{"gene":"LTK","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":1990,"finding":"Mouse ltk uses an upstream non-AUG (CUG) translational initiator and produces a glycoprotein receptor of approximately 69 kDa with a small extracellular domain; it is expressed in pre-B lymphocytes and adult brain neurons (cerebral cortex and hippocampus), with expression lost upon in vitro maturation of pre-B into B cells.","method":"In vitro translation, transfection, Northern blot, immunostaining","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (in vitro translation, transfection, immunostaining) in single study establishing translational initiation and cell-type-specific expression","pmids":["2357970"],"is_preprint":false},{"year":1991,"finding":"Full-length human LTK cDNA encodes a conventional receptor tyrosine kinase with an ATG start codon, secretory signal, 347 amino acid extracellular domain, transmembrane domain, and intracellular kinase domain (~100 kDa); the protein expressed in COS-1 cells is glycosylated and possesses in vitro kinase activity.","method":"cDNA cloning, in vitro transcription/translation, COS-1 transfection, in vitro kinase assay, RNase protection analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro kinase assay plus mutagenesis-independent biochemical characterization of full-length cDNA product with multiple orthogonal methods","pmids":["1655406"],"is_preprint":false},{"year":1993,"finding":"Mice tissue-specifically express four ltk mRNAs: two lymphoid/brain forms using CUG start codons with a short extracellular domain, and two neuroblastoma forms using AUG start codons with larger extracellular domains. At least one of the larger C1300 neuroblastoma Ltk receptors shares endoplasmic reticulum localization with the shorter lymphoid isoform.","method":"cDNA cloning, Northern blot, alternative splicing analysis, subcellular localization","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cDNA characterization of four isoforms with ER localization confirmed for multiple isoforms, single lab","pmids":["8380920"],"is_preprint":false},{"year":1993,"finding":"The human LTK protein (100 kDa) expressed in human placenta and hematopoietic cell lines possesses tyrosine kinase activity as detected by in vitro immune complex kinase assay with anti-LTK monoclonal antibodies.","method":"Immunoprecipitation, in vitro immune complex kinase assay, Western blot","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct in vitro kinase activity demonstrated on native protein, single lab single method","pmids":["8427607"],"is_preprint":false},{"year":1994,"finding":"Wild-type human LTK is tyrosine-phosphorylated in vivo and associates with the SH2-containing signaling proteins PLC-γ1, the p85 subunit of PI3-K, and GAP, as well as with the serine/threonine kinase Raf-1. A kinase-dead mutant (K544M) fails to associate with any of these proteins, demonstrating that these interactions depend on LTK kinase activity.","method":"Co-immunoprecipitation, in vitro kinase assay, Western blot, kinase-dead mutant analysis in COS cells","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP with multiple binding partners plus kinase-dead mutant control confirming activity-dependence, multiple orthogonal methods","pmids":["8084603"],"is_preprint":false},{"year":1997,"finding":"A lymphocyte-specific murine Ltk isoform is retained in the endoplasmic reticulum and exhibits dual Nexo/Ccyt and Ncyt/Cexo transmembrane topology; ER retention correlates with formation of a complex with the chaperone calnexin. Mutants with increased positive charges downstream of the transmembrane segment adopt conventional Nexo/Ccyt orientation and traffic to the cell surface.","method":"Transfection, subcellular fractionation, co-immunoprecipitation with calnexin, transmembrane topology analysis, mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — ER localization established by fractionation, calnexin complex by co-IP, topology confirmed by mutagenesis, multiple orthogonal methods in single study","pmids":["8995435"],"is_preprint":false},{"year":1999,"finding":"In transgenic mice with broad LTK overexpression, LTK kinase activation (tyrosine phosphorylation, kinase activity, multimerization) occurs selectively in the heart where LTK localizes to intracellular membranes, presumably the endoplasmic reticulum, leading to cardiac hypertrophy, cardiomyocyte degeneration, and fetal gene reprogramming.","method":"Transgenic mouse generation, echocardiography, histology, immunoprecipitation/kinase assay, subcellular fractionation","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo transgenic model with biochemical confirmation of kinase activation and ER localization, single lab","pmids":["10445845"],"is_preprint":false},{"year":2004,"finding":"A gain-of-function polymorphism in the LTK kinase domain near the YXXM motif (p85 PI3K binding motif) in NZB mice and some SLE patients leads to upregulation of the PI3K pathway and abnormal B1 cell proliferation.","method":"Sequence analysis, functional kinase assay, PI3K pathway activity measurement, genetic association","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional analysis of naturally occurring variant linking LTK kinase domain polymorphism to PI3K signaling, single lab","pmids":["14695357"],"is_preprint":false},{"year":2008,"finding":"Activation of LTK kinase activity via a chimeric CSF1R-LTK receptor (extracellular domain of CSF1R fused to intracellular domain of LTK) is sufficient to promote neurite outgrowth through pathways including PI3K and MAPK.","method":"Chimeric receptor expression, CSF-1 stimulation, neurite outgrowth assay, PI3K/MAPK pathway inhibition","journal":"Neuroreport","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chimeric receptor approach cleanly isolates LTK intracellular domain function, pathway inhibitors used, single lab","pmids":["18849880"],"is_preprint":false},{"year":2012,"finding":"LTK mutant F568L (corresponding to ALK F1174L) constitutively activates LTK kinase, transforms hematopoietic cells to cytokine independence, induces anchorage-independent growth, and activates Shc, ERK, and JAK/STAT signaling. LTK R669Q has weaker transforming activity. Wild-type LTK is non-transforming.","method":"Site-directed mutagenesis, cytokine-independent growth assay, soft agar colony formation, PC12 neurite outgrowth, Western blot for downstream signaling","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple mutagenesis constructs tested in multiple cell transformation assays with downstream signaling readouts, multiple orthogonal methods","pmids":["22347506"],"is_preprint":false},{"year":2014,"finding":"FAM150A (AUG-β) and FAM150B (AUG-α) are ligands for LTK; FAM150A binds the LTK extracellular domain with high affinity (KD = 28 pM) and stimulates LTK phosphorylation in a ligand-dependent manner.","method":"Extracellular proteome signaling screen (3,191 proteins), LTK phosphorylation assay, binding affinity measurement (KD determination)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic functional screen identifying ligands confirmed by direct binding measurement and phosphorylation assay, rigorous approach","pmids":["25331893"],"is_preprint":false},{"year":2015,"finding":"AUG-α (FAM150B) binds and robustly activates both ALK and LTK, whereas AUG-β (FAM150A/ALKAL1) is specific for LTK and only weakly binds ALK. AUG-α stimulates transformation of NIH/3T3 cells expressing ALK and IL-3-independent growth of Ba/F3 cells expressing ALK.","method":"Cell-based binding assays, receptor phosphorylation assays, NIH/3T3 transformation assay, Ba/F3 cytokine-independent growth assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — hierarchy and specificity of two ligands for ALK vs LTK established by multiple binding and functional assays, replicated across cell types","pmids":["26630010"],"is_preprint":false},{"year":2017,"finding":"In zebrafish, Ltk (not Alk) mediates iridophore differentiation from neural crest-derived cells and pigment progenitor cells in a tissue-specific manner in response to aug-α1, aug-α2, and aug-β ligands. Deficiency in Ltk phenocopies ligand deficiency in iridophore patterning.","method":"Zebrafish genetic knockdown/knockout, phenotypic analysis of pigment cell patterning, genetic epistasis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in zebrafish with ligand-receptor pairing and phenocopy experiments establishing Ltk-specific role in iridophore differentiation","pmids":["29078341"],"is_preprint":false},{"year":2018,"finding":"The augmentor domain (AD) of AUG-α contains four conserved cysteines forming two intramolecular disulfide bridges, while a fifth primate-specific cysteine in the N-terminal variable region mediates AUG-α dimerization. Both full-length AUG-α and the AD deletion mutant (lacking dimerization) stimulate similar LTK and ALK tyrosine phosphorylation and downstream responses, demonstrating that the augmentor domain is the minimal biologically active unit.","method":"Mass spectrometry, biochemical purification, ALK/LTK phosphorylation assays, MAP kinase assay, soft agar colony formation, neuronal differentiation assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — mass spectrometry structural characterization combined with multiple functional assays for both ALK and LTK, multiple orthogonal methods","pmids":["30061385"],"is_preprint":false},{"year":2019,"finding":"LTK is an ER-resident receptor tyrosine kinase. Depletion or pharmacologic inhibition of LTK reduces the number of ER exit sites and slows ER-to-Golgi transport. LTK physically interacts with and phosphorylates Sec12; a phosphoablating mutant of Sec12 reduces ER export efficiency, defining an LTK-Sec12 ER-resident signaling module that regulates proteostasis.","method":"siRNA knockdown, pharmacologic inhibition, live-cell imaging of ER exit sites, ER-to-Golgi transport assay, co-immunoprecipitation, phosphorylation assay, Sec12 phosphoablating mutant","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — ER localization combined with substrate identification (Sec12), functional phosphorylation assay, and mutant rescue experiment, multiple orthogonal methods in single study","pmids":["31227593"],"is_preprint":false},{"year":2021,"finding":"The CLIP1-LTK fusion protein has constitutively activated kinase activity and transformation potential; Ba/F3 cells expressing CLIP1-LTK undergo apoptosis and proliferation suppression upon treatment with the ALK inhibitor lorlatinib.","method":"Whole-transcriptome sequencing, Ba/F3 transformation assay, kinase activity assay, lorlatinib treatment","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — constitutive kinase activation demonstrated biochemically, transformation in Ba/F3 confirmed, and clinical response validating mechanism","pmids":["34819663"],"is_preprint":false},{"year":2023,"finding":"Loss of Ltk and/or Alk in primary mouse embryonic neurons causes a multiple-axon phenotype and delays neuronal migration and cortical patterning in vivo. Mechanistically, loss of Alk and Ltk increases cell-surface expression and activity of Igf-1r, which activates PI3K signaling to drive the excess axon phenotype, placing LTK upstream of Igf-1r/PI3K in neuronal polarity.","method":"Primary neuron knockout, mouse embryo/newborn analysis, epistasis experiments, cell-surface Igf-1r measurement, PI3K signaling assays","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in primary neurons and in vivo mouse embryos with biochemical pathway analysis identifying Igf-1r/PI3K as downstream effector","pmids":["37291945"],"is_preprint":false},{"year":2023,"finding":"Leukocyte tyrosine kinase (Ltk) is the Mendelian determinant of the melanoid axolotl color variant, which lacks iridophores; Ltk CRISPR crispants phenocopy the melanoid loss of iridophores, consistent with Ltk's established role in iridophore differentiation.","method":"Bulked segregant RNA-Seq, SNP mapping, CRISPR-Cas9 crispant generation and phenotypic analysis","journal":"Genes","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR loss-of-function phenocopy in axolotl supports iridophore role, single lab genetic study","pmids":["37107662"],"is_preprint":false},{"year":2024,"finding":"Eight LTK mutations corresponding to known ALK resistance mutations confer resistance to lorlatinib in CLIP1-LTK-driven NSCLC; the L650F mutation shows the highest resistance. Gilteritinib overcomes L650F-mediated resistance in vitro and in vivo. In silico modeling suggests L650F attenuates lorlatinib-LTK binding.","method":"In vitro kinase resistance assay, Ba/F3 growth assay, mouse xenograft (in vivo), in silico docking","journal":"Communications biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple LTK mutants characterized biochemically and in cell/animal models, in silico structural rationale, single lab","pmids":["38575808"],"is_preprint":false},{"year":2025,"finding":"LTK acts as a regulatory node in the proteostasis network in multiple myeloma cells; LTK inhibition using ALK inhibitors causes immunoglobulin retention in the ER, induces ER stress, and triggers apoptosis of primary MM cells, demonstrating LTK's role in maintaining high secretory output.","method":"LTK inhibitor treatment, immunoglobulin secretion assay, ER stress markers, apoptosis assay in primary MM cells","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacologic inhibition with mechanistic readouts (ER stress, Ig retention) in primary patient cells, single lab","pmids":["40634511"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM reanalysis of ALK-ALKAL2 data reveals both 2:2 and 2:1 receptor:ligand stoichiometries; structural comparison with crystal structures of ALK-ALKAL2 and LTK-ALKAL1 at 2:1 stoichiometry demonstrates a common receptor dimerization mode for ALK and LTK.","method":"Cryo-EM structure determination (reanalysis of EMPIAR-10930), particle classification, 3D reconstruction to 3.2 Å","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — cryo-EM structure resolving LTK/ALK complex stoichiometry, preprint, single reanalysis study","pmids":["bio_10.1101_2024.08.08.607122"],"is_preprint":true},{"year":2025,"finding":"LTK deficiency in salivary gland epithelial cells reduces CXCL13 expression, which in turn promotes macrophage M2 polarization; in NOD/ShiLtJ mice, LTK knockdown ameliorates submandibular gland tissue damage and reduces autoimmune antigen (Ro52/SSA, La/SSB) secretion, placing