Affinage

TTI1

TELO2-interacting protein 1 homolog · UniProt O43156

Length
1089 aa
Mass
122.1 kDa
Annotated
2026-04-28
13 papers in source corpus 9 papers cited in narrative 9 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TTI1 is the central scaffold subunit of the TTT (TELO2–TTI1–TTI2) co-chaperone complex that is essential for the folding, stabilization, and assembly of all six PIKK family kinases (mTOR, ATM, ATR, DNA-PKcs, SMG-1, TRRAP) (PMID:20427287). Cryo-EM structures show that TTI1 HEAT repeats organize the complex, with TELO2 binding its central region and TTI2 its C-terminus, while TTI1 N- and C-terminal segments directly contact the FAT domain and N-terminal HEAT repeats of client PIKKs; the TTT complex delivers clients to the R2TP–HSP90 chaperone machinery by engaging the PIKK kinase domain and modulating RUVBL1/2 ATPase activity (PMID:34838521, PMID:34233195). TTI1 is required for mTORC1 and mTORC2 assembly and signaling, and upon growth-factor withdrawal CK2 phosphorylates TTI1, targeting it for SCF^Fbxo9-mediated ubiquitination and degradation to selectively inactivate mTORC1 (PMID:23263282); CK2-mediated phosphorylation of the TTT complex is enhanced by the inositol pyrophosphate IP7, which stabilizes DNA-PKcs and ATM to activate p53-dependent apoptosis (PMID:24657168). Bi-allelic loss-of-function variants in TTI1 cause a neurodevelopmental disorder with impaired mTOR signaling in patient-derived cells (PMID:36724785).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2010 High

    Identifying TTI1 as a constitutive PIKK-binding partner resolved how all six PIKKs are co-stabilized and how mTORC1/mTORC2 assembly depends on a shared chaperone adaptor.

    Evidence Co-IP, gel filtration, and siRNA knockdown in mammalian cells showing loss of all PIKK proteins and mTOR complex activity upon TTI1 depletion

    PMID:20427287

    Open questions at the time
    • No structural information on how TTI1 contacts PIKKs
    • Mechanism of selectivity for PIKK family over other kinases unknown
    • Role beyond protein stability (e.g., kinase activation) not addressed
  2. 2013 High

    Demonstrating that CK2-primed SCF^Fbxo9 ubiquitination degrades TTI1 upon growth-factor withdrawal established the first regulated proteolytic switch that selectively inactivates mTORC1 while sustaining mTORC2/Akt signaling.

    Evidence Reconstituted SCF^Fbxo9 ubiquitin ligase activity, CK2 kinase assays, and siRNA in mammalian cells

    PMID:23263282

    Open questions at the time
    • Specific CK2 phosphosites on TTI1 not mapped at residue resolution
    • Whether other E3 ligases target TTI1 under different stress conditions unknown
  3. 2014 High

    Showing that IP7 enhances CK2-mediated TTT complex phosphorylation to stabilize ATM/DNA-PKcs and activate p53-dependent apoptosis linked metabolite signaling to PIKK homeostasis through TTI1.

    Evidence In vitro CK2 kinase assays with IP7, IP6K2 manipulation, and western blot for PIKK levels and p53 Ser15 phosphorylation

    PMID:24657168

    Open questions at the time
    • Direct IP7 binding site on TTI1/TTT versus CK2 not resolved
    • Context-dependent outcome (mTORC1 degradation vs. ATM stabilization) of TTI1 phosphorylation not mechanistically reconciled
  4. 2019 Medium

    Genetic destabilization of the TTT complex in fission yeast confirmed that TTT integrity is required for ATR checkpoint signaling and telomere maintenance, extending the mammalian findings to a conserved function.

    Evidence Genetic screen with tel2 mutant, co-IP for TTT stability, Rad3 phospho-signaling, and telomere length assay in S. pombe

    PMID:31332096

    Open questions at the time
    • Tti1-specific contributions versus Tel2/Tti2 not separated in this system
    • Telomere maintenance mechanism (via ATR vs. other PIKKs) not distinguished
  5. 2021 High

    Cryo-EM structures of the TTT complex alone and bound to R2TP revealed that TTI1 HEAT repeats serve as the central scaffold bridging TELO2 and TTI2, directly contacting PIKK FAT-domain and kinase-domain surfaces, and delivering clients to the R2TP–HSP90 system while inhibiting RUVBL1/2 ATPase activity.

    Evidence Cryo-EM at 4.2 Å (TTT–ATM) and of R2TP–TTT, deletion mutagenesis, crosslinking mass spectrometry, ATPase assays

    PMID:34233195 PMID:34838521

    Open questions at the time
    • High-resolution map of TTI1–PIKK interface residues needed for drug targeting
    • How ATPase inhibition of RUVBL1/2 is relieved to complete client handoff is unknown
    • Structure of TTT bound to PIKKs other than mTOR/ATM not determined
  6. 2021 Medium

    Yeast suppressor analysis showed Tti1 can function independently of the full TTT complex, with Tti1 overexpression alone rescuing PIKK levels after Hsp70-cochaperone depletion, implying an intrinsic PIKK-specific chaperone capacity.

