{"gene":"SLC15A4","run_date":"2026-06-10T07:46:32","timeline":{"discoveries":[{"year":2020,"finding":"SLC15A4 physically interacts with TASL (encoded by CXorf21) on the endolysosome; TASL localization and function depend on this interaction. Loss of either SLC15A4 or TASL specifically abrogates IRF5 pathway activation downstream of TLR7, TLR8, and TLR9 without affecting NF-κB or MAPK signaling. TASL contains a conserved pLxIS motif that recruits and activates IRF5.","method":"Co-immunoprecipitation, extensive mutagenesis of TASL, genetic deletion of SLC15A4 or TASL in primary and transformed human immune cells, cytokine/IFN reporter assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, mutagenesis, multiple cell types and orthogonal functional readouts in a single rigorous study","pmids":["32433612"],"is_preprint":false},{"year":2010,"finding":"A loss-of-function mutation in Slc15a4 (feeble) abolishes both TLR7- and TLR9-induced type I IFN and proinflammatory cytokine production specifically in plasmacytoid dendritic cells (pDCs), while leaving TLR responses intact in conventional dendritic cells. Slc15a4 was identified as part of a membrane-trafficking pathway (together with AP-3, BLOC-1, and BLOC-2 Hermansky-Pudlak proteins) uniquely required for endosomal TLR signaling in pDCs.","method":"Forward genetic ENU screen, positional cloning of feeble mutation, cytokine assays in pDCs vs. cDCs from mutant mice, genetic epistasis with AP-3/BLOC knockout mice","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic screen, positional cloning, epistasis with multiple pathway members, replicated across multiple cell types","pmids":["21045126"],"is_preprint":false},{"year":2014,"finding":"SLC15A4 is a lysosome-resident proton-coupled transporter that moves histidine and oligopeptides from the lysosomal lumen to the cytosol. Its transporter activity is required for TLR7/9-triggered IFN-I production in B cells. SLC15A4 loss disrupts endolysosomal pH regulation and v-ATPase integrity, leading to failure of the mTOR pathway and consequently the IRF7-IFN-I regulatory circuit.","method":"Genetic deletion of Slc15a4 in mice, transporter-activity rescue experiments with transporter-dead mutants, endolysosomal pH measurements, mTOR pathway biochemical assays, mouse lupus model","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 / Strong — KO mice with defined phenotype, transporter-activity mutagenesis rescue, pH/mTOR mechanistic follow-up, multiple orthogonal methods","pmids":["25238095"],"is_preprint":false},{"year":2022,"finding":"SLC15A4 is required for trafficking and colocalization of nucleic acid-sensing TLRs and their ligands to endolysosomes, and for formation of the LAMP2+VAMP3+ hybrid compartment where IFN-I production is initiated. SLC15A4's endolysosomal trafficking and function depend on the AP-3 complex. SLC15A4, but not its homolog SLC15A3, is required for TLR-driven IFN-I production in pDCs.","method":"Live-cell imaging of TLR/ligand trafficking, genetic deletion of SLC15A4 vs. SLC15A3 in mice, AP-3 complex requirement tested by genetics, confocal colocalization assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization experiments with functional consequence, genetic epistasis with AP-3, negative control with SLC15A3 paralog, multiple orthogonal methods","pmids":["35349343"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of human SLC15A4 reveal apo monomeric and dimeric outward-facing conformations and a TASL-bound complex. In the TASL-bound state, SLC15A4 undergoes a conformational change from outward-facing to inward-facing, forming a binding pocket into which the N-terminal helix of TASL inserts. The dimeric apo form has an extensive interface involving four cholesterol molecules.","method":"Cryo-EM structure determination of apo and TASL-bound SLC15A4","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution cryo-EM structure with functional complex, multiple conformational states resolved","pmids":["37863913"],"is_preprint":false},{"year":2023,"finding":"The small molecule feeblin binds SLC15A4 in a lysosomal outward-open conformation that is incompatible with TASL binding on the cytoplasmic side. Feeblin binding leads to proteostatic degradation of TASL, thereby inhibiting TLR7/8-IRF5 signaling. Cryo-EM structure of feeblin-bound SLC15A4 defines the inhibitor-binding site.","method":"Phenotypic screen for TASL degradation, cryo-EM structure of feeblin-SLC15A4 complex, cytokine/IFN assays in human immune cells from SLE patients","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with inhibitor, functional validation of mechanism in patient-derived cells, multiple orthogonal methods","pmids":["37863876"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structure of SLC15A4 (PHT1) in an outward-open conformation, combined with biochemical and structural modeling, shows that the first 16 N-terminal TASL residues fold into a helical structure that binds in the central cavity of the inward-open conformation of SLC15A4.","method":"Cryo-EM structure determination, biochemical binding assays, structural modeling of SLC15A4-TASL complex","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure plus biochemical validation, single lab but multiple orthogonal methods","pmids":["37709742"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of SLC15A3 (apo) and SLC15A4 (apo and substrate-bound, and TASL-bound) reveal the specific dipeptide recognition mechanism. Both protomers adopt outward-facing conformations in the apo state. The N-terminal region of TASL forms a helical structure that inserts into the inward-facing cavity of SLC15A4, providing the structural basis for selective TASL recruitment by SLC15A4 but not SLC15A3.","method":"Cryo-EM structure determination of SLC15A3 and SLC15A4 in multiple states including substrate-bound and TASL-bound","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures of multiple states including substrate-bound and adaptor-bound complexes, directly resolves recognition mechanism","pmids":["39719710"],"is_preprint":false},{"year":2021,"finding":"SLC15A4 is required for M1-prone metabolic shifts in macrophages after TLR9 stimulation. SLC15A4 loss causes insufficient pyruvate biotransformation to the TCA cycle while increasing glutaminolysis. SLC15A4 is in close proximity to AMPK and mTOR (by proximity-dependent biotin identification), and SLC15A4 deficiency impairs TLR-mediated AMPK activation. SLC15A4 also acts as a gatekeeper that