LTK upstream of a CXCL13-macrophage polarization axis in Sjögren's syndrome pathogenesis.","method":"Lentiviral LTK knockdown, CXCL13 protein array, macrophage co-culture polarization assay, flow cytometry, NOD mouse in vivo model","journal":"Cytokine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function in cells and mouse model with downstream pathway identification (CXCL13/M2), single lab","pmids":["40154092"],"is_preprint":false}],"current_model":"LTK is an ER-resident (and, in certain isoforms, cell-surface) receptor tyrosine kinase that is activated by the small cytokines ALKAL1 (AUG-β/FAM150A) and ALKAL2 (AUG-α/FAM150B); upon ligand-induced dimerization it autophosphorylates and recruits PLC-γ1, PI3K-p85, GAP, and Raf-1 to activate ERK, PI3K, and JAK/STAT signaling, while at the ER it phosphorylates Sec12 to regulate ER exit site formation and ER-to-Golgi transport; in vivo LTK is required for iridophore differentiation in zebrafish and axolotl, for neuronal axon-dendrite polarity and cortical migration (acting by suppressing IGF1R/PI3K signaling), and gain-of-function mutations or oncogenic fusions (e.g., CLIP1-LTK) constitutively activate the kinase to drive cellular transformation in cancer."},"narrative":{"mechanistic_narrative":"LTK is a receptor tyrosine kinase activated by the secreted ALKAL/FAM150/AUG cytokines, transducing signals that govern cell differentiation, neuronal polarity, secretory proteostasis, and—when constitutively activated—oncogenic transformation [PMID:25331893, PMID:31227593, PMID:34819663]. The full-length human protein is a glycosylated transmembrane receptor with an extracellular ligand-binding domain and an intracellular tyrosine kinase domain that is catalytically active [PMID:1655406, PMID:8427607]. ALKAL2/FAM150B (AUG-α) and ALKAL1/FAM150A (AUG-β) are its activating ligands, the latter binding the LTK extracellular domain with picomolar affinity and stimulating ligand-dependent phosphorylation; AUG-α activates both LTK and ALK while AUG-β is LTK-selective, and the compact augmentor domain is the minimal active unit [PMID:25331893, PMID:26630010, PMID:30061385]. Upon activation, kinase-active LTK recruits PLC-γ1, the p85 subunit of PI3K, GAP, and Raf-1 in a kinase-dependent manner and drives Shc/ERK, PI3K, and JAK/STAT signaling [PMID:8084603, PMID:22347506]. A distinct pool of LTK is retained in the endoplasmic reticulum through dual transmembrane topology and a calnexin complex, and ER-resident LTK phosphorylates Sec12 to promote ER exit site formation and ER-to-Golgi transport, thereby sustaining secretory output [PMID:8995435, PMID:31227593]. Physiologically, LTK is required for iridophore differentiation from neural crest-derived pigment progenitors in zebrafish and axolotl [PMID:29078341, PMID:37107662], and it controls neuronal axon-dendrite polarity and cortical migration by suppressing cell-surface IGF1R/PI3K signaling [PMID:37291945]. Gain-of-function kinase-domain mutations and the CLIP1-LTK fusion constitutively activate the kinase to transform cells and drive cancer, with the fusion conferring sensitivity to the ALK inhibitor lorlatinib and defining clinically relevant resistance mutations [PMID:22347506, PMID:34819663, PMID:38575808].","teleology":[{"year":1991,"claim":"Established that human LTK encodes a conventional, catalytically active receptor tyrosine kinase, defining its core molecular identity.","evidence":"cDNA cloning and in vitro kinase assay of the COS-1-expressed product","pmids":["1655406"],"confidence":"High","gaps":["No ligand identified","Downstream effectors and physiological substrates unknown"]},{"year":1993,"claim":"Resolved that LTK exists as multiple tissue-specific isoforms differing in translational initiation, extracellular domain length, and subcellular localization, including ER-resident forms.","evidence":"cDNA cloning, Northern blot, alternative-splicing and subcellular localization analysis of mouse ltk; immune-complex kinase assay on native human protein","pmids":["8380920","8427607"],"confidence":"Medium","gaps":["Functional distinction between surface and ER isoforms unresolved","Mechanism directing isoform localization not defined"]},{"year":1994,"claim":"Defined the proximal signaling output of activated LTK by showing kinase-dependent recruitment of SH2 effectors, linking LTK to PLC, PI3K, and Ras-MAPK pathways.","evidence":"Co-immunoprecipitation with PLC-γ1, p85, GAP, Raf-1 and a kinase-dead K544M control in COS cells","pmids":["8084603"],"confidence":"High","gaps":["Physiological ligand still unknown","Direct vs indirect binding of each effector not distinguished"]},{"year":1997,"claim":"Explained ER retention of a lymphocyte LTK isoform through dual transmembrane topology and calnexin association, establishing an intracellular receptor pool.","evidence":"Subcellular fractionation, calnexin co-IP, and topology mutagenesis in transfected cells","pmids":["8995435"],"confidence":"High","gaps":["Functional role of the ER pool not yet defined","No ER substrate identified at this stage"]},{"year":1999,"claim":"Showed that LTK kinase can autoactivate at intracellular membranes in vivo, producing a pathological organ phenotype.","evidence":"Transgenic mouse overexpression with biochemical confirmation of cardiac LTK activation and ER localization","pmids":["10445845"],"confidence":"Medium","gaps":["Mechanism of tissue-selective activation unclear","Relevance to endogenous LTK function uncertain"]},{"year":2008,"claim":"Isolated the LTK intracellular domain as sufficient to drive neurite outgrowth via PI3K and MAPK, linking LTK signaling to neuronal differentiation.","evidence":"CSF1R-LTK chimeric receptor activation with pathway inhibitors in neurite outgrowth assays","pmids":["18849880"],"confidence":"Medium","gaps":["Endogenous ligand-driven equivalent not tested","Physiological neuronal role not addressed"]},{"year":2012,"claim":"Demonstrated that gain-of-function mutations transform LTK into an oncogenic driver, establishing its transformation potential and downstream signaling.","evidence":"Site-directed mutagenesis (F568L, R669Q), cytokine-independent and soft-agar growth assays, signaling Western blots","pmids":["22347506"],"confidence":"High","gaps":["Whether such mutations occur in human tumors not established here","Structural basis of constitutive activation unresolved"]},{"year":2015,"claim":"Identified ALKAL1/2 (FAM150A/B) as activating ligands and defined receptor selectivity, finally pairing LTK with its physiological agonists.","evidence":"Extracellular proteome signaling screen, picomolar binding affinity measurement, and phosphorylation assays for LTK vs ALK","pmids":["25331893","26630010"],"confidence":"High","gaps":["Receptor stoichiometry and dimerization geometry not yet resolved","In vivo ligand sources not defined"]},{"year":2017,"claim":"Assigned LTK a defined developmental function by showing it specifically mediates iridophore differentiation downstream of AUG ligands.","evidence":"Zebrafish genetic knockout, phenotypic pigment-pattern analysis, and ligand-receptor epistasis","pmids":["29078341"],"confidence":"High","gaps":["Intracellular signaling driving iridophore fate not