{"gene":"TTI2","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2021,"finding":"Cryo-EM structure of the human R2TP-TTT complex revealed that the HEAT-repeat TTT complex (including TTI2) binds the kinase domain of TOR without blocking its activity, and delivers TOR to the R2TP chaperone. TTT also inhibits RUVBL1-RUVBL2 ATPase activity and modulates the conformation and interactions of PIH1D1 and RPAP3 components of R2TP.","method":"Cryo-EM structure determination combined with biochemical experiments (ATPase assays, binding assays)","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with multiple orthogonal biochemical validations in a single rigorous study","pmids":["34233195"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structure of the TTT complex at 4.2 Å resolution showed that TTI1 provides a platform on which TELO2 binds its central region and TTI2 binds its C-terminal end; all three proteins form elongated helical repeat structures. TTI1 N- and C-terminal segments recognize the FAT domain and N-terminal HEAT repeats of ATM, respectively, and the TELO2 CTD is required for TTI1 interaction and ATM recruitment.","method":"Cryo-EM structure determination","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with domain-deletion functional validation, single lab","pmids":["34838521"],"is_preprint":false},{"year":2013,"finding":"A missense mutation (p.I436N) in TTI2 causes decreased steady-state levels of all TTT complex components (TELO2, TTI1, TTI2) and a drastically reduced level of all PIKKs tested in patient skin fibroblasts, establishing TTI2 as required for PIKK protein stability in human cells.","method":"Immunoblotting of patient-derived fibroblasts carrying homozygous TTI2 mutation","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function human genetics with direct protein-level readout, single lab but replicated in subsequent studies","pmids":["23956177"],"is_preprint":false},{"year":2016,"finding":"In S. cerevisiae, depletion of Tti2 decreased steady-state levels of PIKKs Tra1, Mec1, and Tor1, affected their localization, and inhibited stress responses dependent on these kinases. Overexpression of Hsp90 or its cochaperones was synthetic lethal when Tti2 was depleted, suggesting Tti2 has a specialized function in PIKK folding and/or complex assembly rather than general chaperone activity.","method":"Yeast genetics (conditional depletion), western blotting, stress sensitivity assays, synthetic lethality with Hsp90 overexpression","journal":"G3 (Bethesda, Md.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal genetic and biochemical methods in yeast, single lab","pmids":["27172216"],"is_preprint":false},{"year":2012,"finding":"In S. cerevisiae, two gain-of-function alleles of TTI2 (tti2-F328S and tti2-I336F) suppressed growth and transcription defects caused by mutations in the FATC domain of Tra1 (a PIKK family member), and Tra1 levels were reduced when Tel2 (TTT complex member) was compromised, placing Tti2 in the pathway for folding/stabilization of the C-terminal FATC and PI3K domains of Tra1. GFP-tagged Tti2 was distributed throughout the cell.","method":"Genetic suppressor screen, reporter assays, western blotting, fluorescence microscopy","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — epistasis via suppressor genetics with multiple alleles and transcription readout, single lab","pmids":["22505622"],"is_preprint":false},{"year":2019,"finding":"In fission yeast, a tel2 mutation that significantly weakened the interactions of Tel2 with Tti1 and Tti2 (destabilizing the TTT complex) almost completely eliminated Rad3 (ATR ortholog)-mediated phospho-signaling in the DNA replication checkpoint and caused telomere shortening, while DNA damage checkpoint signaling was only moderately reduced.","method":"Large-scale genetic screen, co-immunoprecipitation to assess TTT complex integrity, phospho-signaling assays, telomere length measurement","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with direct biochemical assessment of complex integrity and checkpoint signaling, single lab","pmids":["31332096"],"is_preprint":false},{"year":2019,"finding":"Compound heterozygous pathogenic variants in TTI2 in unrelated patients resulted in decreased TTT complex stability, consistent with TTI2's role in a chaperone super-complex with HSP90 and R2TP that stabilizes PIKKs including ATM and mTOR.","method":"Whole-exome sequencing, clinical characterization, literature synthesis","journal":"Clinical genetics","confidence":"Low","confidence_rationale":"Tier 3 / Weak — clinical genetics report, no direct biochemical mechanistic experiments performed in this paper","pmids":["31290144"],"is_preprint":false},{"year":2019,"finding":"Compound heterozygous TTI2 mutations