AIGR Gene Hypothesis Deep Research — AKTIP (Q9H8T0)

Hypothesis slug: noncatalytic-k63-ubiquitination-and-lesion-bypass
Focus type: function_assignment
Target: Human AKTIP/FTS (UniProt Q9H8T0), Homo sapiens (NCBITaxon:9606)
GO terms adjudicated: GO:0070534 (protein K63-linked ubiquitination), GO:0006301 (DNA damage tolerance / postreplication repair / "lesion bypass")

Summary

The seed hypothesis proposes that human AKTIP/FTS participates noncatalytically in K63-linked ubiquitination (GO:0070534) and in DNA damage tolerance / lesion bypass (GO:0006301), and asks whether any defined complex establishes an AKTIP contribution to K63-chain synthesis despite loss of intrinsic E2 catalytic activity. After three iterations combining sequence and structural-family analysis, GO annotation-provenance checks, interaction-network mining, and primary-literature review, the answer is that neither GO term is supported as a direct, core function of AKTIP. Both are best explained as paralog-driven, phylogenetically inferred (IBA) over-annotation projected onto AKTIP from the genuinely K63/DDT-active UEV clade (UBE2V1/UEV1A and UBE2V2/MMS2), from which AKTIP is a divergent outgroup.

Four convergent lines of evidence drive this verdict. First, AKTIP is a catalytically dead UEV (ubiquitin E2 variant) — it adopts a UBC-like fold but lacks the E2 active-site cysteine required for any thioester/transfer chemistry (only two cysteines total in 292 aa, neither at the active-site position). Second, the current GO annotations for both hypothesis terms are IBA-only (ECO:0000318) — inferred from a family-wide phylogeny, not from any experiment on AKTIP — and there is an explicit manual NOT annotation denying ubiquitin-like protein transferase activity (GO:0019787, NOT|enables). Third, no primary study and no curated interaction (IntAct, 200 interactions) places AKTIP in a complex with UBE2N/UBC13, UBE2V1, UBE2V2, RAD18, or PCNA-ubiquitination machinery; its actual partners are the FHF cargo-adaptor complex and replication-fork proteins (RPA1, POLA2, PCNA). Fourth, at subfamily resolution AKTIP falls outside the tight UBE2V1/UBE2V2 K63-DDT clade (those two are ~90% identical to each other; AKTIP is only ~41–45% identical to either), and CDD assigns it a dedicated domain model (cd23814).

The better-supported interpretation is that AKTIP is a catalytically dead UEV acting as a telomere-replication scaffold (binding TRF1/TRF2/telomeric DNA/PCNA/RPA70, suppressing replication stress) and, in the cytoplasm, a cargo-adaptor scaffold in the FTS–Hook–FHIP (FHF) complex — not a K63-chain builder or a lesion-bypass factor. The hypothesis is refuted / over-annotated for both proposed terms; the curator's most defensible action is to remove or down-qualify GO:0070534 and GO:0006301 as non-core, retain the NOT-transferase call, and anchor the review on the scaffold functions.

Key Findings

Finding 1 — AKTIP is a catalytically dead UEV; no direct evidence it synthesizes K63 chains

AKTIP/Ft1 is homologous to the E2-variant ubiquitin-conjugating (UEV) enzymes, as established by Burla et al. 2015 (PMID: 26110528): "AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes." By definition, UEV proteins adopt the UBC fold but lack the catalytic cysteine of true E2 enzymes and therefore cannot form the ubiquitin–thioester intermediate needed for conjugation or chain elongation.

An in-run pairwise alignment of AKTIP (292 aa) against UBE2N, UBE2V1, UBE2V2, and UBE2D1 gave ~41–45% aligned identity, confirming UEV-family membership, and critically showed that the residue aligning to the UBE2N catalytic Cys87 is not a cysteine in AKTIP. AKTIP contains only two cysteines in the entire sequence, neither at the E2 active site. This is a mechanistic-impossibility argument for direct catalysis: without the active-site Cys, AKTIP cannot itself catalyze ubiquitin transfer or K63-chain building.

