acn1 (SPBC3H7.05c / O74380) — curation notes

Fission yeast (Schizosaccharomyces pombe 972 / ATCC 24843). UniProt entry name YNV5_SCHPO.
PomBase standard name acn1; systematic name SPBC3H7.05c; UniProt accession O74380.

Summary / bottom line

acn1 is an understudied ("conserved unknown") gene encoding a polytopic
integral membrane protein of the membrane-bound O-acyltransferase (MBOAT)
superfamily. PomBase characterisation status is conserved unknown and UniProt
PE level is 4: Predicted. There is no dedicated functional/biochemical study
of acn1; essentially all annotations are either sequence/domain-based inference or
high-throughput screen phenotypes. The confident statements one can make are about
its domain architecture and localization; its specific molecular substrate,
biological process, and physiological role remain unknown.

Identity and provenance

Domain / sequence analysis (from UniProt O74380 record, inline)

Existing GO annotations (from acn1-goa.tsv; 4 GOA lines)

  1. GO:0031966 mitochondrial membrane — IEA, GO_REF:0000044 (UniProtKB-SubCell
    mapping SL-0171). Derived from the UniProt subcellular-location statement
    "Mitochondrion membrane {ECO:0000305}; Multi-pass membrane protein {ECO:0000305}".
    ECO:0000305 = curator inference (not direct assay). The primary evidence for a
    mitochondrial localization is the ORFeome GFP screen PMID:16823372.
  2. GO:0005575 cellular_component — ND, GO_REF:0000015 (root "no data" placeholder).
  3. GO:0008150 biological_process — ND, GO_REF:0000015 (root "no data" placeholder).
  4. GO:0016747 acyltransferase activity, transferring groups other than amino-acyl
    groups     — ISS, GO_REF:0000024, WITH/FROM: PomBase:SPAC24H6.01c. Transferred by
    curator judgment of sequence similarity from gup1 (SPAC24H6.01c, Q09758), an
    MBOAT (product "membrane bound O-acyltransferase, MBOAT Gup1"; annotated to
    GO:0006506 GPI anchor biosynthetic process). NOTE: gup1 uses Pfam PF03062
    (MBOAT_fam) whereas acn1 uses PF13813 (Wax_synthase_dom) — related superfamily,
    different subfamily; the ISS supports family-level acyltransferase activity but
    NOT gup1's specific GPI-anchor function.

Also present in the UniProt DR block (not in GOA TSV): GO:0005739 mitochondrion
HDA:PomBase (from the ORFeome localization dataset), and the now-obsolete
GO:0008374 O-acyltransferase (superseded by GO:0016747). Verified GO:0008374 is
obsolete via QuickGO.

Localization evidence

Phenotype data (all high-throughput or overexpression; PomBase FYPO)

KNOWN vs NOT-known

KNOWN (well-supported):
- It is a polytopic integral membrane protein (9 TM helices; UniProt/DeepTMHMM).
- It belongs to the MBOAT superfamily, wax-synthase (PF13813/IPR032805) branch.
- It localizes to mitochondria / mitochondrial membrane (ORFeome GFP screen; single
moderate-confidence line of evidence).
- Deletion is viable with no gross morphology defect (FYPO:0002177), consistent
with a non-essential gene.
- It has no apparent S. cerevisiae ortholog but is conserved across fungi/eukaryotes.

NOT known (knowledge gaps):
- Molecular function / catalytic activity: whether acn1 is a catalytically active
acyltransferase, and if so its acyl-donor and acceptor substrates, are unknown. The
MF annotation is ISS-from-paralog at the general "acyltransferase, non-amino-acyl"
level only.
- Biological process: unknown. The "acn1 = ACetylation of Nsf" name is a hypothesis,
not established. No BP annotation with experimental support exists.
- Physiological role / pathway: unknown. Phenotypes are broad pleiotropic
screen/overexpression hits that do not converge on a mechanism.
- Localization detail: sub-mitochondrial location and topology unverified beyond a
single genome-scale GFP dataset; a mitochondrial-membrane MBOAT is atypical.
- Direct interactors / regulation: none experimentally established for function.

Candidate MF for core_functions

Falcon deep research (acn1-deep-research-falcon.md, Edison, 26 citations)

The falcon report (completed 2026-07-06, 1659 s) independently reached the same
conclusion and adds useful comparative context. Key points (all secondary/inferential,
about the MBOAT family, NOT direct acn1 data):
- No primary research article has characterized acn1/SPBC3H7.05c function, activity, or
role; all inference is domain/homology-based. (converges with my analysis.)
- MBOAT superfamily = integral-membrane acyltransferases with conserved catalytic His
(in a TM stretch) + upstream Asn; three functional subgroups (sterol/DAG acylation
e.g. ACAT/DGAT1; protein/peptide acylation e.g. PORCN/HHAT/GOAT; lyso-phospholipid
re-acylation e.g. LPCAT/MBOAT7). The wax-synthase (WS) branch acylates fatty
alcohols with fatty-acyl-CoA to make wax esters, and DGAT1-type members make TAG.
- Predicted acn1 activity: acyl-CoA-dependent acyltransferase, most consistent with
wax-ester-synthase and/or DGAT1-type chemistry — but substrate is unknown.
- S. pombe neutral-lipid context: TAG synthesis is by dga1 (DGAT2 fold, structurally
UNRELATED to MBOAT) and plh1 (PDAT); sterol esterification by Are1/Are2 homologs
(Zhang et al. 2003, JBC). acn1 (MBOAT/DGAT1-type) is therefore a distinct enzyme
class and, if catalytically active, would be an additional/accessory acyltransferase.
- Predicted localization by MBOAT analogy: endoplasmic reticulum (DGAT1/ACAT are
ER-resident). NOTE this CONFLICTS with the only experimental localization datum for
acn1 (mitochondrion, ORFeome GFP screen PMID:16823372). This discrepancy is itself a
knowledge gap — the family prior says ER, the single S. pombe screen says mitochondrion.
- CAVEAT on the report: it treats "Wax_synthase_dom (IPR032805)" and "MBOAT_2 (PF13813)"
as two domains; they are the SAME domain (InterPro IPR032805 is the InterPro entry for
Pfam PF13813). Do not double-count. The falcon DOI citations are family reviews, not
acn1-specific; I will NOT cite them as supporting acn1 function directly.

ND literature-search citations (2026-11-06)