Affinage

TTI2

TELO2-interacting protein 2 · UniProt Q6NXR4

Length
508 aa
Mass
56.9 kDa
Annotated
2026-06-10
13 papers in source corpus 10 papers cited in narrative 10 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

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).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2012 Medium

    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

    PMID:22505622

    Open questions at the time
    • Mechanism inferred from suppressor genetics, not direct structural contact
    • Limited to the Tra1 client; generality across PIKKs not tested here
  2. 2013 Medium

    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

    PMID:23956177

    Open questions at the time
    • Single mutation/patient lineage
    • Does not distinguish folding defect from assembly or degradation
  3. 2016 Medium

    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

    PMID:27172216

    Open questions at the time
    • Functional separation of folding vs complex-assembly roles not achieved
    • Single-organism, single-lab
  4. 2019 Medium

    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

    PMID:31332096

    Open questions at the time
    • Differential effect on replication vs damage checkpoint mechanistically unexplained
    • TTI2-specific contribution not isolated from TELO2
  5. 2019 Medium

    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

    PMID:31290144 PMID:31737043

    Open questions at the time
    • Clinical genetics without mechanistic reconstitution in the lower-confidence report
    • Genotype-phenotype correlation across variants not resolved
  6. 2021 High

    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

    PMID:34233195 PMID:34838521

    Open questions at the time
    • Structures capture defined states, not the full dynamic folding cycle
    • TTI2's individual contribution to client binding vs scaffolding not separately resolved
  7. 2022 Medium

    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

    PMID:35377872

    Open questions at the time
    • Which PIKK pathway mediates the metabolic and neurogenic phenotypes is unresolved
    • Single model, heterozygous only

Open questions

Synthesis pass · forward-looking unresolved questions
  • 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.
  • No per-client kinetic or structural dissection of TTI2's binding contribution
  • Mechanism coupling RUVBL ATPase inhibition to client release not defined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0044183 protein folding chaperone 3 GO:0140096 catalytic activity, acting on a protein 2 GO:0098772 molecular function regulator activity 1
Localization
GO:0005829 cytosol 2 GO:0005634 nucleus 1
Pathway
R-HSA-392499 Metabolism of proteins 3 R-HSA-8953897 Cellular responses to stimuli 2 R-HSA-73894 DNA Repair 1
Partners
PIH1D1RPAP3RUVBL1RUVBL2TELO2TTI1
Complex memberships
R2TP-HSP90 chaperone systemTTT complex (TTI2-TTI1-TELO2)

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2021 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. Cryo-EM structure determination combined with biochemical experiments (ATPase assays, binding assays) Cell reports High 34233195
2021 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. Cryo-EM structure determination Journal of molecular biology High 34838521
2013 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. Immunoblotting of patient-derived fibroblasts carrying homozygous TTI2 mutation Human mutation Medium 23956177
2016 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. Yeast genetics (conditional depletion), western blotting, stress sensitivity assays, synthetic lethality with Hsp90 overexpression G3 (Bethesda, Md.) Medium 27172216
2012 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. Genetic suppressor screen, reporter assays, western blotting, fluorescence microscopy Genetics Medium 22505622
2019 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. Large-scale genetic screen, co-immunoprecipitation to assess TTT complex integrity, phospho-signaling assays, telomere length measurement Molecular and cellular biology Medium 31332096
2019 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. Whole-exome sequencing, clinical characterization, literature synthesis Clinical genetics Low 31290144
2019 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. Whole-exome sequencing, immunoblotting of patient lymphocytes Frontiers in genetics Medium 31737043
2022 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. Targeted frameshift mutation in rat model, quantification of hippocampal neurogenesis, metabolic phenotyping PLoS genetics Medium 35377872
2017 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. Transposable element tagging, subcellular localization (fluorescence), yeast two-hybrid interaction assays Proceedings of the National Academy of Sciences of the United States of America Low 28461460

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2021 Structure of the TELO2-TTI1-TTI2 complex and its function in TOR recruitment to the R2TP chaperone. Cell reports 29 34233195
2017 Maize defective kernel mutant generated by insertion of a Ds element in a gene encoding a highly conserved TTI2 cochaperone. Proceedings of the National Academy of Sciences of the United States of America 23 28461460
2013 Mutation in TTI2 reveals a role for triple T complex in human brain development. Human mutation 23 23956177
2016 Saccharomyces cerevisiae Tti2 Regulates PIKK Proteins and Stress Response. G3 (Bethesda, Md.) 20 27172216
2012 Genetic evidence links the ASTRA protein chaperone component Tti2 to the SAGA transcription factor Tra1. Genetics 17 22505622
2019 Confirmation that variants in TTI2 are responsible for autosomal recessive intellectual disability. Clinical genetics 13 31290144
2021 Structure of the Human TELO2-TTI1-TTI2 Complex. Journal of molecular biology 11 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
2020 Whole-exome sequencing identifies homozygous mutation in TTI2 in a child with primary microcephaly: a case report. BMC neurology 9 32061250
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
2019 Novel Compound Heterozygous Mutations in TTI2 Cause Syndromic Intellectual Disability in a Chinese Family. Frontiers in genetics 6 31737043
2022 Systems genetics in the rat HXB/BXH family identifies Tti2 as a pleiotropic quantitative trait gene for adult hippocampal neurogenesis and serum glucose. PLoS genetics 5 35377872
2022 Novel Homozygous TTI2 Variant Causing Autosomal Recessive Syndromic Intellectual Disability and Primary Microcephaly from Pakistan: A Case Report (Exome Report). Case reports in genetics 0 35990009

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