The proteins that actually synthesize K63 chains for DNA damage tolerance are the UEVs UBE2V1 (UEV1A) and UBE2V2 (MMS2), partnering with the catalytic E2 UBE2N/UBC13. These canonical UEVs are moreover functionally redundant for PCNA polyubiquitination: Zhang et al. 2008 (PMID: 18284681) report that "the additional knockdown of a UEV1A (MMS2 homolog) does not deplete PCNA polyubiquitination." A dedicated literature search returned no study connecting AKTIP to UBE2N or to K63 polyubiquitin chain assembly.

Finding 2 — AKTIP's DNA-related role is telomere/replication scaffolding and replication-stress suppression, not lesion bypass (GO:0006301)

Burla et al. 2015 (PMID: 26110528) characterized AKTIP as a shelterin-interacting telomeric factor. It binds telomeric DNA, TRF1 and TRF2, and physically interacts with the replication factors PCNA and RPA70. RNAi depletion in human primary fibroblasts produces telomere dysfunction-induced foci (TIFs), defective PCNA recruitment, and intra-S checkpoint activation: "AKTIP-depleted human primary fibroblasts showed that they are defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint."

These are replication-stress and telomere-replication phenotypes, not evidence of a defined lesion-bypass event. GO:0006301 is mechanistically defined by translesion synthesis (TLS) or template-switch bypass of a specific replication-blocking lesion. Xu et al. 2014 (PMID: 24674630) state that "error-free PRR utilizes noncanonical K63-linked polyubiquitinated PCNA to signal lesion bypass through template switch, a process thought to be dependent on Mms2-Ubc13." The classical DDT/lesion-bypass apparatus — RAD6–RAD18 (PCNA mono-ubiquitination) plus UBC13–MMS2–RAD5 (K63 poly-ubiquitination) — does not include AKTIP (PMID: 18284681; PMID: 18757916). No reviewed paper demonstrates AKTIP mediating TLS or template-switch bypass of a defined lesion. Suppressing replication stress by stabilizing fork progression at telomeres is a distinct, upstream role.

Finding 3 — Both hypothesis GO annotations are phylogenetic (IBA); a manual NOT annotation denies transferase activity

QuickGO records for Q9H8T0 (retrieved in-run) show that GO:0070534 (protein K63-linked ubiquitination) and GO:0006301 (DNA repair / postreplication repair) are both annotated involved_in with evidence code ECO:0000318 (IBA — inferred from biological ancestor / phylogeny), not from any experiment on AKTIP. The molecular-function term GO:0061631 (ubiquitin-conjugating enzyme activity) is likewise IBA-only.

Decisively, GO:0019787 (ubiquitin-like protein transferase activity) carries a NOT|enables qualifier (ECO:0000303) — an explicit curator assertion that AKTIP lacks transferase activity. InterPro/CDD classifies the domain specifically as cd23814, "ubiquitin E2 variant (UEV) domain of AKT-interacting protein and related proteins" (UBC-like fold, IPR000608) — a dedicated subfamily rather than the canonical K63-active UEV group.

IntAct (200 interactions, in-run) reinforces this: among 53 distinct partners are the FHF complex (HOOK1/2/3, FHIP1A/1B/2A/2B, DCTN1) and replication factors RPA1 and POLA2 — but no UBE2N/UBC13, UBE2V1, UBE2V2, RAD18, or PCNA-ubiquitination machinery. This is consistent with AKTIP being a replication-fork/telomere scaffold (its RPA70 partner appears as RPA1) and removes the empirical basis for the K63/UBE2N complex-participation route posited by the hypothesis.

Finding 4 — AKTIP is a distinct UEV subfamily (outgroup), not part of the UBE2V1/UBE2V2 K63-DDT clade

A pairwise identity matrix computed in-run (global alignment; crude aligner, so absolute values are approximate) shows that the two K63/DDT UEVs UBE2V1 (UEV1A) and UBE2V2 (MMS2) are ~90.3% identical to each other, forming a tight functional clade, whereas AKTIP is only ~41.5% identical to UBE2V1 and ~44.8% to UBE2V2 — comparable to its ~41% identity to the catalytic E2 UBE2N and ~45% to the generic E2 UBE2D1. AKTIP is therefore roughly equidistant from the K63-active UEVs and from generic E2s, positioning it as an outgroup rather than a member of the DDT clade.

