pos5 (SPAC323.01c, UniProt Q9UT98) — review notes

2026-11-01 — initial review, centred on Nishihara et al. 2026 (PMID:41926382)

Identity and sequence-level evidence

• UniProt Q9UT98, "NADH kinase pos5, mitochondrial", 386 aa, PE 3 (inferred from
  homology). Curated function is by similarity to S. cerevisiae Pos5
  [file:SCHPO/pos5/pos5-uniprot.txt "Phosphorylates both NADH and NAD(+), with a preference for NADH."].
• Family: NAD kinase family (Pfam PF01513 NAD_kinase, PF20143 NAD_kinase_C; PANTHER
  PTHR20275:SF26 "NADH KINASE POS5, MITOCHONDRIAL"; HAMAP MF_00361)
  [file:SCHPO/pos5/pos5-uniprot.txt "Belongs to the NAD kinase family."].
• Predicted transit peptide 1..42 (ECO:0000255). The paper instead aligns the S. pombe
  N-terminus to the 62-residue ScPos5 transit peptide and places the corresponding
  boundary at residue 83 PMID:41926382.
• 37% identity with ScPos5, conserved NAD kinase motifs (GGDG ATP-binding motif)
  PMID:41926382.

S. cerevisiae Pos5 (ortholog; background, not S. pombe evidence)

• Outten & Culotta 2003 (PMID:12727869, abstract only): recombinant ScPos5 has NADH
  kinase activity and is in the matrix
  PMID:12727869
  PMID:12727869;
  required for oxidative stress resistance, arginine biosynthesis and mitochondrial iron
  homeostasis PMID:12727869.
• These ScPos5 data are the PAINT seed (SGD:S000006109) of the three IBA rows.

What Nishihara et al. 2026 showed — S. pombe results

All results below are in S. pombe unless marked [Sc].

1. CoQ10 deficiency of Δpos5. Found in a Bioneer deletion-library screen; a
   freshly constructed Δpos5 (RYP7) has 0.2-fold of WT CoQ10
   PMID:41926382.
2. Not a secondary effect of respiratory failure (Δcyc1 control)
   PMID:41926382.
   Authors still cannot exclude an indirect Fe-S route
   PMID:41926382.
3. Phenotypes of CoQ-deficient S. pombe: no growth on glycerol/ethanol, H2O2
   sensitivity, slow growth on minimal medium
   PMID:41926382;
   high sulfide (brown colonies on CuSO4)
   PMID:41926382;
   round cell morphology PMID:41926382.
4. Arginine auxotrophy partially rescued by arginine (S1 Fig), attributed to NADPH
   use by Arg11 PMID:41926382.
5. ScPOS5 complements Δpos5 CoQ (chromosomally integrated, P41nmt1)
   PMID:41926382.
6. Mitochondrial NADP(H) is reduced in mitochondria-enriched fractions of Δpos5
   PMID:41926382.
   Caveat: modest reduction; crude mitochondrial fraction; no enzyme assay of SpPos5.
7. Localisation. Endogenous C-terminal GFP fusion was mislocalised and
   non-functional PMID:41926382;
   plasmid-expressed Pos5-GFP co-localised with MitoTracker
   PMID:41926382.
   No sub-mitochondrial fractionation.
8. Mitochondrial NAD kinase activity is what matters: N-terminally truncated Pos5
   (ΔMTS83) and cytosolic ScUtr1 fail, while ScUtr1 fused to the SpCoq3 MTS rescues
   PMID:41926382.
   This is the strongest (indirect) evidence that the S. pombe protein acts as an
   NAD(H) kinase in mitochondria: the defect is rescued by an unrelated NAD kinase
   only if it is targeted to mitochondria.
9. Coq6 is not the limiting step: coq6 overexpression does not help
   PMID:41926382;
   VA tends to help (not significant)
   PMID:41926382;
   PHB partially restores
   PMID:41926382.
   atd1 overexpression: slight, non-significant increase (S7 Fig).
10. Authors' conclusion: the NADPH-dependent step is upstream, in quinone precursor
    synthesis, and remains unidentified
    PMID:41926382.