limits glutamine use to protect macrophages from metabolic stress.","method":"Proximity-dependent biotin identification (BioID), fluxome (13C metabolic flux) analysis, AMPK/mTOR phosphorylation assays, SLC15A4 KO macrophages","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — BioID proximity mapping combined with fluxome analysis and signaling biochemistry, multiple orthogonal methods","pmids":["34385317"],"is_preprint":false},{"year":2021,"finding":"Human SLC15A4 possesses pH- and temperature-dependent transport activity for dipeptides and tripeptides. It localizes constitutively to LAMP1+ endolysosomal compartments and associates constitutively with Raptor and LAMTORs. SLC15A4 knockdown in human pDC line CAL-1 impairs TLR7/8- and TLR9-triggered IFN-I production and mTORC1 activity. SLC15A4 is required for autophagy sustainability but not induction, and is critical for mitochondrial membrane potential under starvation.","method":"Transport assays in cells, immunofluorescence/co-IP with Raptor and LAMTORs, shRNA knockdown in CAL-1, IFN-I ELISA, mTORC1 assays, autophagy flux assay, mitochondrial membrane potential assay","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods in human cell line with KD, direct transport activity and organelle localization validated","pmids":["33560415"],"is_preprint":false},{"year":2017,"finding":"SLC15A4 is required for mast cell secretory-granule homeostasis. In Slc15a4-/- mast cells, reduced mTORC1 activity increases TFEB expression and nuclear translocation, causing secretory granules to degranulate more potently and enhancing FcεRI-mediated and IL-33-triggered inflammatory responses both in vitro and in vivo.","method":"Slc15a4 KO mice, mast cell granule analysis by electron microscopy and degranulation assays, mTORC1 and TFEB activity assays, FcεRI crosslinking assays","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — KO mice with defined cellular phenotype, mTORC1-TFEB pathway mechanistic follow-up, in vitro and in vivo validation","pmids":["29155995"],"is_preprint":false},{"year":2018,"finding":"PHT1 (SLC15A4) is expressed on endosomal membranes of macrophages (whereas PEPT2 is on the plasma membrane). PHT1 KO in mice reduces cytokine responses to bacterially derived peptide ligands (muramyl dipeptide), implicating PHT1 in transporting bacterial peptides to the cytosol to enhance NOD-dependent innate immune responses.","method":"Fluorescent MDP-rhodamine imaging for subcellular localization, Pht1 KO mice, cytokine production assays after bacterial peptide stimulation","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct endosomal localization by fluorescent imaging, KO mice with defined cytokine phenotype, multiple orthogonal methods","pmids":["29784761"],"is_preprint":false},{"year":2024,"finding":"Chemoproteomics identified first-in-class functional inhibitors of SLC15A4 that suppress SLC15A4-mediated endolysosomal TLR7-9 and NOD signaling in human and mouse immune cells, suppress inflammation in vivo, and show activity in clinical settings. Mechanistic studies indicate inhibitor binding to SLC15A4 disrupts its transporter function.","method":"Chemical proteomics (activity-based protein profiling), functional TLR/NOD cytokine assays, in vivo mouse models, SLC15A4 target engagement assays","journal":"Nature chemical biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — integrated chemical proteomics plus functional assays in multiple cell types and in vivo, replicated across human and mouse systems","pmids":["38191941"],"is_preprint":false},{"year":2016,"finding":"PHT1 (SLC15A4) plays an important role in L-histidine transport in brain parenchyma. Pht1 null mice show 28–48% lower L-His uptake in brain parenchyma after intravenous administration. PHT1 ablation leads to compensatory ~2-fold upregulation of Pept2 in brain regions.","method":"In vitro brain slice uptake assays, in vivo pharmacokinetic biodistribution in Pht1 null vs. wildtype mice, immunoblot and PCR for transporter expression","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mice with direct pharmacokinetic and brain slice functional assays, single lab","pmids":["27845049"],"is_preprint":false},{"year":2014,"finding":"Slc15a4 is required for intact class switch recombination to IgG2c in response to TLR9 stimulation. Splenic cDCs and B cells from feeble (Slc15a4-deficient) mice are defective in TLR9 ligation responses ex vivo, indicating a cell-intrinsic role beyond pDCs.","method":"Vaccination of feeble mice with CpG-adjuvanted antigens, ex vivo TLR9 stimulation of cDCs and B cells, isotype ELISA","journal":"Immunology and cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic model with defined B cell/cDC phenotype, ex vivo functional validation, single lab","pmids":["25310967"],"is_preprint":false},{"year":2019,"finding":"PHT1 (SLC15A4) mediates uptake of carnosine (β-alanyl-L-histidine) in glioblastoma cells. siRNA-mediated knockdown of PHT1 significantly reduces carnosine uptake, and L-histidine (a PHT1/2 inhibitor) competitively inhibits uptake.","method":"siRNA knockdown of PHT1 in glioblastoma cell lines, HPLC-MS uptake assays with competitive inhibitors","journal":"Amino acids","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA KD with direct substrate uptake measurement, competitive inhibition assay, single lab","pmids":["31073693"],"is_preprint":false},{"year":2011,"finding":"Human PHT1 (SLC15A4) mediates proton-dependent, concentration- and time-dependent uptake of histidine and Gly-Sar in stably transfected COS-7 cells, with mixed-uptake kinetics. Gly-Sar is a substrate based on efflux from hPHT1-COS-7 cells.","method":"Stable transfection of hPHT1 in COS-7 cells, uptake kinetic assays with radiolabeled or fluorescent substrates","journal":"Revista mexicana de ciencias farmaceutica","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct transport assay in transfected cells, but single lab and limited methods","pmids":["23888104"],"is_preprint":false},{"year":2025,"finding":"Two conformation-selective antibodies against human SLC15A4 were identified: clone 107 selectively binds SLC15A4 in a TASL binding-incompatible luminal-open state, whereas clone 235 stabilizes SLC15A4 in a TASL binding-competent cytoplasmic-open state, confirming that SLC15A4 function in TASL recruitment is conformation-dependent.","method":"Systematic antibody screening and validation, conformation-selective binding assays, functional TLR7/8/9 pathway