This is precisely why the IBA projection is over-broad: the GO:0070534 / GO:0006301 annotations derive from a family-wide UBC phylogeny, but at subfamily resolution AKTIP is not within the UBE2V1/UBE2V2 clade that actually performs K63-linked DNA-damage tolerance. CDD's dedicated cd23814 model formalizes this distinction.

Mechanistic Model / Interpretation

The seed hypothesis conflates two things the data keep separate: (a) being a UEV-fold protein at replication forks, and (b) building K63 chains for lesion bypass. The canonical K63-DDT pathway and AKTIP's actual role can be laid out as follows.

 CANONICAL K63 / DNA-DAMAGE-TOLERANCE PATHWAY (does NOT include AKTIP)
 --------------------------------------------------------------------
   Replication-blocking lesion
    │
   RAD6–RAD18  ── monoUb ─────────►  PCNA-Ub (K164)  ──► TLS (Polη/Polζ)
    │
   UBC13(UBE2N) + MMS2(UBE2V2)/UEV1A(UBE2V1) + RAD5/HLTF/SHPRH
    │  builds K63-linked poly-Ub on PCNA
    ▼
   PCNA-K63-polyUb ──► error-free lesion bypass via TEMPLATE SWITCH   ← GO:0006301 / GO:0070534
                                                          defined mechanistic step

 AKTIP / FTS ACTUAL ROLE (scaffold; replication-stress suppression)
 --------------------------------------------------------------------
   Telomere / replication fork
    │  binds TRF1, TRF2, telomeric DNA
   AKTIP ── binds ──► PCNA, RPA70/RPA1, POLA2   (recruitment / fork support)
    │
   Loss of AKTIP ──► TIFs, defective PCNA recruitment, intra-S checkpoint,
     S-phase arrest  (REPLICATION STRESS — upstream of, and
     distinct from, a defined lesion-bypass step)

   Separate, experimentally supported role:
   AKTIP = FTS core of FTS–Hook–FHIP (FHF) complex ─► dynein cargo/AP-4/ATG9A transport (GO:0070695)

Key interpretive points:

• Catalysis is impossible; scaffolding is unsupported. The absence of the active-site cysteine rules out direct catalysis. The hypothesis's fallback — noncatalytic scaffold participation in a K63-building complex — is legitimate in principle, but the specific empirical hook it requires (a physical/functional link to UBE2N–UBE2V machinery or PCNA-polyubiquitination) is absent from both literature and curated interactomes. So this route is unsupported, not merely untested.
• Replication stress ≠ lesion bypass. AKTIP depletion causes intra-S checkpoint activation and defective PCNA recruitment — consistent with a fork-support/recruitment scaffold, but GO:0006301 requires evidence of a bypass event (TLS or template switch) across a defined lesion, which no study provides for AKTIP.
• The annotation is a phylogeny artifact. IBA propagation from the UBC/UEV family projected the K63-DDT function onto an outgroup with a dedicated CDD model and a manual NOT-transferase call.

Evidence Matrix

GO Curation Implications (leads — require curator verification)

• GO:0070534 (protein K63-linked ubiquitination, BP): Remove or down-qualify (non-core). IBA-only (ECO:0000318), no experimental backing, conflicts with the manual NOT|enables GO:0019787 call. Homology to UBE2V1/UBE2V2 is insufficient; treat as paralog-driven.
• GO:0061631 (ubiquitin-conjugating enzyme activity, MF): Remove/NOT. IBA-only and inconsistent with the catalytically-dead UEV fold and the NOT-transferase annotation.
• GO:0006301 (DNA damage tolerance / postreplication repair, BP): Remove as lesion-bypass — IBA-only. Evidence supports a more general telomeric-replication role. Better-supported alternatives (curator to map exact IDs/evidence codes from PMID 26110528): telomere maintenance / regulation of telomere maintenance / DNA replication (IMP/IPI).
• GO:0019787 (ubiquitin-like protein transferase activity, MF) — NOT|enables: Retain the NOT. Consistent with all evidence.
• Experimentally supported core functions to elevate: GO:0070695 (FHF complex) and associated dynein/transport BP terms (PMID: 32073997); shelterin/chromosome telomeric region (CC) and TRF1 binding / PCNA binding / telomeric DNA binding (MF, IPI) (PMID: 26110528). Prefer these specific terms over a bare "protein binding".