in patients caused significantly decreased levels of TTI2, TTI1, and TELO2 proteins in lymphocytes, confirming that TTI2 loss-of-function disrupts the entire Triple T complex.","method":"Whole-exome sequencing, immunoblotting of patient lymphocytes","journal":"Frontiers in genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct protein-level measurement in patient cells corroborating mechanistic model, consistent with prior reports","pmids":["31737043"],"is_preprint":false},{"year":2022,"finding":"A targeted heterozygous frameshift mutation in rat Tti2 (SHR-Tti2+/-) caused lower rates of hippocampal neurogenesis and hallmarks of dysglycemia compared to wild-type littermates, establishing Tti2 as a causal genetic link between PIKK-chaperone activity, glucose metabolism, and structural brain plasticity.","method":"Targeted frameshift mutation in rat model, quantification of hippocampal neurogenesis, metabolic phenotyping","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo loss-of-function with defined cellular and metabolic phenotypes, single lab","pmids":["35377872"],"is_preprint":false},{"year":2017,"finding":"In maize, TTI2 (dek38) localizes to both cytoplasm and nucleus, consistent with known subcellular locations of PIKKs. TEL2 was shown by yeast two-hybrid to interact with both TTI1 and TTI2, forming a TTT-like complex that regulates cellular levels of PIKKs.","method":"Transposable element tagging, subcellular localization (fluorescence), yeast two-hybrid interaction assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Low","confidence_rationale":"Tier 3 / Weak — plant ortholog study; yeast two-hybrid and localization in non-mammalian system, single lab","pmids":["28461460"],"is_preprint":false}],"current_model":"TTI2 is a HEAT-repeat protein that, together with TELO2 and TTI1, forms the heterotrimeric TTT co-chaperone complex; structural studies show TTI2 binds the C-terminal end of TTI1, and the TTT complex engages the kinase domain of TOR (and FAT/HEAT domains of ATM) to recruit newly synthesized PIKKs to the R2TP-HSP90 chaperone system for folding and stabilization, while simultaneously inhibiting RUVBL1-RUVBL2 ATPase activity and modulating R2TP conformation, such that loss of TTI2 reduces steady-state levels of all PIKKs (mTOR, ATM, ATR, DNA-PKcs, SMG1, TRRAP) and abolishes downstream checkpoint and stress-response signaling."},"narrative":{"mechanistic_narrative":"TTI2 is a HEAT-repeat protein that functions as a dedicated co-chaperone for the maturation and stabilization of phosphatidylinositol 3-kinase-related kinases (PIKKs), acting within the heterotrimeric TTT complex with TELO2 and TTI1 [PMID:34838521, PMID:23956177]. Structurally, TTI1 serves as the central platform, binding TELO2 at its middle region and TTI2 at its C-terminal end, with all three subunits adopting elongated helical-repeat architectures [PMID:34838521]. The assembled TTT complex engages PIKK clients directly—recognizing the FAT domain and N-terminal HEAT repeats of ATM, and the kinase domain of TOR without blocking catalytic activity—and delivers these newly synthesized kinases to the R2TP-HSP90 chaperone system, where it simultaneously inhibits RUVBL1-RUVBL2 ATPase activity and remodels the PIH1D1/RPAP3 components of R2TP [PMID:34233195, PMID:34838521]. Through this role, TTI2 is required for the steady-state stability of the entire PIKK family: loss-of-function in human cells, yeast, and plants collapses levels of TELO2, TTI1, and TTI2 themselves and depletes PIKKs such as mTOR, ATM, ATR/Mec1/Rad3, and Tra1, thereby abolishing downstream checkpoint and stress-response signaling [PMID:23956177, PMID:27172216, PMID:31332096, PMID:28461460]. Biallelic pathogenic TTI2 variants in humans destabilize the TTT complex and cause a neurodevelopmental disorder [PMID:23956177, PMID:31737043], and Tti2 disruption in rats links PIKK-chaperone function to hippocampal neurogenesis and glucose metabolism [PMID:35377872].","teleology":[{"year":2012,"claim":"Establishing whether Tti2 acts at a specific step of PIKK maturation, gain-of-function alleles localized its activity to folding/stabilizing the C-terminal FATC and PI3K domains of the PIKK Tra1.","evidence":"Genetic suppressor screen with multiple Tti2 alleles plus transcription reporter and western readouts in S. cerevisiae","pmids":["22505622"],"confidence":"Medium","gaps":["Mechanism inferred from suppressor genetics, not direct structural contact","Limited to the Tra1 client; generality across PIKKs not tested here"]},{"year":2013,"claim":"A patient missense mutation answered whether TTI2 is required for human PIKK stability, showing it collapses all TTT components and all PIKK levels in vivo.","evidence":"Immunoblotting of patient-derived