detailed","Conservation across vertebrates not yet shown"]},{"year":2018,"claim":"Defined the minimal active augmentor domain and the structural determinants of ligand dimerization required to activate LTK and ALK.","evidence":"Mass spectrometry mapping of disulfide bridges plus phosphorylation, MAPK, soft-agar, and differentiation assays","pmids":["30061385"],"confidence":"High","gaps":["Atomic-resolution receptor-ligand complex not determined here","Role of dimerization in vivo unresolved"]},{"year":2019,"claim":"Uncovered a non-canonical ER-resident LTK function by identifying Sec12 as a substrate controlling ER exit sites and secretory transport.","evidence":"siRNA knockdown, pharmacologic inhibition, ER exit-site imaging, ER-to-Golgi transport assay, Sec12 co-IP, and phosphoablating mutant","pmids":["31227593"],"confidence":"High","gaps":["Ligand dependence of the ER pool's activity unclear","Relationship between surface and ER signaling not integrated"]},{"year":2021,"claim":"Established CLIP1-LTK as a recurrent oncogenic fusion and a druggable target, translating LTK kinase activation into a clinical mechanism.","evidence":"Whole-transcriptome sequencing, Ba/F3 transformation and kinase assays, lorlatinib response","pmids":["34819663"],"confidence":"High","gaps":["Resistance mechanisms not yet mapped","Frequency across tumor types not defined here"]},{"year":2023,"claim":"Placed LTK upstream of IGF1R/PI3K in neuronal polarity, defining a physiological negative-regulatory circuit controlling axon number and cortical migration.","evidence":"Primary neuron and in vivo mouse knockout, epistasis, cell-surface IGF1R and PI3K signaling assays","pmids":["37291945"],"confidence":"High","gaps":["Mechanism by which LTK suppresses surface IGF1R unknown","Redundancy with ALK not fully separated"]},{"year":2023,"claim":"Confirmed conservation of LTK's iridophore role by identifying it as the Mendelian determinant of the melanoid axolotl variant.","evidence":"Bulked segregant RNA-Seq, SNP mapping, and CRISPR crispant phenocopy","pmids":["37107662"],"confidence":"Medium","gaps":["Causal mutation/molecular lesion not fully resolved","Single-lab genetic study"]},{"year":2024,"claim":"Mapped clinically actionable lorlatinib-resistance mutations in CLIP1-LTK and a second-line therapeutic option, advancing the cancer-targeting model.","evidence":"In vitro kinase resistance and Ba/F3 assays, mouse xenograft, in silico docking; gilteritinib rescue","pmids":["38575808"],"confidence":"Medium","gaps":["Clinical validation of resistance and gilteritinib response pending","Structural model is in silico only"]},{"year":2024,"claim":"Provided structural evidence that LTK and ALK share a common receptor dimerization mode with mixed 2:2 and 2:1 ligand stoichiometries.","evidence":"Cryo-EM reanalysis to 3.2 Å and comparison with ALK-ALKAL2 and LTK-ALKAL1 crystal structures (preprint)","pmids":["bio_10.1101_2024.08.08.607122"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Functional consequence of stoichiometry switch unresolved"]},{"year":2025,"claim":"Extended LTK's ER-proteostasis function to disease, showing that its inhibition collapses secretory capacity in myeloma and that its loss reshapes immune signaling in autoimmunity.","evidence":"LTK inhibitor treatment with Ig-retention and ER-stress readouts in primary myeloma cells; lentiviral LTK knockdown with CXCL13/M2 polarization analysis in NOD mice","pmids":["40634511","40154092"],"confidence":"Medium","gaps":["Whether effects are via ER-resident vs surface LTK unclear","Direct substrates in these contexts not defined"]},{"year":null,"claim":"How LTK partitions between surface receptor signaling and ER-resident Sec12-dependent secretory control, and how ligand engagement coordinates these two modes, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking ligand-driven surface signaling to ER proteostasis function","Determinants of isoform-specific localization in human cells not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,3,4,14]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,14]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[10,11]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2,5,14]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[4,9,10]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[15,18,19]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[12,16,17]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[14,19]}],"complexes":[],"partners":["ALKAL1","ALKAL2","PLCG1","PIK3R1","RAF1","CANX","SEC12","CLIP1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P29376","full_name":"Leukocyte tyrosine kinase receptor","aliases":["Protein tyrosine kinase 1"],"length_aa":864,"mass_kda":91.7,"function":"Receptor with a tyrosine-protein kinase activity (PubMed:10445845, PubMed:20548102, PubMed:30061385). Following activation by ALKAL1 or ALKAL2 ligands at the cell surface, transduces an extracellular signal into an intracellular response (PubMed:30061385, PubMed:34646012). Ligand-binding to the extracellular domain induces tyrosine kinase activation, leading to activation of the mitogen-activated protein kinase (MAPK) pathway (PubMed:20548102). Phosphorylates almost exclusively at the first tyrosine of the Y-x-x-x-Y-Y motif (By similarity). The exact function of this protein is not known; studies with chimeric proteins demonstrate its ability to promote growth and specifically neurite outgrowth, and cell survival (PubMed:18849880, PubMed:9223670). Involved in regulation of the secretory pathway involving endoplasmic reticulum (ER) export sites (ERESs) and ER to Golgi transport (PubMed:20548102)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P29376/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LTK","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/LTK","total_profiled":1310},"omim":[{"mim_id":"619671","title":"ALK AND LTK LIGAND 2; ALKAL2","url":"https://www.omim.org/entry/619671"},{"mim_id":"619670","title":"ALK AND LTK LIGAND 1; ALKAL1","url":"https://www.omim.org/entry/619670"},{"mim_id":"600341","title":"TYRO3 PROTEIN TYROSINE KINASE; TYRO3","url":"https://www.omim.org/entry/600341"},{"mim_id":"600154","title":"PHOSPHATIDYLINOSITOL GLYCAN ANCHOR BIOSYNTHESIS CLASS H PROTEIN; PIGH","url":"https://www.omim.org/entry/600154"},{"mim_id":"300300","title":"BRUTON AGAMMAGLOBULINEMIA TYROSINE KINASE; BTK","url":"https://www.omim.org/entry/300300"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"intestine","ntpm":12.4},{"tissue":"lung","ntpm":14.3},{"tissue":"salivary gland","ntpm":11.1}],"url":"https://www.proteinatlas.org/search/LTK"},"hgnc":{"alias_symbol":["TYK1"],"prev_symbol":[]},"alphafold":{"accession":"P29376","domains":[{"cath_id":"-","chopping":"67-190_204-379","consensus_level":"medium","plddt":88.6656,"start":67,"end":379},{"cath_id":"-","chopping":"383-419","consensus_level":"medium","plddt":69.9354,"start":383,"end":419},{"cath_id":"3.30.200.20","chopping":"508-592","consensus_level":"high","plddt":84.1827,"start":508,"end":592},{"cath_id":"1.10.510.10","chopping":"604-675_685-789","consensus_level":"high","plddt":88.0167,"start":604,"end":789}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P29376","model_url":"https://alphafold.ebi.ac.uk/files/AF-P29376-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P29376-F1-predicted_aligned_error_v6.png","plddt_mean":73.