GO Decision Table

Paralog-distance provenance (this run): Pairwise % identity (crude global aligner — absolute values approximate, contrast robust): UBE2V1 vs UBE2V2 = 90.3% (tight K63-DDT clade) vs AKTIP = 41.5% / 44.8% to those two — no closer than AKTIP's ~41% to catalytic UBE2N. AKTIP is an outgroup UEV subfamily (CDD cd23814), so the family-wide UBC IBA tree over-projects UBE2V1/UBE2V2-specific K63/DDT functions onto it.

Mechanistic Scope

The immediate molecular function tested is whether AKTIP directly enables or noncatalytically contributes to K63-linked ubiquitin chain assembly and to lesion bypass. Direct, supported activities: (1) protein/DNA scaffolding at telomeres — binds telomeric dsDNA, shelterin (TRF1/TRF2), and replication factors PCNA/RPA70, facilitating PCNA loading/telomere replication; (2) cargo-adaptor scaffolding in the cytoplasmic FHF complex (dynein/AP-4/ATG9A trafficking). Downstream phenotypes (not the direct activity): S-phase/intra-S checkpoint arrest, telomere fragility/TIFs, senescence, altered AKT survival signaling. K63-chain synthesis and lesion bypass are inferred from domain identity/phenotype, never from a direct AKTIP activity.

Conflicts and Alternatives

• Paralog confusion (primary risk): The K63/DDT UEV functions belong to UBE2V1/UBE2V2 + UBE2N, not AKTIP. AKTIP is a divergent UEV branch (~41–45% aligned identity) that has never been shown to bind UBE2N.
• Alternative core function: AKTIP/FTS is the scaffolding core of the FHF complex (dynein cargo adaptor) — a well-supported, mechanistically distinct role arguing against a primary Ub-enzyme identity.
• Replication-stress ≠ lesion bypass: The intra-S arrest/PCNA phenotype reflects impaired (telomere) replication and checkpoint signaling, which can arise without any postreplication-repair/lesion-bypass activity.
• No contradicting positive evidence found: No primary paper reported AKTIP–UBE2N interaction, AKTIP-dependent K63 chain formation, or AKTIP-dependent TLS/template switch. The absence is consistent across literature and IntAct.

Limitations and Knowledge Gaps

1. Approximate sequence identities. The pairwise identity matrix used a crude in-run global aligner; absolute percentages are approximate. The qualitative outgroup conclusion is corroborated by the independent CDD subfamily assignment, but a formal phylogeny (MAFFT/MUSCLE + IQ-TREE) would harden it.
2. Does AKTIP physically associate with UBE2N/UBC13 or K63 machinery? Checked: PubMed and IntAct — none found. This is the crux of GO:0070534. Absence of interactome coverage cannot fully exclude a transient/condition-specific scaffold role. Resolve with co-IP / proximity-labeling and in-vitro K63 chain-assembly assays (AKTIP ± UBE2N vs UBE2V2 control).
3. Is AKTIP required for PCNA K63-polyubiquitination or lesion-induced DDT? Checked: no assays exist. Matters for GO:0006301. Resolve with PCNA-Ub Westerns after UV/MMS in AKTIP-KO cells and TLS/template-switch reporter assays.
4. Isoform / PTM considerations. Whether any AKTIP isoform gains a distinct ubiquitin-pathway role was not exhaustively assessed; current evidence gives no such indication.

Discriminating Tests

• UBE2N interaction + in-vitro K63 assay: purified AKTIP + UBE2N + E1/Ub — does it build K63 chains? (UBE2V2+UBE2N as positive control). Distinguishes true UEV-partner function from fold-only homology.
• AKTIP-KO PCNA-ubiquitination assay after UV/MMS ± aphidicolin: tests GO:0006301 lesion bypass directly vs. replication-stress-only.
• Domain-swap / active-site rescue: does the AKTIP UEV domain complement mms2Δ in yeast DDT? A negative result cements paralog non-equivalence.
• Proximity interactome (BioID/TurboID) of AKTIP to score telomere/shelterin + FHF partners vs. DDT/UBE2N machinery, ideally under replication stress.
• Rigorous UEV-family phylogeny (MAFFT + IQ-TREE with bootstrap) to formally place AKTIP as an outgroup to UBE2V1/UBE2V2.

