TPP1 (gene: ACD) — telomerase-recruiter subunit of shelterin

Disambiguation. “TPP1” is used for two completely unrelated proteins:

This page — TPP1 (gene: ACD), the shelterin subunit that recruits telomerase. Relevant to aging and telomere attrition.
Tripeptidyl peptidase 1 (gene: CLN2 / TPP1) — a lysosomal exopeptidase whose loss causes neuronal ceroid lipofuscinosis (Batten disease). Unrelated to telomere biology.

All content below concerns the shelterin subunit. For the lysosomal peptidase, see cln2-tpp1 (stub).

TPP1 is the shelterin subunit that bridges pot1 (single-strand telomere binder) to TIN2 and positions telomerase at chromosome ends. Its defining functional feature is the TEL patch — a cluster of seven solvent-exposed charged and hydrophobic residues on the OB-fold surface (E168, E169, E171, R180, L183, L212, E215) that contacts the TEN domain of tert to recruit telomerase ¹. TPP1 also boosts telomerase processivity: POT1-TPP1 together increase the number of repeats added per binding cycle approximately 2–4 fold in biochemical assays, with the mechanism involving both reduced primer dissociation and enhanced translocation ² ³. Germline mutations in ACD cause dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome. Loss-of-function of the TEL patch (separation-of-function point mutations) eliminates telomerase recruitment and telomere elongation in vivo while preserving telomere capping ¹.

Identity

UniProt: Q96AP0 (ACD_HUMAN; Swiss-Prot manually curated entry)
Gene symbol: ACD (adrenocortical dysplasia homolog)
NCBI Gene ID: 65057
HGNC: 25070
Ensembl: ENSG00000102977
Sequence length: 458 amino acids (canonical isoform)
Synonyms (gene-product level): PTOP, TINT1, PIP1 — historically used names before ACD became standard; see aliases above
GenAge entry: not found (ACD not curated in GenAge-human as of 2026-05-07) needs-canonical-id
Mouse ortholog: Acd (one-to-one; TEL patch residues conserved across mammals; mouse acd hypomorphic alleles cause DC-like phenotypes — see Keegan 2005, Tejera 2010, Jones 2014 — but these are distinct alleles, not TEL-patch point mutations; no K170Δ knock-in mouse published as of 2026)

Domain architecture

Domain	Residues (approx.)	Function
N-terminal OB fold	~87–250	Telomerase recruitment (TEL patch) + processivity enhancement; structurally homologous to ciliate TEBP-beta ²
POT1-binding domain (PBD)	~158–251	Directly contacts pot1 OB fold; required for POT1 telomere localization (UniProt Q96AP0; ²)
Disordered linker	~265–368, 387–406	Low-complexity spacer regions; limited structural characterization (UniProt Q96AP0)
TIN2-binding domain	~337–544 (C-terminal)	Tethers TPP1-POT1 sub-complex to TIN2, integrating into full shelterin assembly; residue P491 resides in this domain ⁴

Note: the first 87 N-terminal residues are dispensable for core functions in human cells per ² (Wang 2007 used TPP1(90–544) as the functional construct).

The OB fold carries two distinct surfaces: the TEL patch (telomerase-facing) and the POT1-binding face. Separation-of-function mutations isolating each surface have been experimentally characterized (Nandakumar 2012 ¹).

TEL patch — mechanism of telomerase recruitment

The TEL patch (Telomerase Enhancing Loop patch) is a cluster of charged and hydrophobic residues on the outer face of the OB fold. Seven critical residues were identified by scanning mutagenesis: E168, E169, E171 (acidic cluster), R180, L183 (surface-exposed), and L212, E215 (hydrophobic and acidic). Their mutation to alanine produces separation-of-function alleles: full telomere capping (POT1 interaction intact) but complete loss of telomerase recruitment and telomere elongation in vivo ¹. Notably, double mutants E169A/E171A and L212A showed the strongest defects in processivity stimulation (similar to no TPP1 added); single mutants at adjacent residues (e.g., S106A, S111A, R175V) were tolerated ¹.