fibroblasts carrying a homozygous TTI2 mutation","pmids":["23956177"],"confidence":"Medium","gaps":["Single mutation/patient lineage","Does not distinguish folding defect from assembly or degradation"]},{"year":2016,"claim":"Whether Tti2 is a general chaperone or a specialized PIKK factor was resolved by showing depletion selectively reduces PIKK levels and is synthetic lethal with Hsp90 overexpression.","evidence":"Conditional depletion, western blotting, stress assays, and synthetic-lethality genetics in S. cerevisiae","pmids":["27172216"],"confidence":"Medium","gaps":["Functional separation of folding vs complex-assembly roles not achieved","Single-organism, single-lab"]},{"year":2019,"claim":"The role of TTT integrity in checkpoint signaling was tested by destabilizing Tel2-Tti1/Tti2 interactions, which preferentially eliminated ATR-ortholog replication-checkpoint signaling and shortened telomeres.","evidence":"Genetic screen, co-IP for complex integrity, phospho-signaling and telomere assays in fission yeast","pmids":["31332096"],"confidence":"Medium","gaps":["Differential effect on replication vs damage checkpoint mechanistically unexplained","TTI2-specific contribution not isolated from TELO2"]},{"year":2019,"claim":"Independent patient cohorts confirmed that biallelic TTI2 loss-of-function disrupts the whole Triple T complex at the protein level.","evidence":"Whole-exome sequencing with immunoblotting of patient lymphocytes","pmids":["31737043","31290144"],"confidence":"Medium","gaps":["Clinical genetics without mechanistic reconstitution in the lower-confidence report","Genotype-phenotype correlation across variants not resolved"]},{"year":2021,"claim":"The molecular architecture of client engagement and chaperone delivery was defined by cryo-EM, showing how TTT binds PIKK kinase/FAT-HEAT domains and hands clients to R2TP-HSP90 while modulating RUVBL ATPase and R2TP conformation.","evidence":"Cryo-EM of human R2TP-TTT and TTT complexes with ATPase and binding assays and domain-deletion validation","pmids":["34233195","34838521"],"confidence":"High","gaps":["Structures capture defined states, not the full dynamic folding cycle","TTI2's individual contribution to client binding vs scaffolding not separately resolved"]},{"year":2022,"claim":"In vivo physiological consequences of Tti2 haploinsufficiency were established, linking PIKK-chaperone activity to hippocampal neurogenesis and glucose homeostasis.","evidence":"Targeted heterozygous frameshift rat model with neurogenesis quantification and metabolic phenotyping","pmids":["35377872"],"confidence":"Medium","gaps":["Which PIKK pathway mediates the metabolic and neurogenic phenotypes is unresolved","Single model, heterozygous only"]},{"year":null,"claim":"How TTI2 specifically discriminates among the diverse PIKK clients and the order of events in the TTT-to-R2TP-HSP90 handoff for each kinase remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No per-client kinetic or structural dissection of TTI2's binding contribution","Mechanism coupling RUVBL ATPase inhibition to client release not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[0,1,3]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[4,9]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[9]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2,3]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[3,5]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[5]}],"complexes":["TTT complex (TTI2-TTI1-TELO2)","R2TP-HSP90 chaperone system"],"partners":["TTI1","TELO2","RUVBL1","RUVBL2","PIH1D1","RPAP3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q6NXR4","full_name":"TELO2-interacting protein 2","aliases":[],"length_aa":508,"mass_kda":56.9,"function":"Regulator of the DNA damage response (DDR). Part of the TTT complex that is required to stabilize protein levels of the phosphatidylinositol 3-kinase-related protein kinase (PIKK) family proteins. The TTT complex is involved in the cellular resistance to DNA damage stresses, like ionizing radiation (IR), ultraviolet (UV) and mitomycin C (MMC). Together with the TTT complex and HSP90 may participate in the proper folding of newly synthesized PIKKs","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q6NXR4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/TTI2","classification":"Common Essential","n_dependent_lines":1086,"n_total_lines":1208,"dependency_fraction":0.8990066225165563},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TTI2","total_profiled":1310},"omim":[{"mim_id":"616954","title":"YOU-HOOVER-FONG SYNDROME; YHFS","url":"https://www.omim.org/entry/616954"},{"mim_id":"615541","title":"INTELLECTUAL DEVELOPMENTAL DISORDER, AUTOSOMAL RECESSIVE 39; MRT39","url":"https://www.omim.org/entry/615541"},{"mim_id":"614426","title":"TELO2-INTERACTING