assays","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conformation-selective antibodies with functional validation, single lab, single publication","pmids":["40781080"],"is_preprint":false},{"year":2023,"finding":"A non-coding intronic variant (rs35907548) in SLC15A4 acts as an allele-specific enhancer; the risk allele T increases SLC15A4 regulatory activity and expression. CRISPR/Cas9 knockout of the locus dysregulates endolysosomal pH, directly linking SLC15A4 expression level to endolysosomal pH regulation.","method":"Luciferase reporter assay, ChIP-qPCR, 3C-qPCR chromatin conformation capture, CRISPR/Cas9 KO in HL60 cells, endolysosomal pH measurement","journal":"Frontiers in lupus","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (luciferase, ChIP, 3C, CRISPR KO), direct functional pH readout, single lab","pmids":["38317862"],"is_preprint":false}],"current_model":"SLC15A4 (PHT1/PTR4) is an endolysosomal proton-coupled histidine/oligopeptide transporter that resides in LAMP1+ compartments of immune cells, where it serves a dual role: as a transporter moving histidine and di/tripeptides from the lysosomal lumen to the cytosol (supporting mTORC1 activity via association with Raptor/LAMTORs), and as a scaffold protein that undergoes a conformational switch from an outward-facing to an inward-facing state to recruit the innate immune adaptor TASL via its N-terminal helix into the transporter's central cavity; TASL then activates IRF5 via a pLxIS motif to drive type I interferon production downstream of endolysosomal TLR7, TLR8, and TLR9, with SLC15A4's endolysosomal trafficking dependent on the AP-3 complex, and its loss disrupting endolysosomal pH, mTOR/AMPK signaling, M1-prone metabolic reprogramming, and mast cell granule homeostasis via the mTORC1-TFEB axis."},"narrative":{"mechanistic_narrative":"SLC15A4 (PHT1) is an endolysosomal proton-coupled transporter of histidine and di/tripeptides that has been co-opted as a central organizing node for nucleic acid-sensing innate immunity in immune cells [PMID:25238095, PMID:33560415, PMID:23888104]. It resides constitutively in LAMP1+ endolysosomal compartments and its transport activity moves histidine and oligopeptides from the lysosomal lumen to the cytosol, with bona fide substrates including histidine, Gly-Sar, dipeptides, and carnosine [PMID:33560415, PMID:31073693, PMID:23888104]; this transport supports endolysosomal pH and v-ATPase integrity and sustains mTORC1 activity through constitutive association with Raptor and the LAMTORs [PMID:25238095, PMID:33560415, PMID:38317862]. Beyond transport, SLC15A4 acts as a conformation-dependent scaffold: cryo-EM structures show that switching from an outward-facing to an inward-facing state creates a central cavity into which the N-terminal helix of the innate immune adaptor TASL inserts, an interaction SLC15A4 mediates but its paralog SLC15A3 does not [PMID:37863913, PMID:37709742, PMID:39719710]. Recruited TASL then activates IRF5 via its pLxIS motif to drive type I interferon production downstream of endolysosomal TLR7, TLR8, and TLR9, while leaving NF-κB and MAPK signaling intact [PMID:32433612]. SLC15A4 is also required for trafficking nucleic-acid-sensing TLRs and ligands into the LAMP2+VAMP3+ hybrid compartment where IFN-I is initiated, a function dependent on the AP-3 complex [PMID:21045126, PMID:35349343]. Through its control of mTORC1/AMPK signaling SLC15A4 governs M1-prone metabolic reprogramming in macrophages and, via the mTORC1-TFEB axis, mast cell secretory-granule homeostasis [PMID:34385317, PMID:29155995]. Conformation-selective inhibitors, antibodies, and a lupus-risk enhancer variant link SLC15A4 expression and conformational state directly to TLR-driven inflammation and disease [PMID:37863876, PMID:38191941, PMID:40781080, PMID:38317862].","teleology":[{"year":2010,"claim":"Establishing that a transporter-class protein was selectively required for endosomal TLR signaling answered whether nucleic-acid-sensing innate immunity depends on dedicated membrane-trafficking machinery.","evidence":"Forward ENU genetic screen, positional cloning of the feeble mutation, and epistasis with AP-3/BLOC mutants in pDCs vs cDCs","pmids":["21045126"],"confidence":"High","gaps":["Molecular function of SLC15A4 protein not yet defined","Mechanism linking trafficking pathway to TLR signaling unresolved"]},{"year":2014,"claim":"Defining SLC15A4 as a proton-coupled lysosomal histidine/oligopeptide transporter whose activity is required for IFN-I production connected its biochemical function to endolysosomal pH, v-ATPase integrity, and the mTOR-IRF7 circuit.","evidence":"Slc15a4 KO mice, transporter-dead rescue mutagenesis, endolysosomal pH measurement, mTOR assays, and a mouse lupus model; complemented by a B cell/cDC class-switch phenotype","pmids":["25238095","25310967"],"confidence":"High","gaps":["Direct molecular partner mediating IFN-I output not yet identified","How transport activity couples to pH regulation mechanistically unresolved"]},{"year":2016,"claim":"Quantifying brain L-histidine uptake in null mice tested whether SLC15A4 functions as a physiological histidine transporter outside the immune system.","evidence":"Brain slice uptake assays and in vivo pharmacokinetics in Pht1-null vs wildtype mice with transporter expression profiling","pmids":["27845049"],"confidence":"Medium","gaps":["Single lab","Physiological consequence of reduced brain histidine uptake not established","Compensatory Pept2 upregulation complicates interpretation"]},{"year":2018,"claim":"Showing endosomal SLC15A4 enhances NOD responses to bacterial muramyl dipeptide extended its substrate role to transporting pathogen-derived peptides into the cytosol.","evidence":"Fluorescent MDP imaging for localization and cytokine assays in Pht1 KO mice","pmids":["29784761"],"confidence":"High","gaps":["Direct transport of MDP not biochemically demonstrated","Relationship to TLR/IRF5 arm not addressed"]},{"year":2020,"claim":"Identifying the SLC15A4-TASL interaction and TASL's pLxIS-dependent IRF5 activation resolved how SLC15A4 mechanistically drives IFN-I selectively, without engaging NF-κB or MAPK.","evidence":"Reciprocal Co-IP, extensive TASL mutagenesis, genetic deletion of SLC15A4 or TASL in primary and transformed human immune cells, and IFN reporter assays","pmids":["32433612"],"confidence":"High","gaps":["Structural basis of recruitment not yet resolved","Whether transport and scaffolding are coupled unclear"]},{"year":2021,"claim":"Linking