIT1 in Xenopus oocytes with electrophysiology and radiolabeled uptake; independently corroborated by mouse XT3s1 (with non-functional XT3 as negative control)","pmids":["15632147","15689184"],"confidence":"High","gaps":["Substrate specificity defined in oocytes only, not native epithelia","No structural model of the transport mechanism"]},{"year":2005,"claim":"Extended the functional repertoire by indicating SLC6A20 also contributes to Na+-dependent neutral amino acid transport in a renal epithelial context.","evidence":"Antisense RNA and siRNA/shRNA knockdown of oSIT1 in opossum kidney cells with uptake assays","pmids":["16234310"],"confidence":"Medium","gaps":["Single-lab loss-of-function; broader substrate claim not reconstituted in defined heterologous system","Relationship to canonical imino specificity unresolved"]},{"year":2006,"claim":"Clarified pathway placement by separating SLC6A20 (IMINO/betaine carrier) from the proton-coupled SLC36A1/PAT1 system that only appears Na+-dependent in intact epithelia.","evidence":"Comparative integration of prior heterologous expression and pharmacological/kinetic data","pmids":["17123464"],"confidence":"Medium","gaps":["Review-level synthesis, no new primary experiment","In vivo division of labor between the two systems not measured directly"]},{"year":2010,"claim":"Linked SLC6A20 regulation to a physiological phenotype by showing that delayed neonatal apical expression underlies transient iminoglycinuria.","evidence":"qPCR, Western blot, and immunofluorescence confocal microscopy in developing mouse kidney correlated with urinary amino acid profiles","pmids":["20377526"],"confidence":"Medium","gaps":["Co-regulated with Slc6a18/Slc6a19/Slc36a2, so individual contribution not isolated","Mechanism controlling developmental timing of expression unknown"]},{"year":2021,"claim":"Defined a brain function by demonstrating that SLC6A20A controls extracellular glycine/proline and thereby NMDAR activity, with bidirectional genetic and pharmacological control.","evidence":"Mouse overexpression and knockout models, ASO knockdown, in vivo microdialysis, NMDAR electrophysiology, and heterologous transport assays with rescue","pmids":["33428810"],"confidence":"High","gaps":["Cell types and circuits mediating the NMDAR effect not fully resolved","Relative weighting of glycine vs proline contribution to NMDAR modulation not separated"]},{"year":2023,"claim":"Consolidated the model that SLC6A20 requires ACE2 for membrane trafficking and that its mutations associate with iminoglycinuria/hyperglycinuria.","evidence":"Review integrating functional expression, genetic association, and biochemical data across labs","pmids":["37940347"],"confidence":"Medium","gaps":["ACE2 dependence summarized, not newly tested here","Genotype-phenotype relationship for specific human mutations not detailed"]},{"year":null,"claim":"How SLC6A20 selects imino acids versus glycine at the atomic level, and how its expression and ACE2-dependent trafficking are regulated across tissues, remains open.","evidence":"No structural or trafficking-mechanism study present in the corpus","pmids":[],"confidence":"Low","gaps":["No structure of the transporter","Molecular basis of ACE2-assisted membrane targeting uncharacterized","Transcriptional control of developmental and tissue-specific expression unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,5]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,6]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,1,4]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[5]}],"complexes":[],"partners":["ACE2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NP91","full_name":"Sodium- and chloride-dependent transporter XTRP3","aliases":["Sodium/imino-acid transporter 1","Solute carrier family 6 member 20","Transporter rB21A homolog"],"length_aa":592,"mass_kda":65.9,"function":"Mediates the Na(+)- and Cl(-)-dependent uptake of imino acids such as L-proline, N-methyl-L-proline and pipecolate as well as N-methylated amino acids (PubMed:15632147, PubMed:19033659, PubMed:33428810). Also transports glycine, regulates proline and glycine homeostasis in the brain playing a role in the modulation of NMDAR currents (PubMed:33428810)","subcellular_location":"Apical cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9NP91/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC6A20","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/SLC6A20","total_profiled":1310},"omim":[{"mim_id":"610300","title":"SOLUTE CARRIER FAMILY 6 (NEUROTRANSMITTER TRANSPORTER), MEMBER 18; SLC6A18","url":"https://www.omim.org/entry/610300"},{"mim_id":"608893","title":"SOLUTE CARRIER FAMILY 6 (NEUROTRANSMITTER TRANSPORTER), MEMBER 19; SLC6A19","url":"https://www.omim.org/entry/608893"},{"mim_id":"608331","title":"SOLUTE CARRIER FAMILY 36 (PROTON/AMINO ACID SYMPORTER), MEMBER 2; SLC36A2","url":"https://www.omim.org/entry/608331"},{"mim_id":"605616","title":"SOLUTE CARRIER FAMILY 6 (PROLINE IMINO TRANSPORTER), MEMBER 20; SLC6A20","url":"https://www.omim.org/entry/605616"},{"mim_id":"242600","title":"IMINOGLYCINURIA","url":"https://www.omim.org/entry/242600"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"choroid plexus","ntpm":47.2},{"tissue":"intestine","ntpm":23.8}],"url":"https://www.proteinatlas.org/search/SLC6A20"},"hgnc":{"alias_symbol":["XT3","Xtrp3","SIT1","IMINO"],"prev_symbol":[]},"alphafold":{"accession":"Q9NP91","domains":[{"cath_id":"-","chopping":"12-111_211-403_456-581","consensus_level":"high","plddt":93.3154,"start":12,"end":581}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NP91","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NP91-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NP91-F1-predicted_aligned_error_v6.png","plddt_mean":93.