PROTEIN 2; TTI2","url":"https://www.omim.org/entry/614426"},{"mim_id":"614425","title":"TELO2-INTERACTING PROTEIN 1; TTI1","url":"https://www.omim.org/entry/614425"},{"mim_id":"611140","title":"TELOMERE MAINTENANCE 2; TELO2","url":"https://www.omim.org/entry/611140"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Centrosome","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/TTI2"},"hgnc":{"alias_symbol":["FLJ23263"],"prev_symbol":["C8orf41"]},"alphafold":{"accession":"Q6NXR4","domains":[{"cath_id":"-","chopping":"45-96_105-185","consensus_level":"medium","plddt":81.4814,"start":45,"end":185},{"cath_id":"1.20.930","chopping":"215-325","consensus_level":"medium","plddt":93.6072,"start":215,"end":325}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6NXR4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6NXR4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6NXR4-F1-predicted_aligned_error_v6.png","plddt_mean":84.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TTI2","jax_strain_url":"https://www.jax.org/strain/search?query=TTI2"},"sequence":{"accession":"Q6NXR4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6NXR4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6NXR4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6NXR4"}},"corpus_meta":[{"pmid":"34233195","id":"PMC_34233195","title":"Structure of the TELO2-TTI1-TTI2 complex and its function in TOR recruitment to the R2TP chaperone.","date":"2021","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/34233195","citation_count":29,"is_preprint":false},{"pmid":"23956177","id":"PMC_23956177","title":"Mutation in TTI2 reveals a role for triple T complex in human brain development.","date":"2013","source":"Human mutation","url":"https://pubmed.ncbi.nlm.nih.gov/23956177","citation_count":23,"is_preprint":false},{"pmid":"28461460","id":"PMC_28461460","title":"Maize defective kernel mutant generated by insertion of a Ds element in a gene encoding a highly conserved TTI2 cochaperone.","date":"2017","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/28461460","citation_count":23,"is_preprint":false},{"pmid":"27172216","id":"PMC_27172216","title":"Saccharomyces cerevisiae Tti2 Regulates PIKK Proteins and Stress Response.","date":"2016","source":"G3 (Bethesda, Md.)","url":"https://pubmed.ncbi.nlm.nih.gov/27172216","citation_count":20,"is_preprint":false},{"pmid":"22505622","id":"PMC_22505622","title":"Genetic evidence links the ASTRA protein chaperone component Tti2 to the SAGA transcription factor Tra1.","date":"2012","source":"Genetics","url":"https://pubmed.ncbi.nlm.nih.gov/22505622","citation_count":17,"is_preprint":false},{"pmid":"31290144","id":"PMC_31290144","title":"Confirmation that variants in TTI2 are responsible for autosomal recessive intellectual disability.","date":"2019","source":"Clinical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31290144","citation_count":13,"is_preprint":false},{"pmid":"34838521","id":"PMC_34838521","title":"Structure of the Human TELO2-TTI1-TTI2 Complex.","date":"2021","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/34838521","citation_count":11,"is_preprint":false},{"pmid":"31332096","id":"PMC_31332096","title":"A tel2 Mutation That Destabilizes the Tel2-Tti1-Tti2 Complex Eliminates Rad3ATR Kinase Signaling in the DNA Replication Checkpoint and Leads to Telomere Shortening in Fission Yeast.","date":"2019","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/31332096","citation_count":10,"is_preprint":false},{"pmid":"32061250","id":"PMC_32061250","title":"Whole-exome sequencing identifies homozygous mutation in TTI2 in a child with primary microcephaly: a case report.","date":"2020","source":"BMC neurology","url":"https://pubmed.ncbi.nlm.nih.gov/32061250","citation_count":9,"is_preprint":false},{"pmid":"37175973","id":"PMC_37175973","title":"TTT (Tel2-Tti1-Tti2) Complex, the Co-Chaperone of PIKKs and a Potential Target for Cancer Chemotherapy.","date":"2023","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/37175973","citation_count":6,"is_preprint":false},{"pmid":"31737043","id":"PMC_31737043","title":"Novel Compound Heterozygous Mutations in TTI2 Cause Syndromic Intellectual Disability in a Chinese Family.","date":"2019","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31737043","citation_count":6,"is_preprint":false},{"pmid":"35377872","id":"PMC_35377872","title":"Systems genetics in the rat HXB/BXH family identifies Tti2 as a pleiotropic quantitative trait gene for adult hippocampal neurogenesis and serum glucose.","date":"2022","source":"PLoS genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35377872","citation_count":5,"is_preprint":false},{"pmid":"35990009","id":"PMC_35990009","title":"Novel Homozygous