TAP65 / p65 (Tetrahymena thermophila, UniProt W7X6T2) — curation notes

Automated deep research was unavailable for this gene. The configured providers all
failed (perplexity is not registered; falcon returns an authentication error; openai
returns invalid_api_key). What follows is a manual literature review assembled from
the cached publications in publications/, the UniProt flat file
(TAP65-uniprot.txt), and QuickGO term/annotation lookups. No file named
*-deep-research-*.md was created, per CLAUDE.md.

1. Identity

• UniProt W7X6T2 (LARP7_TETTS), 542 aa, gene TAP65 (TTHERM_000318539),
  "Telomerase-associated protein of 65 kDa", p65.
• Domain architecture (UniProt features): HTH La-type RNA-binding domain 112–239,
  RRM 247–335, xRRM 374–542. PANTHER PTHR22792:SF62 (LA-RELATED PROTEIN 7).
• Family: LARP7 (La-related protein 7). The Tetrahymena protein is the founding
  structural example of the xRRM domain, which was subsequently recognised in genuine
  La and LARP7 proteins PMID:22705372.

2. Core biology established in Tetrahymena

2.1 p65 is a stable, stoichiometric subunit of the telomerase holoenzyme

p65 was identified by affinity purification of endogenously assembled Tetrahymena
telomerase, and reciprocal immunodepletion showed it is not a loose associate:
PMID:15131081. RNase sensitivity mapping later showed p65 is the subunit that stays behind
when the RNA is destroyed — i.e. everything else in the holoenzyme hangs off TER
PMID:19941821.

Architecturally, p65 belongs to the catalytic core, a ternary TERT–TER–p65 complex:
PMID:22705372; PMID:23552895; PMID:19941821. The full holoenzyme is TERT + TER + six proteins
PMID:26472759.

2.2 p65 binds TER specifically — not RNA polymerase III transcripts generally

This is the key point distinguishing p65 from genuine La. Affinity-purified p65 recovers
essentially only TER PMID:15131081,
and knockdown depletes TER without touching other Pol III transcripts
PMID:15131081.

In vitro the affinity is high and the specificity is sequence-based:
PMID:15696174;
PMID:15696174; and a size-matched, structured control RNA does not compete
PMID:15696174. Four separable modules of the protein contribute
PMID:16507983, with La-RRM1 reading the 3′ tail PMID:26472759.

2.3 Three separable jobs: 3′-end protection, RNA folding, TERT loading

The clearest statement of the division of labour is in the structural paper, which lists
them explicitly: PMID:22705372,
PMID:22705372, and the
bending of stem-loop IV to present it to TERT.

• 3′-end protection / RNA stabilization. In vivo, p65 is what sets the steady-state TER
  level: PMID:15131081;
  PMID:15131081;
  PMID:22705372. TERT accumulation also falls, but as a secondary consequence of
  TER loss PMID:19941821.
• RNA folding chaperone. p65 binding remodels TER:
  PMID:22705372;
  PMID:17322903;
  PMID:20713447.
  Critically, this is chaperone-like in the strict sense — it rescues misfolding, not
  merely stabilises: PMID:20713447. It also persists in the assembled particle
  PMID:20713447.
• Hierarchical TERT loading. PMID:15696174; quantitatively
  PMID:15696174, by a defined mechanism
  PMID:15696174. The order of events is fixed
  PMID:16507983, and the xRRM alone suffices for this step
  PMID:16507983. The cryo-EM structure shows the same thing
  at the holoenzyme level PMID:26472759.

2.4 Physiological output: telomere length maintenance

TAP65 is essential PMID:15131081, and
knockdown gives the telomerase-loss phenotype
PMID:15131081.

3. Curation judgements and the traps checked for

3.1 Is catalytic activity transferred onto a non-catalytic subunit?

No. p65 carries no GO:0003721/GO:0003720 (telomerase / RNA-directed DNA polymerase)
annotation in GOA, which is correct: the active site is TERT's, and p65 does not alter
primer-extension chemistry — PMID:15696174. The
COMPLEX_ACTIVITY_TRANSFER failure mode that CLAUDE.md warns about has not occurred
here; no action is needed, and none was invented.

3.2 Is human LARP7 biology imported onto the ciliate protein?

No. There is no GO:0097322 (7SK snRNA binding), no GO:0032786/GO:0034244
(transcription-elongation regulation), no GO:0000494/snRNA processing and no Cajal-body
term in the GOA. The three IEA rows are family-generic nucleic-acid/RNA binding plus a
subcellular-location mapping, all of which are true of the Tetrahymena protein on its own
evidence. This Track A/B failure pattern did not materialise, and nothing was removed on
speculative grounds.

3.3 Generic binding terms

GO:0003676 nucleic acid binding (InterPro IPR035979) and GO:0003723 RNA binding
(InterPro IPR000504/IPR014886 + PANTHER PTN008582015) are both true but strictly less
specific than what is demonstrated. The specific descendant GO:0070034 telomerase RNA binding is already carried by the gene six times over by IDA, and QuickGO confirms
GO:0070034 has ancestors GO:0003723 and GO:0003676. Per CLAUDE.md these are
MODIFY with GO:0070034 as the replacement, not KEEP_AS_NON_CORE.