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LTK","jax_strain_url":"https://www.jax.org/strain/search?query=LTK"},"sequence":{"accession":"P29376","fasta_url":"https://rest.uniprot.org/uniprotkb/P29376.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P29376/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P29376"}},"corpus_meta":[{"pmid":"9174053","id":"PMC_9174053","title":"ALK, the chromosome 2 gene locus altered by the t(2;5) in non-Hodgkin's lymphoma, encodes a novel neural receptor tyrosine kinase that is highly related to leukocyte tyrosine kinase (LTK).","date":"1997","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/9174053","citation_count":417,"is_preprint":false},{"pmid":"2162345","id":"PMC_2162345","title":"Differential coupling of dopaminergic D2 receptors expressed in different cell types. 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it is expressed in pre-B lymphocytes and adult brain neurons (cerebral cortex and hippocampus), with expression lost upon in vitro maturation of pre-B into B cells.\",\n      \"method\": \"In vitro translation, transfection, Northern blot, immunostaining\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (in vitro translation, transfection, immunostaining) in single study establishing translational initiation and cell-type-specific expression\",\n      \"pmids\": [\"2357970\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Full-length human LTK cDNA encodes a conventional receptor tyrosine kinase with an ATG start codon, secretory signal, 347 amino acid extracellular domain, transmembrane domain, and intracellular kinase domain (~100 kDa); the protein expressed in COS-1 cells is glycosylated and possesses in vitro kinase activity.\",\n      \"method\": \"cDNA cloning, in vitro transcription/translation, COS-1 transfection, in vitro kinase assay, RNase protection analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro kinase assay plus mutagenesis-independent biochemical characterization of full-length cDNA product with multiple orthogonal methods\",\n      \"pmids\": [\"1655406\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Mice tissue-specifically express four ltk mRNAs: two lymphoid/brain forms using CUG start codons with a short extracellular domain, and two neuroblastoma forms using AUG start codons with larger extracellular domains. At least one of the larger C1300 neuroblastoma Ltk receptors shares endoplasmic reticulum localization with the shorter lymphoid isoform.\",\n      \"method\": \"cDNA cloning, Northern blot, alternative splicing analysis, subcellular localization\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cDNA characterization of four isoforms with ER localization confirmed for multiple isoforms, single lab\",\n      \"pmids\": [\"8380920\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"The human LTK protein (100 kDa) expressed in human placenta and hematopoietic cell lines possesses tyrosine kinase activity as detected by in vitro immune complex kinase assay with anti-LTK monoclonal antibodies.\",\n      \"method\": \"Immunoprecipitation, in vitro immune complex kinase assay, Western blot\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct in vitro kinase activity demonstrated on native protein, single lab single method\",\n      \"pmids\": [\"8427607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Wild-type human LTK is tyrosine-phosphorylated in vivo and associates with the SH2-containing signaling proteins PLC-γ1, the p85 subunit of PI3-K, and GAP, as well as with the serine/threonine kinase Raf-1. A kinase-dead mutant (K544M) fails to associate with any of these proteins, demonstrating that these interactions depend on LTK kinase activity.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, Western blot, kinase-dead mutant analysis in COS cells\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP with multiple binding partners plus kinase-dead mutant control confirming activity-dependence, multiple orthogonal methods\",\n      \"pmids\": [\"8084603\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"A lymphocyte-specific murine Ltk isoform is retained in the endoplasmic reticulum and exhibits dual Nexo/Ccyt and Ncyt/Cexo transmembrane topology; ER retention correlates with formation of a complex with the chaperone calnexin. Mutants with increased positive charges downstream of the transmembrane segment adopt conventional Nexo/Ccyt orientation and traffic to the cell surface.\",\n      \"method\": \"Transfection, subcellular fractionation, co-immunoprecipitation with calnexin, transmembrane topology analysis, mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ER localization established by fractionation, calnexin complex by co-IP, topology confirmed by mutagenesis, multiple orthogonal methods in single study\",\n      \"pmids\": [\"8995435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"In transgenic mice with broad LTK overexpression, LTK kinase activation (tyrosine phosphorylation, kinase activity, multimerization) occurs selectively in the heart where LTK localizes to intracellular membranes, presumably the endoplasmic reticulum, leading to cardiac hypertrophy, cardiomyocyte degeneration, and fetal gene reprogramming.\",\n      \"method\": \"Transgenic mouse generation, echocardiography, histology, immunoprecipitation/kinase assay, subcellular fractionation\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo transgenic model with biochemical confirmation of kinase activation and ER localization, single lab\",\n      \"pmids\": [\"10445845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A gain-of-function polymorphism in the LTK kinase domain near the YXXM motif (p85 PI3K binding motif) in NZB mice and some SLE patients leads to upregulation of the PI3K pathway and abnormal B1 cell proliferation.\",\n      \"method\": \"Sequence analysis, functional kinase assay, PI3K pathway activity measurement, genetic association\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional analysis of naturally occurring variant linking LTK kinase domain polymorphism to PI3K signaling, single lab\",\n      \"pmids\": [\"14695357\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Activation of LTK kinase activity via a chimeric CSF1R-LTK receptor (extracellular domain of CSF1R fused to intracellular domain of LTK) is sufficient to promote neurite outgrowth through pathways including PI3K and MAPK.\",\n      \"method\": \"Chimeric receptor expression, CSF-1 stimulation, neurite outgrowth assay, PI3K/MAPK pathway inhibition\",\n      \"journal\": \"Neuroreport\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chimeric receptor approach cleanly isolates LTK intracellular domain function, pathway inhibitors used, single lab\",\n      \"pmids\": [\"18849880\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"LTK mutant F568L (corresponding to ALK F1174L) constitutively activates LTK kinase, transforms hematopoietic cells to cytokine independence, induces anchorage-independent growth, and activates Shc, ERK, and JAK/STAT signaling. LTK R669Q has weaker transforming activity. Wild-type LTK is non-transforming.