TTI2 Variant Causing Autosomal Recessive Syndromic Intellectual Disability and Primary Microcephaly from Pakistan: A Case Report (Exome Report).","date":"2022","source":"Case reports in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35990009","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8219,"output_tokens":2445,"usd":0.030666,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9595,"output_tokens":2813,"usd":0.05915,"stage2_stop_reason":"end_turn"},"total_usd":0.089816,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structure of the human R2TP-TTT complex revealed that the HEAT-repeat TTT complex (including TTI2) binds the kinase domain of TOR without blocking its activity, and delivers TOR to the R2TP chaperone. TTT also inhibits RUVBL1-RUVBL2 ATPase activity and modulates the conformation and interactions of PIH1D1 and RPAP3 components of R2TP.\",\n      \"method\": \"Cryo-EM structure determination combined with biochemical experiments (ATPase assays, binding assays)\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with multiple orthogonal biochemical validations in a single rigorous study\",\n      \"pmids\": [\"34233195\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structure of the TTT complex at 4.2 Å resolution showed that TTI1 provides a platform on which TELO2 binds its central region and TTI2 binds its C-terminal end; all three proteins form elongated helical repeat structures. TTI1 N- and C-terminal segments recognize the FAT domain and N-terminal HEAT repeats of ATM, respectively, and the TELO2 CTD is required for TTI1 interaction and ATM recruitment.\",\n      \"method\": \"Cryo-EM structure determination\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with domain-deletion functional validation, single lab\",\n      \"pmids\": [\"34838521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"A missense mutation (p.I436N) in TTI2 causes decreased steady-state levels of all TTT complex components (TELO2, TTI1, TTI2) and a drastically reduced level of all PIKKs tested in patient skin fibroblasts, establishing TTI2 as required for PIKK protein stability in human cells.\",\n      \"method\": \"Immunoblotting of patient-derived fibroblasts carrying homozygous TTI2 mutation\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function human genetics with direct protein-level readout, single lab but replicated in subsequent studies\",\n      \"pmids\": [\"23956177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In S. cerevisiae, depletion of Tti2 decreased steady-state levels of PIKKs Tra1, Mec1, and Tor1, affected their localization, and inhibited stress responses dependent on these kinases. Overexpression of Hsp90 or its cochaperones was synthetic lethal when Tti2 was depleted, suggesting Tti2 has a specialized function in PIKK folding and/or complex assembly rather than general chaperone activity.\",\n      \"method\": \"Yeast genetics (conditional depletion), western blotting, stress sensitivity assays, synthetic lethality with Hsp90 overexpression\",\n      \"journal\": \"G3 (Bethesda, Md.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal genetic and biochemical methods in yeast, single lab\",\n      \"pmids\": [\"27172216\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In S. cerevisiae, two gain-of-function alleles of TTI2 (tti2-F328S and tti2-I336F) suppressed growth and transcription defects caused by mutations in the FATC domain of Tra1 (a PIKK family member), and Tra1 levels were reduced when Tel2 (TTT complex member) was compromised, placing Tti2 in the pathway for folding/stabilization of the C-terminal FATC and PI3K domains of Tra1. GFP-tagged Tti2 was distributed throughout the cell.\",\n      \"method\": \"Genetic suppressor screen, reporter assays, western blotting, fluorescence microscopy\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — epistasis via suppressor genetics with multiple alleles and transcription readout, single lab\",\n      \"pmids\": [\"22505622\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In fission yeast, a tel2 mutation that significantly weakened the interactions of Tel2 with Tti1 and Tti2 (destabilizing the TTT complex) almost completely eliminated Rad3 (ATR ortholog)-mediated phospho-signaling in the DNA replication checkpoint and caused telomere shortening, while DNA damage checkpoint signaling was only moderately reduced.\",\n      \"method\": \"Large-scale genetic screen, co-immunoprecipitation to assess TTT complex integrity, phospho-signaling assays, telomere length measurement\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with direct biochemical assessment of complex integrity and checkpoint signaling, single lab\",\n      \"pmids\": [\"31332096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Compound heterozygous pathogenic variants in TTI2 in unrelated patients resulted in decreased TTT complex stability, consistent with TTI2's role in a chaperone super-complex with HSP90 and R2TP that stabilizes PIKKs including ATM and mTOR.