The complementary interface on the telomerase side is the TEN domain of tert. Schmidt et al. 2014 identified the key TERT TEN-domain residues as K78 and R132 (basic patches). Substituting these with glutamate (K78E, R132E) disrupted POT1-TPP1-stimulated processivity (RAP stimulation reduced to 68% and 24% of wild-type respectively) without substantially reducing intrinsic telomerase activity. A compensatory charge-swap in the TEL patch — E215K in TPP1 — rescued K78E hTERT RAP stimulation from ~25% back to ~98% of wild-type, establishing the interaction is direct and electrostatically driven between TERT K78 and TPP1 E215 ⁵.

Processivity effect: The POT1-TPP1 heterodimer increases telomerase repeat-addition processivity (RAP) approximately 2–4 fold in biochemical assays (Wang 2007: ~4-fold for primer a5, R1/2 from 0.78 to 3.3 repeats; Latrick 2010: 2.7 ± 0.6 fold). The mechanism involves two steps: (1) POT1-TPP1 bound to the primer decreases the primer dissociation rate from actively synthesizing telomerase, and (2) POT1-TPP1 increases the translocation efficiency (fraction of enzyme completing the RNA-template translocation step) ³. These two contributions are independent and additive. The processivity contribution is distinct from telomerase recruitment and is mechanistically similar to a DNA polymerase clamp, but operates transiently rather than as a stable sliding clamp ² ³. dose-response-unclear — in-cell quantification of telomere extension per cycle is indirect; the in-vitro fold-change depends on primer sequence and assay conditions.

Evolutionary context

The TPP1 OB fold is structurally homologous to the beta subunit of ciliate telomere end-binding protein (TEBP-beta, e.g., Euplotes p43). This is not sequence-level conservation but structural: the OB-fold topology and the surface that contacts the equivalent TERT interface are conserved ⁶. The TEL patch therefore represents a conserved functional surface across distantly related eukaryotes.

Role in shelterin assembly

Within shelterin, TPP1 occupies a connector position:

TRF2 ── TIN2 ── TPP1 ── POT1
  |                       |
TRF1                  3' G-overhang

TIN2 binds TRF1 and TRF2 (dsDNA-binding subunits) and simultaneously binds TPP1’s TIN2-binding region, bridging the dsDNA-binding core to the ssDNA-reading POT1 ⁷. POT1’s binding is regulated by TPP1: PTOP/TPP1 promotes POT1 localization to telomeres and modulates its ssDNA affinity.

For the full structural context of shelterin assembly and end-protection, see shelterin (verified).

Disease: dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome

Germline mutations in ACD (biallelic or dominant-negative) cause:

Dyskeratosis congenita (DC) — characterized by oral leukoplakia, nail dystrophy, abnormal skin pigmentation, and bone marrow failure; driven by accelerated telomere shortening in stem cell compartments. ACD mutations cause autosomal dominant DC (DKCA6) and autosomal recessive DC (DKCB7).
Hoyeraal-Hreidarsson syndrome — a severe DC variant adding cerebellar hypoplasia, immunodeficiency, and intrauterine growth retardation; caused by mutations directly in the TEL patch surface. Kocak et al. 2014 identified a patient with compound heterozygous ACD mutations (one deleting a TEL patch residue) showing extremely short telomeres and severe phenotype ⁴.

Human K170Δ — cell-based validation only. Kocak 2014 identifies the K170Δ allele in a human HH proband (NCI-275-1) as a compound heterozygote with P491T (TIN2-binding domain). Functional experiments were performed in HeLa cells transiently transfected with K170Δ constructs — not a knock-in mouse model. These showed that K170Δ: (1) fails to recruit telomerase (only 3% of Flag-TPP1-K170Δ foci colocalize with TR, vs 54% for wild-type, p=0.02); (2) reduces telomerase processivity (disappearance of high-molecular-weight extension products); and (3) modestly reduces TIN2 interaction. The paper’s Discussion cites independent mouse acd hypomorphic allele studies (Keegan 2005; Tejera 2010; Jones 2014 — complete Tpp1 deletion in HSCs) to contextualize findings, but those are not K170Δ knock-ins. There is no K170Δ knock-in mouse in Kocak 2014 ⁴. needs-replication — whether K170Δ or equivalent TEL-patch loss-of-function recapitulates DC/HH in a mouse knock-in model has not been tested as of 2026.