There are no GO:0005515 protein binding rows in this GOA, so that policy does not apply.

3.4 GO:0000781 chromosome, telomeric region

IEA from the UniProt subcellular-location mapping (SL-0276), which itself rests on
UniProt's ECO:0000305 inference, not on an observation. The holoenzyme demonstrably
elongates telomeres and p65 is a constitutive core subunit, so the claim is reasonable; but
every assayed p65 activity (3′-end protection, TER folding, TERT loading) belongs to RNP
biogenesis, upstream of telomere engagement, and no direct imaging of p65 on telomeric
chromatin in Tetrahymena has been published. Kept, marked non-core.

3.5 The one proposed addition: GO:0140691 RNA folding chaperone

Participation test. p65 is not merely required for TER to fold; it performs the
remodelling itself, by direct contact — the 105° bend is made by its own xRRM helix α3x
wedging the GA bulge open PMID:22705372, and misfolded
TER variants are rescued to activity by p65 PMID:20713447. It does the work. Passes.

Comparator test. QuickGO shows 36 experimental annotations to GO:0140691, including
WRAP53/TCAB1 (Q9BUR4, IDA, PMID:19179534 and PMID:29804836) and mouse Wrap53
(Q8VC51) — i.e. a telomerase-RNP biogenesis factor acting on telomerase RNA already
carries this exact term. The term is also carried by Hfq, Lsm2/3/4/8, IMP3/IMP4 and plant
cold-shock proteins. So this is not a systematic absence that would signal a convention
I have misread; the term is routinely applied to proteins in p65's role.

Non-redundancy. GO:0140691 is an MF whose ancestors are GO:0003674 and (via
part_of) the BP GO:0034337 RNA folding. It is not under GO:0003723 RNA binding, so
it is not entailed by the accepted GO:0070034, and it is not entailed by the BP
GO:1904868. The parent BP GO:0034337 is deliberately not proposed — it would be
redundant with GO:0140691.

3.6 Considered and deliberately NOT proposed

• GO:0000333 telomerase catalytic core complex. p65 is unambiguously in the
  TERT–TER–p65 catalytic core, and this CC is more informative than GO:0005697. But
  QuickGO confirms GO:0000333 is a part_of descendant of GO:0005697, which the gene
  already carries nine times by IDA. CLAUDE.md: "Reject a term that is an ancestor or
  descendant of another you are proposing, or of one the gene already carries." Raised as a
  suggested_questions item instead, since this is a curation/ontology-practice question
  for GO, not a wet-lab unknown.
• GO:1905323 telomerase holoenzyme complex assembly. p65's demonstrated assembly role
  is the catalytic core step (TER + TERT), which GO:1904868 already captures exactly.
  Recruitment of p50/Teb1/p75-p45-p19 is not attributed to p65.
• GO:0034337 RNA folding — redundant with the proposed GO:0140691 (see above).
• Anything about 7SK, P-TEFb or Pol II elongation — human LARP7 biology, not shown here.

4. Open questions carried into the review

• Can the 3′-end-protection and the stem-IV-bending functions be genetically separated?
  The 2005 paper explicitly flags this as unresolved
  PMID:15696174
• Does p65 remain bound at telomeres during elongation in vivo, or does it act only in
  biogenesis? No published Tetrahymena localization data settle this.
• Is there a separate Tetrahymena genuine La? The 2004 paper notes a second La-motif
  locus of unknown status PMID:15131081

5. Coordination note (TERT, O77448)

TERT is being reviewed in parallel. Shared claims that should read the same way in both
files: (a) GO:0005697 telomerase holoenzyme complex is the right complex term for both,
and GO:0000333 was declined on redundancy grounds here; (b) GO:1904868 telomerase catalytic core complex assembly is appropriate for p65 as the agent that drives it — if
TERT also carries it, that is fine, both subunits participate; (c) catalysis
(GO:0003721) belongs to TERT alone and must not appear on p65.

Comparator query behind the GO:0140691 NEW row (2026-10-22)

Recorded so the figures in that row's reason are auditable rather than asserted. QuickGO
annotation search, goId=GO:0140691, goUsage=exact, evidenceCode=ECO:0000269 with
evidenceCodeUsage=descendants (i.e. experimental evidence only):

https://www.ebi.ac.uk/QuickGO/services/annotation/search?goId=GO%3A0140691&goUsage=exact&evidenceCode=ECO%3A0000269&evidenceCodeUsage=descendants&limit=100

Re-run 2026-10-22: numberOfHits: 36 across 32 distinct gene products, including
UniProtKB:Q9BUR4 WRAP53 (IDA) and UniProtKB:Q8VC51 mouse Wrap53 (IDA). Those two are the
load-bearing comparators: telomerase-RNP biogenesis factors acting on telomerase RNA, the same
role p65 occupies, so the term is applied to proteins in p65's position and its absence here
reads as a gap rather than a convention.

The counts move as GO annotation advances; what matters for the row is the presence of the
WRAP53 pair, not the exact total.