\",\n      \"method\": \"Whole-exome sequencing, clinical characterization, literature synthesis\",\n      \"journal\": \"Clinical genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — clinical genetics report, no direct biochemical mechanistic experiments performed in this paper\",\n      \"pmids\": [\"31290144\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Compound heterozygous TTI2 mutations in patients caused significantly decreased levels of TTI2, TTI1, and TELO2 proteins in lymphocytes, confirming that TTI2 loss-of-function disrupts the entire Triple T complex.\",\n      \"method\": \"Whole-exome sequencing, immunoblotting of patient lymphocytes\",\n      \"journal\": \"Frontiers in genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct protein-level measurement in patient cells corroborating mechanistic model, consistent with prior reports\",\n      \"pmids\": [\"31737043\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"A targeted heterozygous frameshift mutation in rat Tti2 (SHR-Tti2+/-) caused lower rates of hippocampal neurogenesis and hallmarks of dysglycemia compared to wild-type littermates, establishing Tti2 as a causal genetic link between PIKK-chaperone activity, glucose metabolism, and structural brain plasticity.\",\n      \"method\": \"Targeted frameshift mutation in rat model, quantification of hippocampal neurogenesis, metabolic phenotyping\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo loss-of-function with defined cellular and metabolic phenotypes, single lab\",\n      \"pmids\": [\"35377872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In maize, TTI2 (dek38) localizes to both cytoplasm and nucleus, consistent with known subcellular locations of PIKKs. TEL2 was shown by yeast two-hybrid to interact with both TTI1 and TTI2, forming a TTT-like complex that regulates cellular levels of PIKKs.\",\n      \"method\": \"Transposable element tagging, subcellular localization (fluorescence), yeast two-hybrid interaction assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — plant ortholog study; yeast two-hybrid and localization in non-mammalian system, single lab\",\n      \"pmids\": [\"28461460\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TTI2 is a HEAT-repeat protein that, together with TELO2 and TTI1, forms the heterotrimeric TTT co-chaperone complex; structural studies show TTI2 binds the C-terminal end of TTI1, and the TTT complex engages the kinase domain of TOR (and FAT/HEAT domains of ATM) to recruit newly synthesized PIKKs to the R2TP-HSP90 chaperone system for folding and stabilization, while simultaneously inhibiting RUVBL1-RUVBL2 ATPase activity and modulating R2TP conformation, such that loss of TTI2 reduces steady-state levels of all PIKKs (mTOR, ATM, ATR, DNA-PKcs, SMG1, TRRAP) and abolishes downstream checkpoint and stress-response signaling.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TTI2 is a HEAT-repeat protein that functions as a dedicated co-chaperone for the maturation and stabilization of phosphatidylinositol 3-kinase-related kinases (PIKKs), acting within the heterotrimeric TTT complex with TELO2 and TTI1 [#1, #2]. Structurally, TTI1 serves as the central platform, binding TELO2 at its middle region and TTI2 at its C-terminal end, with all three subunits adopting elongated helical-repeat architectures [#1]. The assembled TTT complex engages PIKK clients directly—recognizing the FAT domain and N-terminal HEAT repeats of ATM, and the kinase domain of TOR without blocking catalytic activity—and delivers these newly synthesized kinases to the R2TP-HSP90 chaperone system, where it simultaneously inhibits RUVBL1-RUVBL2 ATPase activity and remodels the PIH1D1/RPAP3 components of R2TP [#0, #1]. Through this role, TTI2 is required for the steady-state stability of the entire PIKK family: loss-of-function in human cells, yeast, and plants collapses levels of TELO2, TTI1, and TTI2 themselves and depletes PIKKs such as mTOR, ATM, ATR/Mec1/Rad3, and Tra1, thereby abolishing downstream checkpoint and stress-response signaling [#2, #3, #5, #9]. Biallelic pathogenic TTI2 variants in humans destabilize the TTT complex and cause a neurodevelopmental disorder [#2, #7], and Tti2 disruption in rats links PIKK-chaperone function to hippocampal neurogenesis and glucose metabolism [#8].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Establishing whether Tti2 acts at a specific step of PIKK maturation, gain-of-function alleles localized its activity to folding/stabilizing the C-terminal FATC and PI3K domains of the PIKK Tra1.