Dimension	Status	Notes
Pathway conserved in humans?	yes	TEL patch residues conserved across mammals; DC/HH are human genetic diseases caused by ACD mutations ⁴
Phenotype conserved in humans?	yes	Human ACD mutations (K170Δ, P491T and others) recapitulate telomere attrition syndrome in multiple independent pedigrees
Replicated in humans?	yes	Kocak 2014 (HH/DC); at least one other independent ACD DC pedigree (Savage/Alter lab cohort); cell-based validation across independent labs

Aging relevance

TPP1 is relevant to aging through two mechanisms:

Telomerase gating. In normal somatic stem cells with low-level telomerase activity, TPP1-mediated recruitment is the rate-limiting step for telomere maintenance. Age-associated reductions in TERT expression would be predicted to sensitize cells to TEL-patch-dependent recruitment efficiency — though direct evidence for age-dependent TPP1 expression or activity changes in human tissues is lacking. needs-human-replication
End-protection. As a POT1 partner, TPP1 contributes to ATR-DDR suppression at the 3’ overhang. At critically shortened telomeres in aged cells, partial shelterin destabilization (including TPP1-POT1 sub-complex) contributes to the DDR signal that drives cellular-senescence and the sasp. The shelterin page documents this mechanism; TPP1’s contribution is the ssDNA-binding regulation via POT1.

needs-human-replication — Direct evidence that TPP1 expression or function changes with age in human stem cell compartments (HSCs, intestinal crypts) is not established.

Pathway membership

telomerase-pathway — TPP1 is the telomerase-recruiting component; the central molecular function of this page
cellular-senescence — downstream consequence of TPP1/shelterin dysfunction at shortened telomeres
telomere-attrition — the driving hallmark

Interactors

pot1 — direct heterodimer partner; TPP1 regulates POT1 localization and ssDNA affinity
tert — direct contact via TEL patch (OB fold) + TERT TEN domain; the recruitment interface
tin2 — TPP1 C-terminal region binds TIN2, anchoring TPP1-POT1 into the shelterin scaffold
shelterin — the full six-subunit complex of which TPP1 is a subunit (complex page)

Druggability

Druggability tier: 3 (predicted druggable surface; no clinical drug). The TEL patch is a well-characterized protein-protein interaction (PPI) surface (TPP1-TERT). PPI surfaces of this geometry are challenging but not undruggable: OB-fold surfaces have been targeted in other contexts, and the defined electrostatic character of the interface — anchored by TPP1 E215 making direct contact with TERT K78 ⁵ — provides a precise target for structure-guided small-molecule development. No clinical-stage inhibitor or activator of the TPP1-TERT interaction exists. Aging-context tier is 3 — the TPP1-TEL patch is a research-stage druggable surface without validated clinical compounds for any indication.

Potential therapeutic angles (speculative; uncited):

Telomerase activators via TEL patch stabilization — mimicking TPP1 recruitment to enhance telomere maintenance in disease (DC/HH, HSC exhaustion). no-mechanism for whether small-molecule approaches are feasible.
Telomerase inhibition via competitive blocking of the TEL patch — oncology context; would accelerate telomere shortening in cancer cells. Opposite of the aging-intervention goal.

Limitations and knowledge gaps

#gap/needs-canonical-id — GenAge entry absent for ACD (confirmed absent via direct lookup 2026-05-07); if added in future HAGR release, populate genage-id.
#gap/needs-human-replication — No published data on age-dependent changes in TPP1 expression or TEL-patch function in human aged tissues.
#gap/needs-replication — No K170Δ knock-in mouse exists as of 2026; in-vivo proof that a single TEL-patch point mutation is sufficient to cause DC/HH in mice has not been published. Mouse acd allele studies (Keegan 2005, Tejera 2010, Jones 2014) use hypomorphic or complete-loss alleles, not TEL-patch point mutations.
#gap/dose-response-unclear — processivity enhancement by TPP1 is characterized in reconstituted biochemical systems; in-cell quantification of telomere extension per binding cycle is indirect. Fold-enhancement varies 2–4× across assay conditions and primers.
#gap/unsourced — GTEx aging correlation (gtex-aging-correlation) not populated; requires sops/finding-tissue-expression.md workflow.
#gap/unsourced — Mendelian randomization evidence (mr-causal-evidence) not tested; germline ACD variants in GWAS are rare (DC is a rare disease), limiting instrument availability.

