acn1 (systematic name SPBC3H7.05c) is an uncharacterized, non-essential polytopic integral membrane protein of the fission yeast Schizosaccharomyces pombe. Its sequence carries a single Wax_synthase_dom / MBOAT_2 domain (InterPro IPR032805, Pfam PF13813), placing it in the wax-synthase branch of the membrane-bound O-acyltransferase (MBOAT) superfamily; the protein is predicted to span the membrane with nine transmembrane helices. Characterized MBOAT enzymes are acyl-CoA-dependent acyltransferases that transfer acyl chains onto lipid, sterol, fatty-alcohol, or protein acceptors, and by homology acn1 is inferred to be an acyltransferase, most plausibly acting in neutral-lipid (e.g. wax-ester or diacylglycerol) metabolism. However, no direct biochemical, genetic, or cell-biological study of acn1 has been published: its catalytic activity, physiological substrate, and biological role are all unknown. A genome-scale GFP screen assigned it a mitochondrial membrane localization, which is atypical for an MBOAT enzyme (most localize to the endoplasmic reticulum). It has no apparent Saccharomyces cerevisiae ortholog but is conserved across fungi and more broadly in eukaryotes.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0031966 mitochondrial membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic annotation mapped from the UniProt Subcellular Location statement "Mitochondrion membrane {ECO:0000305}; Multi-pass membrane protein {ECO:0000305}" (SL-0171). The underlying primary evidence is a genome-wide GFP localization screen (PMID:16823372), and PomBase independently records a mitochondrion localization (HDA). This is a reasonable location call for a multi-pass membrane protein and is the only localization datum available, so it is retained; it is marked non-core because a single genome-scale screen is moderate-confidence and a mitochondrial localization is atypical for an MBOAT-family acyltransferase (family members are usually ER-resident), leaving the true compartment and topology unresolved. Reason: Supported location call, but derived from a single high-throughput screen (via UniProt ECO:0000305 inference) and biologically atypical for the family; keep as a location without treating it as a core, mechanism-defining assertion. Supporting Evidence: UniProt:O74380 SUBCELLULAR LOCATION: Mitochondrion membrane |
| GO:0005575 cellular_component | ND GO_REF:0000015 | ACCEPT | Summary: Root-level "no data" placeholder (ND, GO_REF:0000015) recording that no specific cellular-component annotation has been curated. This is a bookkeeping annotation, not a biological statement; it is superseded in practice by the mitochondrial membrane location above. No action needed beyond noting it. Reason: Standard ND root placeholder; correctly indicates absence of curated CC data and should be left as-is per GO_REF:0000015 conventions. |
| GO:0008150 biological_process | ND GO_REF:0000015 | ACCEPT | Summary: Root-level "no data" placeholder (ND, GO_REF:0000015) recording that no biological process has been curated for acn1. This is accurate: the biological process in which acn1 acts is genuinely unknown (see knowledge_gaps). It is a bookkeeping annotation and should remain until an experimentally supported process is established. Reason: Standard ND root placeholder that correctly reflects the absence of any curated, evidence-supported biological process for this dark gene. |
| GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Sequence-similarity (ISS) annotation transferred by a PomBase curator from the paralog gup1 (SPAC24H6.01c; WITH/FROM PomBase:SPAC24H6.01c), an MBOAT annotated to the same term. acn1 carries the Wax_synthase_dom / MBOAT_2 domain (IPR032805 / PF13813) and the MBOAT superfamily is defined by acyl-CoA-dependent acyltransferase activity, so a general acyltransferase MF is domain-appropriate and is retained. It is kept as non-core because (i) the transfer is from a different MBOAT subfamily (gup1 uses Pfam PF03062, MBOAT_fam, and acts in GPI-anchor biosynthesis, whereas acn1 uses the wax-synthase-domain PF13813), so it supports family-level activity but NOT gup1's specific substrate/process; and (ii) no assay has demonstrated that acn1 is catalytically active or identified its acyl donor/acceptor. This is the most specific defensible molecular-function term; anything narrower (wax-ester synthase, DGAT, GPI-anchor acyltransferase) would be over-annotation. Note the current term is GO:0016747 because the older O-acyltransferase term (GO:0008374) used in the UniProt DR line is now obsolete. Reason: Domain/family-appropriate general acyltransferase activity supported by ISS from an MBOAT paralog, but unproven for acn1 and transferred from a different MBOAT subfamily; retain as a reasonable family-level MF without treating it as an established, substrate-defined core function. Supporting Evidence: UniProt:O74380 Pfam; PF13813; MBOAT_2; 1. UniProt:O74380 InterPro; IPR032805; Wax_synthase_dom. |
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Download this section (compressed HTML)Q: Is acn1 a catalytically active acyltransferase, and what acyl-CoA donor and lipid/fatty-alcohol/sterol acceptor does it use?
Q: Does acn1 localize to the endoplasmic reticulum (as expected for an MBOAT) or genuinely to the mitochondrial membrane as suggested by the ORFeome GFP screen?
Q: Does acn1 contribute to neutral-lipid (wax-ester or triacylglycerol/sterol-ester) metabolism in S. pombe, i.e. does an acn1 deletion or overexpression change the lipid droplet / storage-lipid profile?
Experiment: Express recombinant acn1 (in S. pombe or a heterologous host) and assay acyltransferase activity in vitro against a panel of acyl-CoA donors and candidate acceptors (diacylglycerol, fatty alcohols, sterols, lysophospholipids), reading out product formation by mass spectrometry.
Hypothesis: acn1 is an acyl-CoA-dependent acyltransferase of the wax-synthase/DGAT1-type MBOAT branch acting in neutral-lipid biosynthesis.