SLC15A4 to constitutive Raptor/LAMTOR association and to macrophage metabolic reprogramming defined its role as an mTORC1/AMPK-regulating hub controlling inflammatory metabolism.","evidence":"Transport assays, co-IP with Raptor/LAMTORs and shRNA knockdown in CAL-1 (human pDC line); separately BioID proximity mapping, 13C fluxome analysis, and AMPK/mTOR phosphorylation assays in KO macrophages","pmids":["33560415","34385317"],"confidence":"High","gaps":["Whether mTORC1 regulation depends on transport activity or scaffolding not separated","Direct vs indirect proximity to AMPK/mTOR unresolved"]},{"year":2023,"claim":"Cryo-EM structures of apo, substrate-bound, TASL-bound, and inhibitor-bound SLC15A4 established that an outward-to-inward conformational switch creates the TASL-binding cavity, explaining paralog-selective recruitment and providing a druggable mechanism.","evidence":"Cryo-EM structure determination in multiple conformational states including TASL and feeblin complexes, plus biochemical binding and a TASL-degradation phenotypic screen with patient-derived cells","pmids":["37863913","37863876","37709742","39719710"],"confidence":"High","gaps":["Dynamics of the conformational switch in situ not measured","Coupling between substrate translocation cycle and TASL release not fully defined"]},{"year":2024,"claim":"First-in-class chemoproteomic inhibitors that disrupt SLC15A4 transporter function and suppress TLR7-9/NOD signaling in vivo validated SLC15A4 as a tractable anti-inflammatory drug target.","evidence":"Activity-based protein profiling, functional cytokine assays in human and mouse immune cells, target engagement assays, and in vivo inflammation models","pmids":["38191941"],"confidence":"High","gaps":["Selectivity over related transporters in vivo not fully mapped","Therapeutic window not established"]},{"year":2025,"claim":"Conformation-selective antibodies and a lupus-risk enhancer variant tied SLC15A4 conformational state and expression level directly to TASL recruitment and endolysosomal pH dysregulation in disease.","evidence":"Conformation-selective antibody screening with TLR pathway assays; separately luciferase/ChIP/3C/CRISPR KO in HL60 cells with pH readout","pmids":["40781080","38317862"],"confidence":"Medium","gaps":["Single lab for each finding","Causal contribution of the variant to human lupus in patients not established"]},{"year":null,"claim":"It remains unresolved whether SLC15A4's transport cycle and its TASL-scaffolding function are mechanistically coupled or independent activities, and how substrate flux directly shapes the conformational equilibrium that gates TASL recruitment.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No experiment separates transport-dependent from scaffold-dependent IFN-I output","Real-time link between substrate translocation and conformational gating unmeasured"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[2,9,11,13,15,16]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[2,9,16]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,4,6,7]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[2,9,11]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,11]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,1,2,3,11]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,8,9]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[2,9,16]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[1,3]}],"complexes":[],"partners":["TASL","RPTOR","IRF5"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8N697","full_name":"Solute carrier family 15 member 4","aliases":["Peptide transporter 4","Peptide/histidine transporter 1","hPHT1"],"length_aa":577,"mass_kda":62.0,"function":"Proton-coupled amino-acid transporter that mediates the transmembrane transport of L-histidine and some di- and tripeptides from inside the lysosome to the cytosol, and plays a key role in innate immune response (PubMed:16289537, PubMed:25238095, PubMed:29224352, PubMed:39719710). Preferably binds to short peptides with a basic residue at the first position and a hydrophobic residue at the second position (PubMed:39719710). Able to transport a variety of di- and tripeptides, including carnosine and some peptidoglycans (PubMed:29224352, PubMed:31073693). Transporter activity is pH-dependent and maximized in the acidic lysosomal environment (By similarity). Involved in the detection of microbial pathogens by toll-like receptors (TLRs) and NOD-like receptors (NLRs), probably by mediating transport of bacterial peptidoglycans across the endolysosomal membrane: catalyzes the transport of certain bacterial peptidoglycans, such as muramyl dipeptide (MDP), the NOD2 ligand, and L-alanyl-gamma-D-glutamyl-meso-2,6-diaminoheptanedioate (tri-DAP), the NOD1 ligand (PubMed:25238095, PubMed:29224352). Required for TLR7, TLR8 and TLR9-mediated type I interferon (IFN-I) productions in plasmacytoid dendritic cells (pDCs) (PubMed:25238095). Independently of its transporter activity, also promotes the recruitment of innate immune adapter TASL to endolysosome downstream of TLR7, TLR8 and TLR9: TASL recruitment leads to the specific recruitment and activation of IRF5 (PubMed:32433612, PubMed:39719710). Required for isotype class switch recombination to IgG2c isotype in response to TLR9 stimulation (By similarity). Required for mast cell secretory-granule homeostasis by limiting mast cell functions and inflammatory responses (By similarity)","subcellular_location":"Lysosome membrane; Endosome membrane; Early endosome membrane","url":"https://www.uniprot.org/uniprotkb/Q8N697/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC15A4","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000139370","cell_line_id":"CID001382","localizations":[{"compartment":"vesicles","grade":3}],"interactors":[{"gene":"CLCN7","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID001382","total_profiled":1310},"omim":[{"mim_id":"615806","title":"SOLUTE CARRIER FAMILY 15 (OLIGOPEPTIDE TRANSPORTER), MEMBER 4; SLC15A4","url":"https://www.omim.org/entry/615806"},{"mim_id":"610408","title":"SOLUTE CARRIER FAMILY 15 (OLIGOPEPTIDE TRANSPORTER), MEMBER 3; SLC15A3","url":"https://www.omim.org/entry/610408"},{"mim_id":"605956","title":"NUCLEOTIDE-BINDING OLIGOMERIZATION DOMAIN PROTEIN 2; NOD2","url":"https://www.omim.org/entry/605956"},{"mim_id":"603455","title":"RECEPTOR-INTERACTING SERINE/THREONINE KINASE 2; RIPK2","url":"https://www.omim.org/entry/603455"},{"mim_id":"301049","title":"TLR ADAPTOR INTERACTING WITH ENDOLYSOSOMAL SLC15A4; TASL","url":"https://www.omim.org/entry/301049"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SLC15A4"},"hgnc":{"alias_symbol":["PHT1","PTR4"],"prev_symbol":[]},"alphafold":{"accession":"Q8N697","domains":[{"cath_id":"1.20.1250.20","chopping":"30-247","consensus_level":"medium","plddt":89.1127,"start":30,"end":247},{"cath_id":"1.20.1250.20","chopping":"332-354_361-539","consensus_level":"medium","plddt":89.9475,"start":332,"end":539}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N697","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N697-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N697-F1-predicted_aligned_error_v6.png","plddt_mean":84.