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC6A20","jax_strain_url":"https://www.jax.org/strain/search?query=SLC6A20"},"sequence":{"accession":"Q9NP91","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NP91.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NP91/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NP91"}},"corpus_meta":[{"pmid":"2432665","id":"PMC_2432665","title":"Macrophage cytotoxicity: role for L-arginine deiminase and imino nitrogen oxidation to nitrite.","date":"1987","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/2432665","citation_count":1456,"is_preprint":false},{"pmid":"3037381","id":"PMC_3037381","title":"A single mode of DNA base-pair opening drives imino proton exchange.","date":"1987","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/3037381","citation_count":292,"is_preprint":false},{"pmid":"2819069","id":"PMC_2819069","title":"NMR studies of DNA (R+)n.(Y-)n.(Y+)n triple helices in solution: imino and amino proton markers of T.A.T and C.G.C+ base-triple formation.","date":"1989","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/2819069","citation_count":184,"is_preprint":false},{"pmid":"15901676","id":"PMC_15901676","title":"Imino sugar inhibitors for treating the lysosomal glycosphingolipidoses.","date":"2005","source":"Glycobiology","url":"https://pubmed.ncbi.nlm.nih.gov/15901676","citation_count":169,"is_preprint":false},{"pmid":"8134380","id":"PMC_8134380","title":"Secretion of human hepatitis B virus is inhibited by the imino sugar N-butyldeoxynojirimycin.","date":"1994","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/8134380","citation_count":148,"is_preprint":false},{"pmid":"8663612","id":"PMC_8663612","title":"Evidence for an imino intermediate in the DNA polymerase beta deoxyribose phosphate excision reaction.","date":"1996","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/8663612","citation_count":147,"is_preprint":false},{"pmid":"7642635","id":"PMC_7642635","title":"Studies on the catalytic mechanism of five DNA glycosylases. 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When expressed in Xenopus oocytes, rat SIT1 mediated Na+- and Cl--dependent, voltage-dependent electrogenic uptake of imino acids (proline, K0.5 ~0.2 mM; pipecolate) and N-methylated amino acids (MeAIB, sarcosine). Transport was pH-independent, insensitive to alanine or lysine inhibition, and Li+ (but not H+) could substitute for Na+. Human SIT1 also functioned as a Na+-dependent proline transporter.\",\n      \"method\": \"Functional expression in Xenopus laevis oocytes with electrophysiology and radiolabeled substrate uptake assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted transport activity in Xenopus oocytes with detailed kinetic and ion-dependence characterization; replicated for both rat and human orthologs in same study\",\n      \"pmids\": [\"15632147\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mouse SLC6A20 has two homologs (XT3 and XT3s1). Expression of XT3s1 (but not XT3) in Xenopus oocytes induced electrogenic Na+- and Cl--dependent transport of proline, hydroxyproline, betaine, N-methylaminoisobutyric acid, and pipecolic acid, confirming XT3s1 as the IMINO system. XT3s1 was expressed in brain, kidney, small intestine, thymus, spleen and lung; XT3 was expressed mainly in kidney and lung.\",\n      \"method\": \"Functional expression in Xenopus laevis oocytes; electrophysiology; RT-PCR for tissue distribution\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — electrogenic transport reconstituted in Xenopus oocytes; negative control (XT3 non-functional) strengthens specificity; independent of PMID 15632147\",\n      \"pmids\": [\"15689184\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"In opossum kidney (OK) cells, SLC6A20 ortholog (oSIT1) mediates not only imino acid (L-Pro) but also Na+-dependent neutral L-amino acid transport. Antisense RNA to oSIT1 (but not oXT2/SLC6A18) inhibited Na+-dependent neutral amino acid transport from OK cell mRNA injected into Xenopus oocytes; siRNA/shRNA knockdown of oSIT1 in OK cells selectively reduced Na+-dependent neutral L-amino acid uptake.