clk-1 (COQ7 ortholog) — research notes

UniProt: P48376 (COQ7_CAEEL). WormBase: WBGene00000536 / ZC395.2. Gene symbol clk-1
("clock abnormal / clk"). Human ortholog: COQ7. 187 aa precursor with an N-terminal
mitochondrial transit peptide (1–8) cleaved to a mature 9–187 chain.

Summary of what is KNOWN (with provenance)

Direct molecular function: di-iron ubiquinone/CoQ biosynthetic monooxygenase

• clk-1/COQ7 catalyzes the penultimate step of ubiquinone (coenzyme Q) biosynthesis: the
  hydroxylation of 5-demethoxyubiquinone (DMQ) at the C6 (C5 in some numbering) position,
  producing 3-demethylubiquinone. EC 1.14.13.253; RHEA:81211. It uses a carboxylate-bridged
  di-iron center and NAD(P)H (UniProt P48376 CATALYTIC ACTIVITY / COFACTOR: "Binds 2 iron
  ions per subunit"; FT BINDING residues 30/60/63/112/148/151 to Fe cation).
• The enzyme belongs to the COQ7 family / ferritin-like di-iron superfamily
  (Pfam PF03232 COQ7; InterPro IPR011566; PANTHER PTHR11237 "COENZYME Q10 BIOSYNTHESIS
  PROTEIN 7"; HAMAP MF_01658/MF_03194).
• Founding paper established structural + functional conservation of clk-1 as a "timing gene"
  and did mutagenesis of the catalytic Glu148: PMID:9020081 (Science 1997). (Not in GOA
  annotation set but is the founding functional reference; cited in UniProt.)
• Biochemical proof that CLK-1 is required for UQ biosynthesis in the worm:
  PMID:11244089 Also: DMQ9 can substitute as an electron carrier —
  PMID:11244089.
• clk-1 encodes "a hydroxylase involved in the biosynthesis of the redox-active lipid
  ubiquinone (co-enzyme Q), and in clk-1 mutants, ubiquinone is replaced by its biosynthetic
  precursor demethoxyubiquinone" PMID:14517217.

Subcellular location: mitochondrion (inner membrane, matrix face)

• UniProt SUBCELLULAR LOCATION: "Mitochondrion inner membrane; Peripheral membrane protein;
  Matrix side" (so a genuine "extrinsic component of mitochondrial inner membrane",
  GO:0031314, not an integral membrane protein).
• CLK-1-GFP is fully active and localizes to mitochondria of all somatic cells:
  PMID:10202142
• Mitochondrial localization independently observed: PMID:25961505 (mitochondria and nucleus).

Downstream / mutant-phenotype (NON-CORE) biology

The mutant phenotypes are consequences of altered Q/DMQ, not the direct molecular activity.

• Longevity ("Clk" clock phenotype): clk-1 mutations extend lifespan and slow developmental
  and behavioral rates. PMID:10202142 Overexpression shortens
  lifespan and accelerates behavior PMID:10202142.
• Lifespan extension by RNAi of UQ-biosynthesis genes incl. clk-1: PMID:12709403
• Maternal rescue / developmental-timing: PMID:14517217.
• Genetic-interaction lifespan modulation with translation / DR pathways (clk-1 used as a
  long-lived ETC/"clk" mutant partner): PMID:17277769; PMID:19783783 (DR / NLP-7,
  clk-1 as ETC-mutant control; IGI/IMP determination of adult lifespan).
• Oxidative phosphorylation / anesthetic (xenobiotic) sensitivity: complex I–dependent OXPHOS
  correlates with volatile-anesthetic sensitivity, tested across ETC/CoQ mutants including
  clk-1: PMID:16920626
• CLK-1 protein level tracks mitochondrial biogenesis; used as a mito marker reduced on hsp-6
  knockdown: PMID:17189267.