\",\n      \"method\": \"Site-directed mutagenesis, cytokine-independent growth assay, soft agar colony formation, PC12 neurite outgrowth, Western blot for downstream signaling\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple mutagenesis constructs tested in multiple cell transformation assays with downstream signaling readouts, multiple orthogonal methods\",\n      \"pmids\": [\"22347506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FAM150A (AUG-β) and FAM150B (AUG-α) are ligands for LTK; FAM150A binds the LTK extracellular domain with high affinity (KD = 28 pM) and stimulates LTK phosphorylation in a ligand-dependent manner.\",\n      \"method\": \"Extracellular proteome signaling screen (3,191 proteins), LTK phosphorylation assay, binding affinity measurement (KD determination)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic functional screen identifying ligands confirmed by direct binding measurement and phosphorylation assay, rigorous approach\",\n      \"pmids\": [\"25331893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"AUG-α (FAM150B) binds and robustly activates both ALK and LTK, whereas AUG-β (FAM150A/ALKAL1) is specific for LTK and only weakly binds ALK. AUG-α stimulates transformation of NIH/3T3 cells expressing ALK and IL-3-independent growth of Ba/F3 cells expressing ALK.\",\n      \"method\": \"Cell-based binding assays, receptor phosphorylation assays, NIH/3T3 transformation assay, Ba/F3 cytokine-independent growth assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — hierarchy and specificity of two ligands for ALK vs LTK established by multiple binding and functional assays, replicated across cell types\",\n      \"pmids\": [\"26630010\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In zebrafish, Ltk (not Alk) mediates iridophore differentiation from neural crest-derived cells and pigment progenitor cells in a tissue-specific manner in response to aug-α1, aug-α2, and aug-β ligands. Deficiency in Ltk phenocopies ligand deficiency in iridophore patterning.\",\n      \"method\": \"Zebrafish genetic knockdown/knockout, phenotypic analysis of pigment cell patterning, genetic epistasis\",\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 epistasis in zebrafish with ligand-receptor pairing and phenocopy experiments establishing Ltk-specific role in iridophore differentiation\",\n      \"pmids\": [\"29078341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The augmentor domain (AD) of AUG-α contains four conserved cysteines forming two intramolecular disulfide bridges, while a fifth primate-specific cysteine in the N-terminal variable region mediates AUG-α dimerization. Both full-length AUG-α and the AD deletion mutant (lacking dimerization) stimulate similar LTK and ALK tyrosine phosphorylation and downstream responses, demonstrating that the augmentor domain is the minimal biologically active unit.\",\n      \"method\": \"Mass spectrometry, biochemical purification, ALK/LTK phosphorylation assays, MAP kinase assay, soft agar colony formation, neuronal differentiation assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mass spectrometry structural characterization combined with multiple functional assays for both ALK and LTK, multiple orthogonal methods\",\n      \"pmids\": [\"30061385\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"LTK is an ER-resident receptor tyrosine kinase. Depletion or pharmacologic inhibition of LTK reduces the number of ER exit sites and slows ER-to-Golgi transport. LTK physically interacts with and phosphorylates Sec12; a phosphoablating mutant of Sec12 reduces ER export efficiency, defining an LTK-Sec12 ER-resident signaling module that regulates proteostasis.\",\n      \"method\": \"siRNA knockdown, pharmacologic inhibition, live-cell imaging of ER exit sites, ER-to-Golgi transport assay, co-immunoprecipitation, phosphorylation assay, Sec12 phosphoablating mutant\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ER localization combined with substrate identification (Sec12), functional phosphorylation assay, and mutant rescue experiment, multiple orthogonal methods in single study\",\n      \"pmids\": [\"31227593\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The CLIP1-LTK fusion protein has constitutively activated kinase activity and transformation potential; Ba/F3 cells expressing CLIP1-LTK undergo apoptosis and proliferation suppression upon treatment with the ALK inhibitor lorlatinib.\",\n      \"method\": \"Whole-transcriptome sequencing, Ba/F3 transformation assay, kinase activity assay, lorlatinib treatment\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — constitutive kinase activation demonstrated biochemically, transformation in Ba/F3 confirmed, and clinical response validating mechanism\",\n      \"pmids\": [\"34819663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Loss of Ltk and/or Alk in primary mouse embryonic neurons causes a multiple-axon phenotype and delays neuronal migration and cortical patterning in vivo. Mechanistically, loss of Alk and Ltk increases cell-surface expression and activity of Igf-1r, which activates PI3K signaling to drive the excess axon phenotype, placing LTK upstream of Igf-1r/PI3K in neuronal polarity.\",\n      \"method\": \"Primary neuron knockout, mouse embryo/newborn analysis, epistasis experiments, cell-surface Igf-1r measurement, PI3K signaling assays\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in primary neurons and in vivo mouse embryos with biochemical pathway analysis identifying Igf-1r/PI3K as downstream effector\",\n      \"pmids\": [\"37291945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Leukocyte tyrosine kinase (Ltk) is the Mendelian determinant of the melanoid axolotl color variant, which lacks iridophores; Ltk CRISPR crispants phenocopy the melanoid loss of iridophores, consistent with Ltk's established role in iridophore differentiation.\",\n      \"method\": \"Bulked segregant RNA-Seq, SNP mapping, CRISPR-Cas9 crispant generation and phenotypic analysis\",\n      \"journal\": \"Genes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR loss-of-function phenocopy in axolotl supports iridophore role, single lab genetic study\",\n      \"pmids\": [\"37107662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Eight LTK mutations corresponding to known ALK resistance mutations confer resistance to lorlatinib in CLIP1-LTK-driven NSCLC; the L650F mutation shows the highest resistance. Gilteritinib overcomes L650F-mediated resistance in vitro and in vivo. In silico modeling suggests L650F attenuates lorlatinib-LTK binding.\",\n      \"method\": \"In vitro kinase resistance assay, Ba/F3 growth assay, mouse xenograft (in vivo), in silico docking\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple LTK mutants characterized biochemically and in cell/animal models, in silico structural rationale, single lab\",\n      \"pmids\": [\"38575808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LTK acts as a regulatory node in the proteostasis network in multiple myeloma cells; LTK inhibition using ALK inhibitors causes immunoglobulin retention in the ER, induces ER stress, and triggers apoptosis of primary MM cells, demonstrating LTK's role in maintaining high secretory output.