\",\n      \"method\": \"Antisense RNA injection and siRNA/shRNA knockdown in opossum kidney cells and Xenopus oocytes; radiolabeled amino acid uptake assays\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal loss-of-function approaches (antisense RNA, siRNA) in a single lab; functional readout with defined substrate specificity\",\n      \"pmids\": [\"16234310\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SLC6A20 (IMINO transporter) is distinguished from SLC36A1 (PAT1/imino acid carrier) as two distinct intestinal transport systems: SLC6A20 corresponds to the betaine carrier and IMINO transporter (Na+-dependent, proline-specific, alanine-insensitive) identified in hamster/rabbit/guinea pig, while SLC36A1 is a proton-coupled imino acid symporter that cooperates with NHE3 to appear Na+-dependent in intact epithelia.\",\n      \"method\": \"Functional comparison and review of heterologous expression data from prior studies; integration of pharmacological and kinetic properties\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — mechanistic distinction based on integration of multiple prior functional expression studies; no new primary experiment but clear pathway placement\",\n      \"pmids\": [\"17123464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Developmental iminoglycinuria in neonatal mice is caused by delayed post-natal expression and apical membrane targeting of Slc6a20a (along with Slc6a18, Slc6a19, and Slc36a2) in kidney proximal tubule. qPCR and Western blot showed Slc6a20a expression was ~2.4-fold lower at birth than in adult kidney; immunofluorescence confocal microscopy confirmed absence of apical Slc6a20a protein in neonatal kidney, correlating with iminoglycinuria during the first week of life.\",\n      \"method\": \"qPCR, Western blot, immunofluorescence confocal microscopy in developing mouse kidney; correlation with urinary amino acid measurements\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct subcellular localization by immunofluorescence tied to functional consequence (iminoglycinuria), supported by qPCR and Western blot; single lab\",\n      \"pmids\": [\"20377526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SLC6A20A regulates brain glycine and proline homeostasis and NMDAR function in vivo. Mice overexpressing SLC6A20A (via enhanced β-catenin binding to the Slc6a20a gene promoter through mutant PTEN lacking C-terminus) showed reduced extracellular brain proline and glycine and decreased NMDAR currents. Antisense oligonucleotide knockdown of SLC6A20 or competitive GlyT1 antagonist sarcosine normalized NMDAR currents and repetitive behavior. SLC6A20A knockout mice showed increased extracellular glycine and NMDAR currents. Both mouse and human SLC6A20 proteins mediated proline and glycine transport in heterologous expression.\",\n      \"method\": \"Genetic mouse models (overexpression via PTEN mutation, knockout); antisense oligonucleotide knockdown; in vivo microdialysis for extracellular amino acids; electrophysiology for NMDAR currents; heterologous expression for transport characterization\",\n      \"journal\": \"EMBO molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal genetic approaches (gain-of-function, loss-of-function KO, ASO knockdown), in vivo neurochemical measurements, and electrophysiological readouts; rescue experiments performed\",\n      \"pmids\": [\"33428810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SLC6A20 is a sodium-dependent symporter in the SLC6 family that requires the ancillary protein ACE2 to reach the cell membrane, mediates intestinal absorption and renal reabsorption of imino acids (proline, hydroxyproline) and glycine, and mutations in SLC6A20 are associated with iminoglycinuria and/or hyperglycinuria.\",\n      \"method\": \"Review integrating functional expression, genetic association, and biochemical studies from multiple prior publications\",\n      \"journal\": \"Pharmacological reviews\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — review paper summarizing replicated findings from multiple labs including ACE2 requirement for membrane trafficking; no new primary experiment\",\n      \"pmids\": [\"37940347\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC6A20 (SIT1/IMINO transporter) is a Na+- and Cl--dependent electrogenic symporter in the SLC6 family that transports imino acids (proline, hydroxyproline, pipecolic acid) and N-methylated amino acids at the apical membrane of intestinal epithelia and kidney proximal tubule (requiring ACE2 as an ancillary subunit for membrane targeting); in the brain, SLC6A20A regulates extracellular proline and glycine levels to modulate NMDA receptor activity, and its delayed post-natal renal expression underlies developmental iminoglycinuria.