[Sc] results in the same paper: ΔScpos5 CoQ6 is 0.2-fold of WT
PMID:41926382;
ΔScpos5 respiratory deficiency, H2O2 sensitivity and arginine auxotrophy confirmed (Fig 4A).

Not done in the paper: no in vitro kinase assay of SpPos5, no sub-mitochondrial
localisation, no CoQ intermediate profiling.

Supplementary material not accessible: S1-S7 Figs are only available as captions in
the cached text (images not available); S1 Table (primers) and S2 Table (plasmids) are
XLSX files that were not retrieved. None of these is needed for the annotation
decisions, but the S3/S5 localisation images could not be inspected.

Decision on "ubiquinone biosynthetic process" (GO:0006744)

Question: does Pos5 participate in CoQ biosynthesis, or is it necessary for it?

• Who performs the step? No step of CoQ biosynthesis is assigned to Pos5. Pos5
  makes mitochondrial NADP(H); the step(s) that consume it are unknown (Coq6 excluded
  as the limiting step; precursor synthesis suspected but unidentified; atd1
  overexpression did not significantly rescue). The CoQ reduction (to 20%, not zero)
  fits a supplier of a cofactor pool used by many processes.
• Comparator check (QuickGO, GO:0006744 incl. descendants, taxa 284812, 559292,
  9606, run 2026-11-01): the annotated gene products are the Coq enzymes, prenyl
  diphosphate synthases, Coq8/Coq9/Coq11/Coq12 accessory factors, and the
  ferredoxin/ferredoxin-reductase electron donors to Coq6 (S. pombe arh1, etp1;
  S. cerevisiae ARH1, YAH1; human FDX2, FDXR). The ferredoxin system transfers electrons
  directly to the Coq6 reaction — step-level participation. No NADPH-generating enzyme
  carries the term: not ScPOS5 (annotated to NADH kinase activity, matrix, NADP+
  biosynthesis, Fe-S cluster assembly IMP, oxidative stress), not UTR1, ZWF1, human
  NADK/NADK2, G6PD, nor S. pombe zwf1. Note ScPOS5 has an SGD IMP to Fe-S cluster
  assembly (PMID:20889970), so SGD does sometimes annotate a cofactor-supplier IMP;
  but no SGD row exists for CoQ even though this 2026 paper also covers ScPOS5.
• Conclusion: do not add GO:0006744. Pos5's relation to CoQ synthesis is a necessity
  (cofactor supply) relationship whose consuming step is unknown; it is recorded as a
  knowledge gap and a suggested question, and the phenotype is better captured as a
  FYPO phenotype (decreased cellular ubiquinone level) than as a GO process.

Other BP considerations

• NADP+ biosynthetic process: Pos5 catalyses the NAD(H) -> NADP(H) phosphorylation
  that is the definitional step; S. pombe mitochondrial NADP+ drops in Δpos5. The
  IEA row is accepted with PMID:41926382 as organism-specific support (an IMP row
  would be appropriate for PomBase).
• Cellular response to oxidative stress (IBA): supported by H2O2 sensitivity in both
  yeasts, but this is a downstream consequence of NADPH supply (and of CoQ loss);
  kept as non-core.
• Arginine biosynthesis, respiration, Fe-S assembly: necessity phenotypes via NADPH,
  not added.

Annotation decisions (summary)

No NEW rows. The S. pombe IDA (plasmid Pos5-GFP, mitochondrion) and IMP (mitochondrial
NADP+ decrease, NADP+ biosynthetic process) results from PMID:41926382 would justify
experimental rows, but both terms are already in GOA (validation rejects NEW for a term
already present), so they are cited in the reasons of the existing rows and flagged here
for PomBase curation.