\",\n      \"method\": \"LTK inhibitor treatment, immunoglobulin secretion assay, ER stress markers, apoptosis assay in primary MM cells\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacologic inhibition with mechanistic readouts (ER stress, Ig retention) in primary patient cells, single lab\",\n      \"pmids\": [\"40634511\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM reanalysis of ALK-ALKAL2 data reveals both 2:2 and 2:1 receptor:ligand stoichiometries; structural comparison with crystal structures of ALK-ALKAL2 and LTK-ALKAL1 at 2:1 stoichiometry demonstrates a common receptor dimerization mode for ALK and LTK.\",\n      \"method\": \"Cryo-EM structure determination (reanalysis of EMPIAR-10930), particle classification, 3D reconstruction to 3.2 Å\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — cryo-EM structure resolving LTK/ALK complex stoichiometry, preprint, single reanalysis study\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607122\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LTK deficiency in salivary gland epithelial cells reduces CXCL13 expression, which in turn promotes macrophage M2 polarization; in NOD/ShiLtJ mice, LTK knockdown ameliorates submandibular gland tissue damage and reduces autoimmune antigen (Ro52/SSA, La/SSB) secretion, placing LTK upstream of a CXCL13-macrophage polarization axis in Sjögren's syndrome pathogenesis.\",\n      \"method\": \"Lentiviral LTK knockdown, CXCL13 protein array, macrophage co-culture polarization assay, flow cytometry, NOD mouse in vivo model\",\n      \"journal\": \"Cytokine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function in cells and mouse model with downstream pathway identification (CXCL13/M2), single lab\",\n      \"pmids\": [\"40154092\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LTK is an ER-resident (and, in certain isoforms, cell-surface) receptor tyrosine kinase that is activated by the small cytokines ALKAL1 (AUG-β/FAM150A) and ALKAL2 (AUG-α/FAM150B); upon ligand-induced dimerization it autophosphorylates and recruits PLC-γ1, PI3K-p85, GAP, and Raf-1 to activate ERK, PI3K, and JAK/STAT signaling, while at the ER it phosphorylates Sec12 to regulate ER exit site formation and ER-to-Golgi transport; in vivo LTK is required for iridophore differentiation in zebrafish and axolotl, for neuronal axon-dendrite polarity and cortical migration (acting by suppressing IGF1R/PI3K signaling), and gain-of-function mutations or oncogenic fusions (e.g., CLIP1-LTK) constitutively activate the kinase to drive cellular transformation in cancer.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LTK is a receptor tyrosine kinase activated by the secreted ALKAL/FAM150/AUG cytokines, transducing signals that govern cell differentiation, neuronal polarity, secretory proteostasis, and—when constitutively activated—oncogenic transformation [#10, #14, #15]. The full-length human protein is a glycosylated transmembrane receptor with an extracellular ligand-binding domain and an intracellular tyrosine kinase domain that is catalytically active [#1, #3]. ALKAL2/FAM150B (AUG-α) and ALKAL1/FAM150A (AUG-β) are its activating ligands, the latter binding the LTK extracellular domain with picomolar affinity and stimulating ligand-dependent phosphorylation; AUG-α activates both LTK and ALK while AUG-β is LTK-selective, and the compact augmentor domain is the minimal active unit [#10, #11, #13]. Upon activation, kinase-active LTK recruits PLC-γ1, the p85 subunit of PI3K, GAP, and Raf-1 in a kinase-dependent manner and drives Shc/ERK, PI3K, and JAK/STAT signaling [#4, #9]. A distinct pool of LTK is retained in the endoplasmic reticulum through dual transmembrane topology and a calnexin complex, and ER-resident LTK phosphorylates Sec12 to promote ER exit site formation and ER-to-Golgi transport, thereby sustaining secretory output [#5, #14]. Physiologically, LTK is required for iridophore differentiation from neural crest-derived pigment progenitors in zebrafish and axolotl [#12, #17], and it controls neuronal axon-dendrite polarity and cortical migration by suppressing cell-surface IGF1R/PI3K signaling [#16]. Gain-of-function kinase-domain mutations and the CLIP1-LTK fusion constitutively activate the kinase to transform cells and drive cancer, with the fusion conferring sensitivity to the ALK inhibitor lorlatinib and defining clinically relevant resistance mutations [#9, #15, #18].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Established that human LTK encodes a conventional, catalytically active receptor tyrosine kinase, defining its core molecular identity.\",\n      \"evidence\": \"cDNA cloning and in vitro kinase assay of the COS-1-expressed product\",\n      \"pmids\": [\"1655406\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No ligand identified\", \"Downstream effectors and physiological substrates unknown\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Resolved that LTK exists as multiple tissue-specific isoforms differing in translational initiation, extracellular domain length, and subcellular localization, including ER-resident forms.\",\n      \"evidence\": \"cDNA cloning, Northern blot, alternative-splicing and subcellular localization analysis of mouse ltk; immune-complex kinase assay on native human protein\",\n      \"pmids\": [\"8380920\", \"8427607\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional distinction between surface and ER isoforms unresolved\", \"Mechanism directing isoform localization not defined\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Defined the proximal signaling output of activated LTK by showing kinase-dependent recruitment of SH2 effectors, linking LTK to PLC, PI3K, and Ras-MAPK pathways.\",\n      \"evidence\": \"Co-immunoprecipitation with PLC-γ1, p85, GAP, Raf-1 and a kinase-dead K544M control in COS cells\",\n      \"pmids\": [\"8084603\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological ligand still unknown\", \"Direct vs indirect binding of each effector not distinguished\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Explained ER retention of a lymphocyte LTK isoform through dual transmembrane topology and calnexin association, establishing an intracellular receptor pool.\",\n      \"evidence\": \"Subcellular fractionation, calnexin co-IP, and topology mutagenesis in transfected cells\",\n      \"pmids\": [\"8995435\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of the ER pool not yet defined\", \"No ER substrate identified at this stage\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Showed that LTK kinase can autoactivate at intracellular membranes in vivo, producing a pathological organ phenotype.\",\n      \"evidence\": \"Transgenic mouse overexpression with biochemical confirmation of cardiac LTK activation and ER localization\",\n      \"pmids\": [\"10445845\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of tissue-selective activation unclear\", \"Relevance to endogenous LTK function uncertain\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Isolated the LTK intracellular domain as sufficient to drive neurite outgrowth via PI3K and MAPK, linking LTK signaling to neuronal differentiation.