Proposed Follow-up Actions / Curation Leads (require curator verification)

• Candidate reference to cite: PMID: 26110528 — verify snippets: "AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes" and "defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint."
• Candidate action changes: (a) Remove/reject or mark non-core any GO:0070534 K63-ubiquitination annotation for AKTIP; (b) Do not add GO:0006301 as lesion bypass; (c) Add/retain telomere-maintenance (BP), telomeric-region/shelterin (CC), and TRF1/PCNA/DNA-binding (MF, IPI) terms from PMID 26110528; (d) retain FHF/dynein cargo-adaptor terms from PMID: 32073997; (e) retain the GO:0019787 NOT|enables call.
• Suggested curator questions: Are the K63/DDT annotations for AKTIP all IEA/IBA (paralog-derived)? If so they likely stem from UEV-domain projection from UBE2V1/UBE2V2 and should be down-weighted.
• Suggested experiments: the discriminating tests above (UBE2N binding + in-vitro K63 assay; AKTIP-KO PCNA-Ub after genotoxin).

Evidence Base (Key Literature)

• AKTIP/Ft1, a New Shelterin-Interacting Factor Required for Telomere Maintenance. PMID: 26110528 — AKTIP as a UEV-homolog telomere/replication scaffold (TRF1/TRF2/PCNA/RPA70); source of the replication-stress phenotypes. Primary support for the scaffold interpretation; refutes lesion-bypass.
• hMMS2 serves a redundant role in human PCNA polyubiquitination. PMID: 18284681 — Identifies UEV1A/MMS2 as the redundant K63-PCNA UEVs. Names the true actors; excludes AKTIP.
• The Rad5 helicase activity is dispensable for error-free DNA post-replication repair. PMID: 24674630 — Defines K63-polyUb-PCNA template-switch bypass via Mms2-Ubc13. Mechanistic definition of GO:0006301.
• Requirement of Rad5 for DNA polymerase zeta-dependent translesion synthesis. PMID: 18757916 — Rad5 + Mms2-Ubc13 K63-PCNA polyubiquitination in DDT. Confirms machinery without AKTIP.
• Rice UBC13 … required for K63-linked polyubiquitination and tolerance to DNA damage. PMID: 27234244 — OsUbc13 partners Mms2/Uev1A, K63 catalysis in vitro. UBE2V, not AKTIP, is the conserved K63 actor.
• The FTS-Hook-FHIP (FHF) complex interacts with AP-4 … PMID: 32073997 — FTS/AKTIP as core FHF scaffold in trafficking. Alternative experimentally grounded core function.
• Database provenance (in-run): QuickGO Q9H8T0 (IBA annotations + NOT|enables GO:0019787), InterPro/CDD cd23814, IntAct (200 interactions).

Conclusion

The hypothesis that human AKTIP/FTS directly participates in K63-linked ubiquitination (GO:0070534) and DNA damage tolerance / lesion bypass (GO:0006301) is refuted as a direct function assignment and best classified as paralog-driven over-annotation. AKTIP is a catalytically dead UEV outgroup with a manual NOT-transferase call, IBA-only support for both disputed terms, and no interaction or primary-literature link to K63-chain machinery. Its genuine, experimentally supported roles are as a telomere/replication-fork scaffold and a core subunit of the FHF trafficking complex. Both GO:0070534 and GO:0006301 should be removed or down-qualified as non-core, the NOT-transferase annotation retained, and the review anchored on AKTIP's scaffold functions.

Provenance: Sequence retrieval + pairwise alignment executed this run (UniProt Q9H8T0/P61088/Q13404/Q15819): AKTIP has 2 Cys, none aligned to the UBE2N catalytic Cys87 column; ~41–45% aligned identity to UBE2N/UBE2V1/UBE2V2 — consistent with a catalytically inactive UEV. GO/InterPro/IntAct records retrieved via public APIs; literature via PubMed. Where a resource could not be queried it is stated; no results were fabricated.

Artifacts

• OpenScientist final report
• OpenScientist final report