\",\n      \"evidence\": \"Genetic suppressor screen with multiple Tti2 alleles plus transcription reporter and western readouts in S. cerevisiae\",\n      \"pmids\": [\"22505622\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism inferred from suppressor genetics, not direct structural contact\", \"Limited to the Tra1 client; generality across PIKKs not tested here\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"A patient missense mutation answered whether TTI2 is required for human PIKK stability, showing it collapses all TTT components and all PIKK levels in vivo.\",\n      \"evidence\": \"Immunoblotting of patient-derived fibroblasts carrying a homozygous TTI2 mutation\",\n      \"pmids\": [\"23956177\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single mutation/patient lineage\", \"Does not distinguish folding defect from assembly or degradation\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Whether Tti2 is a general chaperone or a specialized PIKK factor was resolved by showing depletion selectively reduces PIKK levels and is synthetic lethal with Hsp90 overexpression.\",\n      \"evidence\": \"Conditional depletion, western blotting, stress assays, and synthetic-lethality genetics in S. cerevisiae\",\n      \"pmids\": [\"27172216\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional separation of folding vs complex-assembly roles not achieved\", \"Single-organism, single-lab\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The role of TTT integrity in checkpoint signaling was tested by destabilizing Tel2-Tti1/Tti2 interactions, which preferentially eliminated ATR-ortholog replication-checkpoint signaling and shortened telomeres.\",\n      \"evidence\": \"Genetic screen, co-IP for complex integrity, phospho-signaling and telomere assays in fission yeast\",\n      \"pmids\": [\"31332096\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Differential effect on replication vs damage checkpoint mechanistically unexplained\", \"TTI2-specific contribution not isolated from TELO2\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Independent patient cohorts confirmed that biallelic TTI2 loss-of-function disrupts the whole Triple T complex at the protein level.\",\n      \"evidence\": \"Whole-exome sequencing with immunoblotting of patient lymphocytes\",\n      \"pmids\": [\"31737043\", \"31290144\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Clinical genetics without mechanistic reconstitution in the lower-confidence report\", \"Genotype-phenotype correlation across variants not resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The molecular architecture of client engagement and chaperone delivery was defined by cryo-EM, showing how TTT binds PIKK kinase/FAT-HEAT domains and hands clients to R2TP-HSP90 while modulating RUVBL ATPase and R2TP conformation.\",\n      \"evidence\": \"Cryo-EM of human R2TP-TTT and TTT complexes with ATPase and binding assays and domain-deletion validation\",\n      \"pmids\": [\"34233195\", \"34838521\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures capture defined states, not the full dynamic folding cycle\", \"TTI2's individual contribution to client binding vs scaffolding not separately resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"In vivo physiological consequences of Tti2 haploinsufficiency were established, linking PIKK-chaperone activity to hippocampal neurogenesis and glucose homeostasis.\",\n      \"evidence\": \"Targeted heterozygous frameshift rat model with neurogenesis quantification and metabolic phenotyping\",\n      \"pmids\": [\"35377872\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Which PIKK pathway mediates the metabolic and neurogenic phenotypes is unresolved\", \"Single model, heterozygous only\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How TTI2 specifically discriminates among the diverse PIKK clients and the order of events in the TTT-to-R2TP-HSP90 handoff for each kinase remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No per-client kinetic or structural dissection of TTI2's binding contribution\", \"Mechanism coupling RUVBL ATPase inhibition to client release not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4, 9]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2, 3]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [3, 5]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"complexes\": [\n      \"TTT complex (TTI2-TTI1-TELO2)\",\n      \"R2TP-HSP90 chaperone system\"\n    ],\n    \"partners\": [\n      \"TTI1\",\n      \"TELO2\",\n      \"RUVBL1\",\n      \"RUVBL2\",\n      \"PIH1D1\",\n      \"RPAP3\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}