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC6A20 (SIT1) is the molecular correlate of the classical System IMINO transporter, an Na+- and Cl--dependent, voltage-dependent electrogenic symporter that mediates uptake of imino acids (proline, hydroxyproline, pipecolate) and N-methylated amino acids (MeAIB, sarcosine, betaine) [#0, #1]. Transport is pH-independent and insensitive to alanine or lysine, distinguishing it from the proton-coupled imino acid carrier SLC36A1/PAT1 [#0, #3]. At the apical membrane of kidney proximal tubule, delayed post-natal expression and membrane targeting of Slc6a20a, together with related transporters, accounts for the transient iminoglycinuria seen in neonatal mice [#4]. In the brain, SLC6A20A transports both proline and glycine and thereby sets extracellular glycine and proline levels that tune NMDA receptor activity: gain-of-function lowers extracellular glycine/proline and NMDAR currents while knockout raises them, and pharmacological or antisense manipulation normalizes NMDAR currents and associated behavior [#5]. Beyond these reconstituted transport and homeostatic roles, the structural basis of substrate recognition has not been characterized in the available corpus.\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Established the molecular identity of the long-described System IMINO activity by showing that SLC6A20/SIT1 is itself an electrogenic Na+/Cl--dependent imino acid transporter.\",\n      \"evidence\": \"Functional expression of rat and human SIT1 in Xenopus oocytes with electrophysiology and radiolabeled uptake; independently corroborated by mouse XT3s1 (with non-functional XT3 as negative control)\",\n      \"pmids\": [\"15632147\", \"15689184\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate specificity defined in oocytes only, not native epithelia\", \"No structural model of the transport mechanism\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Extended the functional repertoire by indicating SLC6A20 also contributes to Na+-dependent neutral amino acid transport in a renal epithelial context.\",\n      \"evidence\": \"Antisense RNA and siRNA/shRNA knockdown of oSIT1 in opossum kidney cells with uptake assays\",\n      \"pmids\": [\"16234310\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab loss-of-function; broader substrate claim not reconstituted in defined heterologous system\", \"Relationship to canonical imino specificity unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Clarified pathway placement by separating SLC6A20 (IMINO/betaine carrier) from the proton-coupled SLC36A1/PAT1 system that only appears Na+-dependent in intact epithelia.\",\n      \"evidence\": \"Comparative integration of prior heterologous expression and pharmacological/kinetic data\",\n      \"pmids\": [\"17123464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review-level synthesis, no new primary experiment\", \"In vivo division of labor between the two systems not measured directly\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Linked SLC6A20 regulation to a physiological phenotype by showing that delayed neonatal apical expression underlies transient iminoglycinuria.\",\n      \"evidence\": \"qPCR, Western blot, and immunofluorescence confocal microscopy in developing mouse kidney correlated with urinary amino acid profiles\",\n      \"pmids\": [\"20377526\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Co-regulated with Slc6a18/Slc6a19/Slc36a2, so individual contribution not isolated\", \"Mechanism controlling developmental timing of expression unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined a brain function by demonstrating that SLC6A20A controls extracellular glycine/proline and thereby NMDAR activity, with bidirectional genetic and pharmacological control.\",\n      \"evidence\": \"Mouse overexpression and knockout models, ASO knockdown, in vivo microdialysis, NMDAR electrophysiology, and heterologous transport assays with rescue\",\n      \"pmids\": [\"33428810\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cell types and circuits mediating the NMDAR effect not fully resolved\", \"Relative weighting of glycine vs proline contribution to NMDAR modulation not separated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Consolidated the model that SLC6A20 requires ACE2 for membrane trafficking and that its mutations associate with iminoglycinuria/hyperglycinuria.\",\n      \"evidence\": \"Review integrating functional expression, genetic association, and biochemical data across labs\",\n      \"pmids\": [\"37940347\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ACE2 dependence summarized, not newly tested here\", \"Genotype-phenotype relationship for specific human mutations not detailed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How SLC6A20 selects imino acids versus glycine at the atomic level, and how its expression and ACE2-dependent trafficking are regulated across tissues, remains open.\",\n      \"evidence\": \"No structural or trafficking-mechanism study present in the corpus\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of the transporter\", \"Molecular basis of ACE2-assisted membrane targeting uncharacterized\", \"Transcriptional control of developmental and tissue-specific expression unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 4]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ACE2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}