    Evidence Genetic suppressor screen, protein level measurements, and rapamycin sensitivity in S. cerevisiae with Sis1 depletion

    PMID:34935410

    Open questions at the time
    • Whether mammalian TTI1 retains TTT-independent chaperone activity not tested
    • Physical interaction between Tti1 and PIKKs in the absence of Tel2/Tti2 not demonstrated biochemically
  7. 2023 Medium

    Patient studies established that bi-allelic TTI1 loss-of-function causes a neurodevelopmental disorder with reduced mTOR signaling, directly linking TTI1 to human Mendelian disease.

    Evidence Functional studies in patient-derived fibroblasts and lymphoblastoid cells, HEK293T reconstitution, rapamycin rescue

    PMID:36724785

    Open questions at the time
    • Impact on non-mTOR PIKKs (ATM, ATR) in patient cells not characterized
    • Genotype-phenotype correlation across different TTI1 variants not established
  8. 2025 Medium

    Demonstrating that TTI1 promotes ATM-dependent DNA damage repair in rectal cancer cells linked TTI1 expression to radioresistance and identified it as a potential therapeutic target.

    Evidence siRNA knockdown and overexpression with comet assay, colony formation, xenograft, organoid, and PDX models

    PMID:40514657

    Open questions at the time
    • Whether TTI1 contribution to radioresistance is solely through ATM or involves other PIKKs not resolved
    • No pharmacological inhibitor of TTI1 tested

Open questions

Synthesis pass · forward-looking unresolved questions
  • The precise mechanism by which TTI1 coordinates opposing phospho-dependent outcomes — mTORC1 degradation versus ATM/DNA-PKcs stabilization — and whether TTI1 has PIKK-independent functions remain unresolved.
  • Site-resolved phospho-regulation of TTI1 determining client-specific fate is unknown
  • No high-resolution structure of TTI1 with a full-length non-mTOR/ATM PIKK client
  • Whether TTI1 functions outside of PIKK biology is unexplored

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0044183 protein folding chaperone 4 GO:0060090 molecular adaptor activity 3
Localization
GO:0005829 cytosol 2
Pathway
R-HSA-162582 Signal Transduction 4 R-HSA-392499 Metabolism of proteins 4 R-HSA-73894 DNA Repair 3 R-HSA-5357801 Programmed Cell Death 1 R-HSA-9612973 Autophagy 1
Partners
ATMATRCSNK2A1FBXO9MTORPRKDCTELO2TTI2
Complex memberships
R2TP–TTT complexTTT complex (TELO2–TTI1–TTI2)