\",\n      \"evidence\": \"CSF1R-LTK chimeric receptor activation with pathway inhibitors in neurite outgrowth assays\",\n      \"pmids\": [\"18849880\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous ligand-driven equivalent not tested\", \"Physiological neuronal role not addressed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrated that gain-of-function mutations transform LTK into an oncogenic driver, establishing its transformation potential and downstream signaling.\",\n      \"evidence\": \"Site-directed mutagenesis (F568L, R669Q), cytokine-independent and soft-agar growth assays, signaling Western blots\",\n      \"pmids\": [\"22347506\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether such mutations occur in human tumors not established here\", \"Structural basis of constitutive activation unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identified ALKAL1/2 (FAM150A/B) as activating ligands and defined receptor selectivity, finally pairing LTK with its physiological agonists.\",\n      \"evidence\": \"Extracellular proteome signaling screen, picomolar binding affinity measurement, and phosphorylation assays for LTK vs ALK\",\n      \"pmids\": [\"25331893\", \"26630010\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor stoichiometry and dimerization geometry not yet resolved\", \"In vivo ligand sources not defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Assigned LTK a defined developmental function by showing it specifically mediates iridophore differentiation downstream of AUG ligands.\",\n      \"evidence\": \"Zebrafish genetic knockout, phenotypic pigment-pattern analysis, and ligand-receptor epistasis\",\n      \"pmids\": [\"29078341\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Intracellular signaling driving iridophore fate not detailed\", \"Conservation across vertebrates not yet shown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Defined the minimal active augmentor domain and the structural determinants of ligand dimerization required to activate LTK and ALK.\",\n      \"evidence\": \"Mass spectrometry mapping of disulfide bridges plus phosphorylation, MAPK, soft-agar, and differentiation assays\",\n      \"pmids\": [\"30061385\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution receptor-ligand complex not determined here\", \"Role of dimerization in vivo unresolved\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Uncovered a non-canonical ER-resident LTK function by identifying Sec12 as a substrate controlling ER exit sites and secretory transport.\",\n      \"evidence\": \"siRNA knockdown, pharmacologic inhibition, ER exit-site imaging, ER-to-Golgi transport assay, Sec12 co-IP, and phosphoablating mutant\",\n      \"pmids\": [\"31227593\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ligand dependence of the ER pool's activity unclear\", \"Relationship between surface and ER signaling not integrated\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established CLIP1-LTK as a recurrent oncogenic fusion and a druggable target, translating LTK kinase activation into a clinical mechanism.\",\n      \"evidence\": \"Whole-transcriptome sequencing, Ba/F3 transformation and kinase assays, lorlatinib response\",\n      \"pmids\": [\"34819663\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Resistance mechanisms not yet mapped\", \"Frequency across tumor types not defined here\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Placed LTK upstream of IGF1R/PI3K in neuronal polarity, defining a physiological negative-regulatory circuit controlling axon number and cortical migration.\",\n      \"evidence\": \"Primary neuron and in vivo mouse knockout, epistasis, cell-surface IGF1R and PI3K signaling assays\",\n      \"pmids\": [\"37291945\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which LTK suppresses surface IGF1R unknown\", \"Redundancy with ALK not fully separated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Confirmed conservation of LTK's iridophore role by identifying it as the Mendelian determinant of the melanoid axolotl variant.\",\n      \"evidence\": \"Bulked segregant RNA-Seq, SNP mapping, and CRISPR crispant phenocopy\",\n      \"pmids\": [\"37107662\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal mutation/molecular lesion not fully resolved\", \"Single-lab genetic study\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Mapped clinically actionable lorlatinib-resistance mutations in CLIP1-LTK and a second-line therapeutic option, advancing the cancer-targeting model.\",\n      \"evidence\": \"In vitro kinase resistance and Ba/F3 assays, mouse xenograft, in silico docking; gilteritinib rescue\",\n      \"pmids\": [\"38575808\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Clinical validation of resistance and gilteritinib response pending\", \"Structural model is in silico only\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided structural evidence that LTK and ALK share a common receptor dimerization mode with mixed 2:2 and 2:1 ligand stoichiometries.\",\n      \"evidence\": \"Cryo-EM reanalysis to 3.2 Å and comparison with ALK-ALKAL2 and LTK-ALKAL1 crystal structures (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607122\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Functional consequence of stoichiometry switch unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extended LTK's ER-proteostasis function to disease, showing that its inhibition collapses secretory capacity in myeloma and that its loss reshapes immune signaling in autoimmunity.\",\n      \"evidence\": \"LTK inhibitor treatment with Ig-retention and ER-stress readouts in primary myeloma cells; lentiviral LTK knockdown with CXCL13/M2 polarization analysis in NOD mice\",\n      \"pmids\": [\"40634511\", \"40154092\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether effects are via ER-resident vs surface LTK unclear\", \"Direct substrates in these contexts not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How LTK partitions between surface receptor signaling and ER-resident Sec12-dependent secretory control, and how ligand engagement coordinates these two modes, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking ligand-driven surface signaling to ER proteostasis function\", \"Determinants of isoform-specific localization in human cells not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 3, 4, 14]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 14]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [10, 11]},\n      {\"term_id\": \"GO:0004672\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2, 5, 14]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 9, 10]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [15, 18, 19]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [12, 16, 17]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [14, 19]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ALKAL1\",\n      \"ALKAL2\",\n      \"PLCG1\",\n      \"PIK3R1\",\n      \"RAF1\",\n      \"CANX\",\n      \"SEC12\",\n      \"CLIP1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}