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC15A4","jax_strain_url":"https://www.jax.org/strain/search?query=SLC15A4"},"sequence":{"accession":"Q8N697","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8N697.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8N697/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N697"}},"corpus_meta":[{"pmid":"32433612","id":"PMC_32433612","title":"TASL is the SLC15A4-associated adaptor for IRF5 activation by TLR7-9.","date":"2020","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/32433612","citation_count":178,"is_preprint":false},{"pmid":"21045126","id":"PMC_21045126","title":"Slc15a4, AP-3, and Hermansky-Pudlak syndrome proteins are required for Toll-like receptor signaling in plasmacytoid dendritic cells.","date":"2010","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/21045126","citation_count":166,"is_preprint":false},{"pmid":"25238095","id":"PMC_25238095","title":"The histidine transporter SLC15A4 coordinates mTOR-dependent inflammatory responses and pathogenic antibody production.","date":"2014","source":"Immunity","url":"https://pubmed.ncbi.nlm.nih.gov/25238095","citation_count":133,"is_preprint":false},{"pmid":"14756304","id":"PMC_14756304","title":"Characterization of two phosphate transporters from barley; evidence for diverse function 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  \"finding\": \"SLC15A4 physically interacts with TASL (encoded by CXorf21) on the endolysosome; TASL localization and function depend on this interaction. Loss of either SLC15A4 or TASL specifically abrogates IRF5 pathway activation downstream of TLR7, TLR8, and TLR9 without affecting NF-κB or MAPK signaling. TASL contains a conserved pLxIS motif that recruits and activates IRF5.\",\n      \"method\": \"Co-immunoprecipitation, extensive mutagenesis of TASL, genetic deletion of SLC15A4 or TASL in primary and transformed human immune cells, cytokine/IFN reporter assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, mutagenesis, multiple cell types and orthogonal functional readouts in a single rigorous study\",\n      \"pmids\": [\"32433612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"A loss-of-function mutation in Slc15a4 (feeble) abolishes both TLR7- and TLR9-induced type I IFN and proinflammatory cytokine production specifically in plasmacytoid dendritic cells (pDCs), while leaving TLR responses intact in conventional dendritic cells. Slc15a4 was identified as part of a membrane-trafficking pathway (together with AP-3, BLOC-1, and BLOC-2 Hermansky-Pudlak proteins) uniquely required for endosomal TLR signaling in pDCs.\",\n      \"method\": \"Forward genetic ENU screen, positional cloning of feeble mutation, cytokine assays in pDCs vs. cDCs from mutant mice, genetic epistasis with AP-3/BLOC knockout mice\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic screen, positional cloning, epistasis with multiple pathway members, replicated across multiple cell types\",\n      \"pmids\": [\"21045126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SLC15A4 is a lysosome-resident proton-coupled transporter that moves histidine and oligopeptides from the lysosomal lumen to the cytosol. Its transporter activity is required for TLR7/9-triggered IFN-I production in B cells. SLC15A4 loss disrupts endolysosomal pH regulation and v-ATPase integrity, leading to failure of the mTOR pathway and consequently the IRF7-IFN-I regulatory circuit.\",\n      \"method\": \"Genetic deletion of Slc15a4 in mice, transporter-activity rescue experiments with transporter-dead mutants, endolysosomal pH measurements, mTOR pathway biochemical assays, mouse lupus model\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — KO mice with defined phenotype, transporter-activity mutagenesis rescue, pH/mTOR mechanistic follow-up, multiple orthogonal methods\",\n      \"pmids\": [\"25238095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SLC15A4 is required for trafficking and colocalization of nucleic acid-sensing TLRs and their ligands to endolysosomes, and for formation of the LAMP2+VAMP3+ hybrid compartment where IFN-I production is initiated. SLC15A4's endolysosomal trafficking and function depend on the AP-3 complex. SLC15A4, but not its homolog SLC15A3, is required for TLR-driven IFN-I production in pDCs.\",\n      \"method\": \"Live-cell imaging of TLR/ligand trafficking, genetic deletion of SLC15A4 vs. SLC15A3 in mice, AP-3 complex requirement tested by genetics, confocal colocalization assays\",\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 localization experiments with functional consequence, genetic epistasis with AP-3, negative control with SLC15A3 paralog, multiple orthogonal methods\",\n      \"pmids\": [\"35349343\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of human SLC15A4 reveal apo monomeric and dimeric outward-facing conformations and a TASL-bound complex. In the TASL-bound state, SLC15A4 undergoes a conformational change from outward-facing to inward-facing, forming a binding pocket into which the N-terminal helix of TASL inserts. The dimeric apo form has an extensive interface involving four cholesterol molecules.\",\n      \"method\": \"Cryo-EM structure determination of apo and TASL-bound SLC15A4\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution cryo-EM structure with functional complex, multiple conformational states resolved\",\n      \"pmids\": [\"37863913\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The small molecule feeblin binds SLC15A4 in a lysosomal outward-open conformation that is incompatible with TASL binding on the cytoplasmic side. Feeblin binding leads to proteostatic degradation of TASL, thereby inhibiting TLR7/8-IRF5 signaling. Cryo-EM structure of feeblin-bound SLC15A4 defines the inhibitor-binding site.