DEBATED / secondary nuclear moonlighting role — treat cautiously

• A distinct nuclear pool of CLK-1/COQ7 was reported to regulate mitochondrial stress
  responses and longevity independently of UQ biosynthesis: PMID:25961505; the nuclear-only,
  MTS-deleted CLK-1 "localised predominantly to the nucleus" and "CLK-1nuc(+) could not" rescue
  UQ biosynthesis PMID:25961505, yet partially rescued ROS, UPRmt, and
  longevity phenotypes. Nuclear COQ7 associates with chromatin PMID:25961505 and modulates
  transcription of ROS/UPRmt genes (sod-2, skn-1, hsp-6, hsp-60, spg-7; human SOD2/NRF2/HMOX1,
  HSPA9/HSPD1/AFG3L2 etc.). This underlies the WormBase IMP annotations to
  negative/positive regulation of transcription by RNA Pol II, regulation of ROS metabolic
  process, and determination of adult lifespan on PMID:25961505.
• An older, isolated in-vitro report: CLK-1 binds the O_L region of mitochondrial DNA:
  PMID:11959146 This is the basis of the IDA
  "transcription cis-regulatory region binding" (GO:0000976) annotation. Note the binding is to
  mtDNA (a replication/transcription origin), a single in-vitro study, not widely replicated —
  a possible moonlighting activity, not the core enzymatic function.

Caveat on the nuclear model: the nuclear localization/role is a genuine experimental report
(Nat Cell Biol 2015) but remains debated in the field; CLK-1's N-terminus and human COQ7's NTS
are not conserved (PMID:25961505), and independent
confirmation of an endogenous, functionally significant nuclear pool in the worm is limited.

What is NOT known (knowledge gaps)

1. How DMQ-vs-Q levels produce the longevity signal. clk-1 mutants accumulate DMQ9 and lack
   UQ9 yet respire near-normally (DMQ9 substitutes as an electron carrier) PMID:11244089, and
   lifespan extension appears "independent of ubiquinone biosynthesis and ATP production"
   PMID:25961505. The mechanistic chain from the specific quinone species (DMQ vs Q, ROS
   output) to the pro-longevity signal is not established.
2. The disputed nuclear/non-mitochondrial role. Whether an endogenous nuclear CLK-1 pool of
   physiological significance exists in the worm, how it is targeted (worm N-terminus lacks the
   COQ7 NTS), whether it binds DNA sequence-specifically, and whether "transcription regulation"
   is a direct molecular function or an indirect consequence, are unresolved.
3. Ontology gap: CLK-1/COQ7 is described as a component of a multi-subunit COQ enzyme
   ("CoQ synthome") complex with a structural stabilizing role (UniProt, By similarity), but GO
   has no "coenzyme Q biosynthesis complex" cellular-component term to capture this membership.

Provenance note on deep research

Falcon deep research (just deep-research-falcon worm clk-1 --fallback perplexity-lite) took
~29 min on the congested shared Edison endpoint and the wrapper recipe ultimately reported a
timeout, but the Edison run did land a real report at the boundary:
clk-1-deep-research-falcon.md (provider: falcon, model: Edison Scientific Literature,
cached: false, 40 cited sources). It was verified as genuine and on-target (di-iron DMQ9->UQ9
hydroxylase, COQ metabolon/CoQ-synthome membership, debated nuclear moonlighting "requires
further confirmation", 2,4-DHB bypass showing phenotypes are attributable to UQ deficiency) and
is committed as corroborating context. The review itself is grounded in the UniProt record, the
GOA TSV, and the 11 cached primary publications (PMID_9020081, 10202142, 11244089, 11959146,
12709403, 14517217, 16920626, 17189267, 17277769, 19783783, 25961505); every supporting_text
is a verbatim PMID quote. The falcon file is cited only once (core_functions corroboration) with
a verbatim quote from the committed file. There are no UNDECIDED calls.

Annotation-review plan (core vs non-core)

• CORE (keep, ACCEPT): 3-demethoxyubiquinone 3-hydroxylase (NADH) activity (GO:0160224);
  ubiquinone biosynthetic process (GO:0006744); mitochondrion (GO:0005739); mitochondrial inner
  membrane (GO:0005743); extrinsic component of mitochondrial inner membrane (GO:0031314);
  general MF parents monooxygenase activity (GO:0004497) and oxidoreductase...NAD(P)H one-oxygen
  (GO:0016709) are correct but less-informative parents — keep, flag as non-core/general.
• NON-CORE (KEEP_AS_NON_CORE): determination of adult lifespan (all evidence); cellular
  respiration; oxidative phosphorylation; adult behavior; positive regulation of behavior;
  positive regulation of growth rate; positive regulation of developmental process; response to
  xenobiotic stimulus; regulation of ROS metabolic process; nucleus (all); neg/pos regulation
  of transcription by RNA Pol II; transcription cis-regulatory region binding (debated in-vitro
  mtDNA-binding moonlighting).
• No REMOVE actions: all experimental annotations are defensible; the debated nuclear/DNA
  functions are experimental (IDA/IMP) so are retained as non-core with caveats rather than
  removed.