Footnotes

doi:10.1038/nature11648 · Nandakumar J et al. (Cech lab) · Nature 2012 · n=not applicable (biochemical + cell-based) · in-vitro + stable human cell lines · model: HeLa-EM2-11ht stable cell lines; 14 single-AA and 2 double-AA mutants of TPP1-OB screened; 7 critical TEL-patch residues identified (E168, E169, E171, R180, L183, L212, E215) — all cluster on OB-fold surface; E169A/E171A and L212A mutants lose processivity stimulation and telomere elongation in vivo (p<0.005) while retaining POT1-DNA binding · PDF verified (OA via PMC3521872) ↩ ↩² ↩³ ↩⁴ ↩⁵
doi:10.1038/nature05454 · Wang F et al. (Lei / Cech labs) · Nature 2007 · n=not applicable (biochemical) · in-vitro + reconstituted · model: recombinant human POT1-TPP1(90-544) + immunopurified telomerase · POT1-TPP1 increases telomerase repeat-addition processivity ~4-fold for primer a5 (R1/2: 0.78 → 3.3 repeats) and ~3-fold in total activity; processivity effect requires the TPP1 OB fold and the POT1-TPP1 interaction; also reports TPP1-OB crystal structure (PDB: 2I46) showing structural homology to Oxyrtricha TEBP-β · PDF locally available ↩ ↩² ↩³ ↩⁴ ↩⁵
doi:10.1038/emboj.2009.409 · Latrick CM, Cech TR · EMBO Journal 2010 · n=not applicable (biochemical) · in-vitro · model: super-telomerase extract (HEK293T overexpressing TERT+TR) + purified recombinant POT1-TPP1 · POT1-TPP1 increases RAP 2.7 ± 0.6 fold; mechanism: (1) decreases primer dissociation rate from actively synthesizing telomerase (koff reduced at least 4-fold at 16°C or 30°C); (2) increases translocation efficiency (57% → 99%); one POT1-TPP1 binding site on the primer is necessary and sufficient; effect is specific (non-specific ssDNA-binding protein gp32 cannot substitute) · PDF verified (OA via PMC2837173) ↩ ↩² ↩³
doi:10.1101/gad.248567.114 · Kocak H et al. (Savage / Nandakumar labs) · Genes & Development 2014 · n=1 proband (human genetic, 1 family) · human genetic + cell-based · model: human HH proband NCI-275-1 with compound heterozygous ACD mutations K170Δ (TEL patch, paternally inherited) + P491T (TIN2-binding domain, maternally inherited); functional validation by IF/FISH and direct telomerase assay in transfected HeLa cells — no mouse knock-in model in this study · PDF verified (diamond OA via PMC4180972) ↩ ↩² ↩³ ↩⁴
doi:10.7554/eLife.03563 · Schmidt JC, Dalby AB, Cech TR · eLife 2014 · n=not applicable (biochemical + cell-based) · in-vitro + stable human cell lines · model: HeLa and HEK293T cells; TERT TEN-domain K78E and R132E mutants lose POT1-TPP1-stimulated RAP without losing catalytic activity (K78E: 68% RAP; R132E: 24% RAP relative to WT); compensatory E215K mutation in TPP1 TEL patch rescues K78E hTERT RAP stimulation to 98% of WT (n=3, mean±SD; p<0.01) — establishes direct electrostatic interaction between TERT K78 and TPP1 E215 · PDF verified (gold OA via PMC4359370) ↩ ↩²
doi:10.1038/nature05469 · Xin H et al. (Songyang lab) · Nature 2007 · n=not applicable (biochemical) · in-vitro + cell lines · model: 293T cells + recombinant proteins; TAP-TERT pull-down; TRAP assays · TPP1 OB fold identified as structural homolog of TEBP-β (Oxyrtricha nova); TPP1 OB fold required for telomerase co-IP (full-length TPP1 brought down ~20% of telomerase activity); TPP1 OB fold + POT1 pull down HA-TERT; TPP1 RD domain (residues 87–240) required for telomerase interaction; TIN2 bridges TPP1-POT1 to TRF1/TRF2 double-strand-binding core · PDF locally available ↩
doi:10.1038/ncb1142 · Liu D et al. (Songyang lab) · Nature Cell Biology 2004 · n=not applicable (biochemical) · in-vitro · model: human cell lines + in-vitro binding · PTOP (TPP1 original name) discovery: interacts with POT1, regulates POT1 telomere localization; POT1 telomere length regulation depends on PTOP · not_oa locally (closed access) ↩

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