\",\n      \"method\": \"Phenotypic screen for TASL degradation, cryo-EM structure of feeblin-SLC15A4 complex, cytokine/IFN assays in human immune cells from SLE patients\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with inhibitor, functional validation of mechanism in patient-derived cells, multiple orthogonal methods\",\n      \"pmids\": [\"37863876\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structure of SLC15A4 (PHT1) in an outward-open conformation, combined with biochemical and structural modeling, shows that the first 16 N-terminal TASL residues fold into a helical structure that binds in the central cavity of the inward-open conformation of SLC15A4.\",\n      \"method\": \"Cryo-EM structure determination, biochemical binding assays, structural modeling of SLC15A4-TASL complex\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure plus biochemical validation, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"37709742\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of SLC15A3 (apo) and SLC15A4 (apo and substrate-bound, and TASL-bound) reveal the specific dipeptide recognition mechanism. Both protomers adopt outward-facing conformations in the apo state. The N-terminal region of TASL forms a helical structure that inserts into the inward-facing cavity of SLC15A4, providing the structural basis for selective TASL recruitment by SLC15A4 but not SLC15A3.\",\n      \"method\": \"Cryo-EM structure determination of SLC15A3 and SLC15A4 in multiple states including substrate-bound and TASL-bound\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures of multiple states including substrate-bound and adaptor-bound complexes, directly resolves recognition mechanism\",\n      \"pmids\": [\"39719710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SLC15A4 is required for M1-prone metabolic shifts in macrophages after TLR9 stimulation. SLC15A4 loss causes insufficient pyruvate biotransformation to the TCA cycle while increasing glutaminolysis. SLC15A4 is in close proximity to AMPK and mTOR (by proximity-dependent biotin identification), and SLC15A4 deficiency impairs TLR-mediated AMPK activation. SLC15A4 also acts as a gatekeeper that limits glutamine use to protect macrophages from metabolic stress.\",\n      \"method\": \"Proximity-dependent biotin identification (BioID), fluxome (13C metabolic flux) analysis, AMPK/mTOR phosphorylation assays, SLC15A4 KO macrophages\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — BioID proximity mapping combined with fluxome analysis and signaling biochemistry, multiple orthogonal methods\",\n      \"pmids\": [\"34385317\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Human SLC15A4 possesses pH- and temperature-dependent transport activity for dipeptides and tripeptides. It localizes constitutively to LAMP1+ endolysosomal compartments and associates constitutively with Raptor and LAMTORs. SLC15A4 knockdown in human pDC line CAL-1 impairs TLR7/8- and TLR9-triggered IFN-I production and mTORC1 activity. SLC15A4 is required for autophagy sustainability but not induction, and is critical for mitochondrial membrane potential under starvation.\",\n      \"method\": \"Transport assays in cells, immunofluorescence/co-IP with Raptor and LAMTORs, shRNA knockdown in CAL-1, IFN-I ELISA, mTORC1 assays, autophagy flux assay, mitochondrial membrane potential assay\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods in human cell line with KD, direct transport activity and organelle localization validated\",\n      \"pmids\": [\"33560415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"SLC15A4 is required for mast cell secretory-granule homeostasis. In Slc15a4-/- mast cells, reduced mTORC1 activity increases TFEB expression and nuclear translocation, causing secretory granules to degranulate more potently and enhancing FcεRI-mediated and IL-33-triggered inflammatory responses both in vitro and in vivo.\",\n      \"method\": \"Slc15a4 KO mice, mast cell granule analysis by electron microscopy and degranulation assays, mTORC1 and TFEB activity assays, FcεRI crosslinking assays\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mice with defined cellular phenotype, mTORC1-TFEB pathway mechanistic follow-up, in vitro and in vivo validation\",\n      \"pmids\": [\"29155995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PHT1 (SLC15A4) is expressed on endosomal membranes of macrophages (whereas PEPT2 is on the plasma membrane). PHT1 KO in mice reduces cytokine responses to bacterially derived peptide ligands (muramyl dipeptide), implicating PHT1 in transporting bacterial peptides to the cytosol to enhance NOD-dependent innate immune responses.\",\n      \"method\": \"Fluorescent MDP-rhodamine imaging for subcellular localization, Pht1 KO mice, cytokine production assays after bacterial peptide stimulation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct endosomal localization by fluorescent imaging, KO mice with defined cytokine phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"29784761\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Chemoproteomics identified first-in-class functional inhibitors of SLC15A4 that suppress SLC15A4-mediated endolysosomal TLR7-9 and NOD signaling in human and mouse immune cells, suppress inflammation in vivo, and show activity in clinical settings. Mechanistic studies indicate inhibitor binding to SLC15A4 disrupts its transporter function.\",\n      \"method\": \"Chemical proteomics (activity-based protein profiling), functional TLR/NOD cytokine assays, in vivo mouse models, SLC15A4 target engagement assays\",\n      \"journal\": \"Nature chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — integrated chemical proteomics plus functional assays in multiple cell types and in vivo, replicated across human and mouse systems\",\n      \"pmids\": [\"38191941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PHT1 (SLC15A4) plays an important role in L-histidine transport in brain parenchyma. Pht1 null mice show 28–48% lower L-His uptake in brain parenchyma after intravenous administration. PHT1 ablation leads to compensatory ~2-fold upregulation of Pept2 in brain regions.