Evidence

Reading pass · 9 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2010 Mammalian TTI1 (KIAA0406) constitutively interacts with mTOR in both mTORC1 and mTORC2, binds Tel2, and is required for stability of all six PIKK family members (mTOR, ATM, ATR, DNA-PKcs, SMG-1, TRRAP). Knockdown of TTI1 causes disassembly of mTORC1 and mTORC2, suppresses phosphorylation of mTORC1 substrates (S6K1, 4E-BP1) and mTORC2 substrate (Akt), and induces autophagy. Co-immunoprecipitation, size-exclusion chromatography, siRNA knockdown, western blot for PIKK levels and substrate phosphorylation The Journal of biological chemistry High 20427287
2013 CK2 phosphorylates TTI1 (and Tel2) within mTORC1 upon growth factor withdrawal, targeting them for ubiquitination and degradation by the SCFFbxo9 E3 ubiquitin ligase, leading to mTORC1-specific inactivation while relieving feedback to sustain PI3K/mTORC2/Akt signaling. Co-immunoprecipitation, ubiquitination assays, CK2 kinase assay, siRNA knockdown, reconstitution of SCFFbxo9 ubiquitin ligase activity Nature cell biology High 23263282
2014 IP7 (generated by IP6K2) binds CK2 and enhances CK2-mediated phosphorylation of the TTT complex (Tel2/TTI1/TTI2), which stabilizes DNA-PKcs and ATM, thereby promoting p53 phosphorylation at serine 15 and apoptosis. In vitro CK2 kinase assay with IP7, co-immunoprecipitation, IP6K2 knockdown/overexpression, western blot for PIKK levels and p53 phosphorylation Molecular cell High 24657168
2021 Cryo-EM structure of the human R2TP-TTT complex reveals that the HEAT-repeat TTT complex binds the kinase domain of TOR (without blocking kinase activity) and delivers TOR to the R2TP chaperone; TTT also inhibits RUVBL1-RUVBL2 ATPase activity and modulates PIH1D1 and RPAP3 conformations within R2TP. Cryo-EM structure determination, biochemical ATPase assays, co-immunoprecipitation, crosslinking mass spectrometry Cell reports High 34233195
2021 Cryo-EM structure of the TTT complex at 4.2 Å resolution shows TTI1 as a central scaffold with TELO2 binding its central region and TTI2 binding its C-terminal end; TTI1 N- and C-terminal segments contact the FAT domain and N-terminal HEAT repeats of ATM respectively, and the TELO2 CTD is required for interaction with TTI1 and ATM recruitment. TTI1 N- and C-terminal segments are required for cell survival after ionizing radiation. Cryo-EM structure determination, deletion mutagenesis, co-immunoprecipitation, cell survival assays after ionizing radiation Journal of molecular biology High 34838521
2019 In fission yeast, destabilization of the TTT complex (Tel2-Tti1-Tti2) by a tel2 mutation nearly abolishes Rad3 (ATR ortholog) checkpoint signaling in the DNA replication checkpoint and causes telomere shortening, demonstrating that TTT complex integrity is required for ATR-mediated checkpoint kinase signaling. Genetic screen, co-immunoprecipitation to assess TTT complex stability, western blot for Rad3-mediated phospho-signaling, telomere length assay Molecular and cellular biology Medium 31332096
2021 In S. cerevisiae, single-residue substitutions in Tti1 suppress lethality caused by Sis1 (Hsp70 cochaperone) depletion; Sis1 depletion reduces levels of essential PIKKs (Mec1/ATR, Tra1/TRRAP, Tor2/mTOR), and overexpression of Tti1 alone (without increasing Tel2 or Tti2) restores growth, indicating Tti1 can function independently of the full TTT complex as a PIKK-specific chaperone. Genetic suppressor analysis, protein level measurement by western blot, rapamycin sensitivity assay, overexpression experiments Molecular biology of the cell Medium 34935410
2023 Bi-allelic loss-of-function variants in TTI1 impair TTT complex integrity and reduce mTOR pathway activity (mTORC1 signaling) in patient-derived cells; rapamycin treatment partially restores mTOR pathway activity, placing TTI1 upstream of mTOR in the signaling cascade in a human disease context. Functional studies in HEK293T cells and patient-derived fibroblasts/lymphoblastoid cells, western blot for mTOR pathway substrates, rapamycin rescue experiment American journal of human genetics Medium 36724785
2025 TTI1 promotes ATM signaling pathway activation in rectal cancer cells; TTI1 knockdown reduces ATM-dependent DNA damage repair after irradiation, increasing radiosensitivity, while TTI1 overexpression enhances repair and radioresistance. siRNA knockdown, overexpression, western blot for ATM pathway activation, comet assay, colony formation assay, in vivo xenograft, organoid and PDX models Journal of translational medicine Medium 40514657

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2010 Tti1 and Tel2 are critical factors in mammalian target of rapamycin complex assembly. The Journal of biological chemistry 204 20427287
2014 Inositol pyrophosphates mediate the DNA-PK/ATM-p53 cell death pathway by regulating CK2 phosphorylation of Tti1/Tel2. Molecular cell 105 24657168
2013 SCFFbxo9 and CK2 direct the cellular response to growth factor withdrawal via Tel2/Tti1 degradation and promote survival in multiple myeloma. Nature cell biology 82 23263282
2021 Structure of the TELO2-TTI1-TTI2 complex and its function in TOR recruitment to the R2TP chaperone. Cell reports 26 34233195
2021 Structure of the Human TELO2-TTI1-TTI2 Complex. Journal of molecular biology 10 34838521
2019 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. Molecular and cellular biology 10 31332096
2022 TTI1 promotes non-small-cell lung cancer progression by regulating the mTOR signaling pathway. Cancer science 9 36403197
2021 Essentiality of Sis1, a J-domain protein Hsp70 cochaperone, can be overcome by Tti1, a specialized PIKK chaperone. Molecular biology of the cell 8 34935410
2023 TTT (Tel2-Tti1-Tti2) Complex, the Co-Chaperone of PIKKs and a Potential Target for Cancer Chemotherapy. International journal of molecular sciences 6 37175973
2021 A role of TTI1 in the colorectal cancer by promoting proliferation. Translational cancer research 6 35116463
2023 Bi-allelic TTI1 variants cause an autosomal-recessive neurodevelopmental disorder with microcephaly. American journal of human genetics 5 36724785
2024 ALKBH5 promotes hepatocellular carcinoma cell proliferation, migration and invasion by regulating TTI1 expression. Biomolecules & biomedicine 3 38501918
2025 TTI1 contributes to radioresistance by activating ATM pathway in rectal cancer. Journal of translational medicine 1 40514657