Type: in vitro enzyme assay
Experiment: Compare untargeted lipidomes of an acn1-deletion strain versus wild type (and versus dga1/plh1 mutants) under standard and lipid-stress conditions to detect changes in triacylglycerol, wax esters, sterol esters, or phospholipid species.
Hypothesis: Loss of acn1 alters a specific neutral-lipid or membrane-lipid pool, revealing its in vivo product.
Type: comparative lipidomics
Experiment: Determine acn1 subcellular localization and topology using endogenous fluorescent tagging with co-localization against ER and mitochondrial markers, and protease-protection / topology mapping of the transmembrane helices.
Hypothesis: acn1 resides in the ER membrane (consistent with the MBOAT family) rather than the mitochondrion.
Type: fluorescence microscopy and membrane topology
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Whether acn1 is a catalytically active acyltransferase, and if so what reaction it catalyzes, is undetermined. No enzymatic assay has been performed on the acn1 protein; the acyltransferase molecular-function annotation is inferred entirely from the MBOAT/wax-synthase domain and from ISS transfer from the paralog gup1.
OPEN BIOLOGY MF_DARK
What is known: acn1 unambiguously carries a Wax_synthase_dom / MBOAT_2 domain (InterPro IPR032805, Pfam PF13813) and is predicted to be a nine-transmembrane integral membrane protein, and the MBOAT superfamily is defined by acyl-CoA-dependent acyltransferase activity with a conserved catalytic histidine. The acn1 sequence retains candidate His/Asn residues, so catalytic competence is plausible. What is NOT established is any measured activity or the enzyme's substrate specificity.
Significance: Determines whether acn1 is a bona fide enzyme (and validates the propagated GO acyltransferase annotation) or a catalytically inactive family member, and defines which branch of lipid/protein acylation it belongs to.
What would resolve it: Heterologous expression of acn1 and in vitro acyltransferase assays with candidate acyl-CoA donors and lipid/fatty-alcohol/sterol acceptors, plus lipidomic comparison of an acn1-deletion versus wild-type S. pombe strain.
Provenance (the field's own admissions):
Gap: The physiological substrate(s) of acn1 (its acyl donor and acyl acceptor) are unknown. It is undetermined whether acn1 acts on diacylglycerol (DGAT-type), a fatty alcohol (wax-ester synthase-type), a sterol, a lysophospholipid, or a protein/peptide acceptor.
OPEN BIOLOGY MF_DARK
What is known: The MBOAT superfamily splits into subgroups by substrate — sterol/diacylglycerol acylation (ACAT/DGAT1), protein/peptide acylation (PORCN, HHAT, GOAT), and lysophospholipid re-acylation (LPCAT, MBOAT7) — and the wax-synthase branch acylates fatty alcohols to make wax esters. In S. pombe, terminal triacylglycerol synthesis is carried out by dga1 (a DGAT2-fold enzyme unrelated to MBOATs) and plh1 (PDAT), so acn1 would be a distinct enzyme class rather than a redundant copy of those. Which subgroup acn1 belongs to has not been tested.
Significance: Substrate identity would define acn1's biochemical reaction and place it in a specific lipid- or protein-acylation pathway, and would let a specific GO molecular-function term replace the current general one.
What would resolve it: Substrate-resolved in vitro assays (panel of acyl-CoA donors versus DAG, fatty alcohols, sterols, lysophospholipids) and untargeted lipidomics of acn1-deletion cells.
Provenance (the field's own admissions):
Gap: The biological process and physiological role of acn1 are unknown. No pathway, developmental role, or cellular process has been established; the only curated biological-process annotation is a root-level "no data" placeholder.
OPEN BIOLOGY BP_DARK
What is known: acn1 deletion is viable with normal cell morphology, and available phenotypes come almost entirely from genome-wide high-throughput screens (broad drug/stress resistance and sensitivity, minor nitrogen/carbon-source growth changes; PMID:37787768) plus low-throughput overexpression phenotypes (septation defects; PMID:23695302). In a lipotoxicity screen, acn1 deletion did not alter growth on pentadecanoic acid (PMID:40424131). These pleiotropic screen hits do not converge on a mechanism. The PomBase gene name (acn1, from a proposed "ACetylation of Nsf" hypothesis) is not backed by any published functional study.
Significance: Assigning acn1 to a biological process (e.g. neutral-lipid storage, membrane lipid remodeling, or an unrelated pathway) is required to move it out of the conserved-unknown set and to interpret its many screen phenotypes.
What would resolve it: Targeted loss-of-function and epistasis analysis under lipid-metabolic and stress conditions, combined with the substrate/activity assays above, to connect a defined biochemical activity to a cellular process.
Provenance (the field's own admissions):
Gap: acn1's subcellular compartment is unresolved. The family prior (MBOAT enzymes are typically ER-resident) conflicts with the single experimental datum for acn1, a genome-wide GFP screen that reported mitochondrial localization.
OPEN BIOLOGY CC_DARK
What is known: UniProt records "Mitochondrion membrane" as a multi-pass membrane protein (ECO:0000305 curator inference from the ORFeome GFP dataset, PMID:16823372) and PomBase records a mitochondrion HDA localization; no ER localization has been reported for acn1 specifically. Characterized MBOAT/DGAT1/ACAT enzymes are ER-membrane proteins, creating a genuine tension between the location prior and the observed screen result.
Significance: Resolving whether acn1 is truly mitochondrial (which would be unusual for an MBOAT and biologically informative) or ER-localized (matching the family) constrains its likely substrates and pathway.
What would resolve it: Endogenous-tagged, high-resolution co-localization microscopy with mitochondrial and ER markers, and/or subcellular fractionation, in S. pombe.
Provenance (the field's own admissions):
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