\",\n      \"method\": \"In vitro brain slice uptake assays, in vivo pharmacokinetic biodistribution in Pht1 null vs. wildtype mice, immunoblot and PCR for transporter expression\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mice with direct pharmacokinetic and brain slice functional assays, single lab\",\n      \"pmids\": [\"27845049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Slc15a4 is required for intact class switch recombination to IgG2c in response to TLR9 stimulation. Splenic cDCs and B cells from feeble (Slc15a4-deficient) mice are defective in TLR9 ligation responses ex vivo, indicating a cell-intrinsic role beyond pDCs.\",\n      \"method\": \"Vaccination of feeble mice with CpG-adjuvanted antigens, ex vivo TLR9 stimulation of cDCs and B cells, isotype ELISA\",\n      \"journal\": \"Immunology and cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic model with defined B cell/cDC phenotype, ex vivo functional validation, single lab\",\n      \"pmids\": [\"25310967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PHT1 (SLC15A4) mediates uptake of carnosine (β-alanyl-L-histidine) in glioblastoma cells. siRNA-mediated knockdown of PHT1 significantly reduces carnosine uptake, and L-histidine (a PHT1/2 inhibitor) competitively inhibits uptake.\",\n      \"method\": \"siRNA knockdown of PHT1 in glioblastoma cell lines, HPLC-MS uptake assays with competitive inhibitors\",\n      \"journal\": \"Amino acids\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA KD with direct substrate uptake measurement, competitive inhibition assay, single lab\",\n      \"pmids\": [\"31073693\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Human PHT1 (SLC15A4) mediates proton-dependent, concentration- and time-dependent uptake of histidine and Gly-Sar in stably transfected COS-7 cells, with mixed-uptake kinetics. Gly-Sar is a substrate based on efflux from hPHT1-COS-7 cells.\",\n      \"method\": \"Stable transfection of hPHT1 in COS-7 cells, uptake kinetic assays with radiolabeled or fluorescent substrates\",\n      \"journal\": \"Revista mexicana de ciencias farmaceutica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct transport assay in transfected cells, but single lab and limited methods\",\n      \"pmids\": [\"23888104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Two conformation-selective antibodies against human SLC15A4 were identified: clone 107 selectively binds SLC15A4 in a TASL binding-incompatible luminal-open state, whereas clone 235 stabilizes SLC15A4 in a TASL binding-competent cytoplasmic-open state, confirming that SLC15A4 function in TASL recruitment is conformation-dependent.\",\n      \"method\": \"Systematic antibody screening and validation, conformation-selective binding assays, functional TLR7/8/9 pathway assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conformation-selective antibodies with functional validation, single lab, single publication\",\n      \"pmids\": [\"40781080\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"A non-coding intronic variant (rs35907548) in SLC15A4 acts as an allele-specific enhancer; the risk allele T increases SLC15A4 regulatory activity and expression. CRISPR/Cas9 knockout of the locus dysregulates endolysosomal pH, directly linking SLC15A4 expression level to endolysosomal pH regulation.\",\n      \"method\": \"Luciferase reporter assay, ChIP-qPCR, 3C-qPCR chromatin conformation capture, CRISPR/Cas9 KO in HL60 cells, endolysosomal pH measurement\",\n      \"journal\": \"Frontiers in lupus\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (luciferase, ChIP, 3C, CRISPR KO), direct functional pH readout, single lab\",\n      \"pmids\": [\"38317862\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC15A4 (PHT1/PTR4) is an endolysosomal proton-coupled histidine/oligopeptide transporter that resides in LAMP1+ compartments of immune cells, where it serves a dual role: as a transporter moving histidine and di/tripeptides from the lysosomal lumen to the cytosol (supporting mTORC1 activity via association with Raptor/LAMTORs), and as a scaffold protein that undergoes a conformational switch from an outward-facing to an inward-facing state to recruit the innate immune adaptor TASL via its N-terminal helix into the transporter's central cavity; TASL then activates IRF5 via a pLxIS motif to drive type I interferon production downstream of endolysosomal TLR7, TLR8, and TLR9, with SLC15A4's endolysosomal trafficking dependent on the AP-3 complex, and its loss disrupting endolysosomal pH, mTOR/AMPK signaling, M1-prone metabolic reprogramming, and mast cell granule homeostasis via the mTORC1-TFEB axis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC15A4 (PHT1) is an endolysosomal proton-coupled transporter of histidine and di/tripeptides that has been co-opted as a central organizing node for nucleic acid-sensing innate immunity in immune cells [#2, #9, #16]. It resides constitutively in LAMP1+ endolysosomal compartments and its transport activity moves histidine and oligopeptides from the lysosomal lumen to the cytosol, with bona fide substrates including histidine, Gly-Sar, dipeptides, and carnosine [#9, #15, #16]; this transport supports endolysosomal pH and v-ATPase integrity and sustains mTORC1 activity through constitutive association with Raptor and the LAMTORs [#2, #9, #18]. Beyond transport, SLC15A4 acts as a conformation-dependent scaffold: cryo-EM structures show that switching from an outward-facing to an inward-facing state creates a central cavity into which the N-terminal helix of the innate immune adaptor TASL inserts, an interaction SLC15A4 mediates but its paralog SLC15A3 does not [#4, #6, #7]. Recruited TASL then activates IRF5 via its pLxIS motif to drive type I interferon production downstream of endolysosomal TLR7, TLR8, and TLR9, while leaving NF-\\u03baB and MAPK signaling intact [#0]. SLC15A4 is also required for trafficking nucleic-acid-sensing TLRs and ligands into the LAMP2+VAMP3+ hybrid compartment where IFN-I is initiated, a function dependent on the AP-3 complex [#1, #3]. Through its control of mTORC1/AMPK signaling SLC15A4 governs M1-prone metabolic reprogramming in macrophages and, via the mTORC1-TFEB axis, mast cell secretory-granule homeostasis [#8, #10]. Conformation-selective inhibitors, antibodies, and a lupus-risk enhancer variant link SLC15A4 expression and conformational state directly to TLR-driven inflammation and disease [#5, #12, #17, #18].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Establishing that a transporter-class protein was selectively required for endosomal TLR signaling answered whether nucleic-acid-sensing innate immunity depends on dedicated membrane-trafficking machinery.\",\n      \"evidence\": \"Forward ENU genetic screen, positional cloning of the feeble mutation, and epistasis with AP-3/BLOC mutants in pDCs vs cDCs\",\n      \"pmids\": [\"21045126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular function of SLC15A4 protein not yet defined\", \"Mechanism linking trafficking pathway to TLR signaling unresolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defining SLC15A4 as a proton-coupled lysosomal histidine/oligopeptide transporter whose activity is required for IFN-I production connected its biochemical function to endolysosomal pH, v-ATPase integrity, and the mTOR-IRF7 circuit.\",\n      \"evidence\": \"Slc15a4 KO mice, transporter-dead rescue mutagenesis, endolysosomal pH measurement, mTOR assays, and a mouse lupus model; complemented by a B cell/cDC class-switch phenotype\",\n      \"pmids\": [\"25238095\", \"25310967\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct molecular partner mediating IFN-I output not yet identified\", \"How transport activity couples to pH regulation mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Quantifying brain L-histidine uptake in null mice tested whether SLC15A4 functions as a physiological histidine transporter outside the immune system.\",\n      \"evidence\": \"Brain slice uptake assays and in vivo pharmacokinetics in Pht1-null vs wildtype mice with transporter expression profiling\",\n      \"pmids\": [\"27845049\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Physiological consequence of reduced brain histidine uptake not established\", \"Compensatory Pept2 upregulation complicates interpretation\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing endosomal SLC15A4 enhances NOD responses to bacterial muramyl dipeptide extended its substrate role to transporting pathogen-derived peptides into the cytosol.\",\n      \"evidence\": \"Fluorescent MDP imaging for localization and cytokine assays in Pht1 KO mice\",\n      \"pmids\": [\"29784761\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct transport of MDP not biochemically demonstrated\", \"Relationship to TLR/IRF5 arm not addressed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identifying the SLC15A4-TASL interaction and TASL's pLxIS-dependent IRF5 activation resolved how SLC15A4 mechanistically drives IFN-I selectively, without engaging NF-\\u03baB or MAPK.\",\n      \"evidence\": \"Reciprocal Co-IP, extensive TASL mutagenesis, genetic deletion of SLC15A4 or TASL in primary and transformed human immune cells, and IFN reporter assays\",\n      \"pmids\": [\"32433612\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of recruitment not yet resolved\", \"Whether transport and scaffolding are coupled unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Linking SLC15A4 to constitutive Raptor/LAMTOR association and to macrophage metabolic reprogramming defined its role as an mTORC1/AMPK-regulating hub controlling inflammatory metabolism.\",\n      \"evidence\": \"Transport assays, co-IP with Raptor/LAMTORs and shRNA knockdown in CAL-1 (human pDC line); separately BioID proximity mapping, 13C fluxome analysis, and AMPK/mTOR phosphorylation assays in KO macrophages\",\n      \"pmids\": [\"33560415\", \"34385317\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mTORC1 regulation depends on transport activity or scaffolding not separated\", \"Direct vs indirect proximity to AMPK/mTOR unresolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Cryo-EM structures of apo, substrate-bound, TASL-bound, and inhibitor-bound SLC15A4 established that an outward-to-inward conformational switch creates the TASL-binding cavity, explaining paralog-selective recruitment and providing a druggable mechanism.\",\n      \"evidence\": \"Cryo-EM structure determination in multiple conformational states including TASL and feeblin complexes, plus biochemical binding and a TASL-degradation phenotypic screen with patient-derived cells\",\n      \"pmids\": [\"37863913\", \"37863876\", \"37709742\", \"39719710\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of the conformational switch in situ not measured\", \"Coupling between substrate translocation cycle and TASL release not fully defined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"First-in-class chemoproteomic inhibitors that disrupt SLC15A4 transporter function and suppress TLR7-9/NOD signaling in vivo validated SLC15A4 as a tractable anti-inflammatory drug target.\",\n      \"evidence\": \"Activity-based protein profiling, functional cytokine assays in human and mouse immune cells, target engagement assays, and in vivo inflammation models\",\n      \"pmids\": [\"38191941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Selectivity over related transporters in vivo not fully mapped\", \"Therapeutic window not established\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Conformation-selective antibodies and a lupus-risk enhancer variant tied SLC15A4 conformational state and expression level directly to TASL recruitment and endolysosomal pH dysregulation in disease.\",\n      \"evidence\": \"Conformation-selective antibody screening with TLR pathway assays; separately luciferase/ChIP/3C/CRISPR KO in HL60 cells with pH readout\",\n      \"pmids\": [\"40781080\", \"38317862\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab for each finding\", \"Causal contribution of the variant to human lupus in patients not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved whether SLC15A4's transport cycle and its TASL-scaffolding function are mechanistically coupled or independent activities, and how substrate flux directly shapes the conformational equilibrium that gates TASL recruitment.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experiment separates transport-dependent from scaffold-dependent IFN-I output\", \"Real-time link between substrate translocation and conformational gating unmeasured\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [2, 9, 11, 13, 15, 16]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [2, 9, 16]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 4, 6, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [2, 9, 11]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 2, 3, 11]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 8, 9]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [2, 9, 16]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"TASL\", \"RPTOR\", \"IRF5\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":7,"faith_pct":85.71428571428571}}