ahk1 (SPAC7D4.03c; UniProt O14260) is a large (886-residue) fungal-specific protein of the UPF0592 family, defined by a single DUF1765 domain (Pfam PF08578; InterPro IPR013887; PANTHER PTHR37988). It is predicted to be a multi-pass membrane protein (three predicted transmembrane helices) and contains a disordered, low-complexity region near its N-terminus, but it carries no recognizable catalytic motif, consistent with a non-catalytic role. The DUF1765 domain has no experimentally assigned biochemical function. Its budding-yeast homolog, Saccharomyces cerevisiae Ahk1 (YDL073W), is an experimentally validated scaffold protein of the HKR1 sub-branch of the Hog1 osmostress-activated MAPK pathway that binds the Hkr1 cytoplasmic domain together with Sho1, Ste11 and Pbs2 and prevents aberrant cross-talk signalling to the Kss1 MAPK. On the basis of that orthology (shared UPF0592 domain family), the fission-yeast protein is proposed to be a putative MAP-kinase-cascade scaffold, though this role has not been tested in fission yeast. The gene is non-essential (ahk1 deletion is viable), and in genome-wide phenotypic screens its deletion is associated with altered stress responses (sensitivity or resistance to osmotic, cell-wall, and genotoxic agents) and abnormalities in mating and shmoo morphology, a fingerprint broadly compatible with a role in stress-responsive signalling. No direct molecular function, binding partner, localization, or specific pathway role has been demonstrated for the fission-yeast protein itself.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Low-resolution, prediction-based localization. UniProt predicts three transmembrane helices and annotates the protein as a multi-pass membrane protein by ECO:0000305 inference. This is consistent with the sequence but is not experimentally demonstrated in S. pombe, and the term is the generic root-level "membrane". Retain as non-core. Reason: Supported by UniProt transmembrane-helix predictions but unverified experimentally and uninformative as to which membrane; a plausible prediction-based localization rather than a demonstrated core function. Supporting Evidence: GO_REF:0000044 Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location |
| GO:0005575 cellular_component | ND GO_REF:0000015 | KEEP AS NON CORE | Summary: Root-level "cellular_component" with the ND (No biological Data) evidence code, a standard PomBase placeholder recording that no specific cellular component is known. It carries no biological content and should be retained only as a placeholder. Reason: Root placeholder term with ND evidence; represents absence of specific localization knowledge rather than a substantive annotation. |
| GO:0000165 MAPK cascade | ISO PMID:26787842 Scaffold Protein Ahk1, Which Associates with Hkr1, Sho1, Ste... | KEEP AS NON CORE | Summary: Process annotation transferred by orthology (ISO, with-from SGD:S000002231) from S. cerevisiae AHK1, which is an experimentally validated scaffold in the Hog1 osmostress MAPK pathway. The orthology rests on the shared UPF0592/DUF1765 domain family (InterPro:IPR013887). The assignment is a plausible, curator-made transfer, but the involvement of the S. pombe protein in a MAPK cascade has not been demonstrated directly; the fission-yeast stress core (Wis1 to Sty1/Spc1) does not reproduce the budding-yeast Hkr1/Kss1 cross-talk architecture. Retain as a reasonable orthology-based hypothesis, but mark non-core and flag as a knowledge gap. Reason: Orthology-based (ISO) transfer from a verified S. cerevisiae scaffold; biologically plausible and consistent with the deletion stress/mating phenotype fingerprint, but unproven in S. pombe. Per PomBase reliability, retained rather than removed; treated as non-core because it is inferred, not demonstrated for this gene. Supporting Evidence: PMID:26787842 Thus, Ahk1 is a scaffold protein in the HKR1 subbranch and prevents incorrect signal flow from Hkr1 to Kss1. |
| GO:0005078 MAP kinase scaffold activity | ISO GO_REF:0000024 | KEEP AS NON CORE | Summary: The single molecular-function annotation, transferred by orthology (ISO, GO_REF:0000024, with-from SGD:S000002231) from S. cerevisiae AHK1. It is more informative than "protein binding" and captures the best available functional hypothesis: that ahk1 acts as a physical scaffold assembling components of a MAPK signalling module. However, the scaffold activity has never been tested for the S. pombe protein and no fission-yeast binding partners are known. Retain as a plausible orthology-based hypothesis; mark non-core and flag as a molecular-function knowledge gap. Reason: Orthology-based (ISO) transfer of an experimentally established S. cerevisiae molecular function; retained per PomBase reliability and because it is the most informative available MF term (preferable to protein binding), but it is inferred rather than demonstrated for S. pombe and so is treated as non-core with an explicit knowledge gap. Supporting Evidence: PMID:26787842 we identified a protein, termed Ahk1 (Associated with Hkr1), that binds to Hkr1-cyto. |
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Download this section (compressed HTML)Q: Does S. pombe ahk1 physically associate with any component of the Sty1/Spc1 stress-activated MAPK pathway (Sty1, Wis1, Win1/Wis4) or with membrane osmosensors, and does its deletion alter stress-induced Sty1 activation?
Suggested experts: Nishimura A, Tatebayashi K
Q: What is the biochemical activity of the DUF1765 domain, and is ahk1 a bona fide scaffold or does it have an as-yet-uncharacterized molecular function?
Experiment: Construct an endogenously tagged ahk1 strain; perform affinity purification-mass spectrometry to identify interactors, live-cell imaging to determine localization, and quantitative immunoblotting of phospho-Sty1 in wild-type vs ahk1-deletion cells across a panel of stresses (osmotic, cell-wall, oxidative, genotoxic).
Hypothesis: ahk1 acts as a membrane-associated scaffold that contributes to Sty1/Spc1 activation under osmotic or cell-wall stress in S. pombe.
Type: interactome, localization, and signalling epistasis
Experiment: Determine the structure of the ahk1 DUF1765 domain (experimentally or via AlphaFold-guided modelling) and test candidate activities biochemically; use structure-guided point mutants to link domain features to the deletion stress/mating phenotypes.
Hypothesis: The DUF1765 domain confers a defined, conserved molecular activity that underlies ahk1 function.
Type: structural biology and structure-function analysis
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: Whether S. pombe ahk1 (SPAC7D4.03c) actually functions as a MAP-kinase scaffold, or has any scaffold activity at all, has never been tested. The molecular function is inferred entirely from the S. cerevisiae ortholog via the shared UPF0592/DUF1765 domain family; no in vivo or in vitro assay of scaffold activity, and no identification of assembled kinase-module components, has been reported for the fission-yeast protein.
OPEN BIOLOGY MF_DARK
What is known: Scaffold activity in the Hog1 osmostress MAPK pathway is experimentally established for S. cerevisiae Ahk1 (YDL073W = SGD:S000002231), which binds Hkr1-cyto, Sho1, Ste11 and Pbs2; the S. pombe protein shares the defining UPF0592/DUF1765 domain (InterPro:IPR013887) and lacks any catalytic motif, so a scaffolding role is plausible but unproven.
Significance: Determines whether the propagated MAP-kinase scaffold activity (GO:0005078) annotation is valid for fission yeast, or whether ahk1 has a distinct (or no) signalling function despite family membership.
What would resolve it: Affinity purification / mass spectrometry of tagged ahk1 in S. pombe to identify direct interactors, combined with epistasis and phospho-readout assays testing whether ahk1 contributes to Sty1/Spc1 (or other MAPK) activation under stress.
Provenance (the field's own admissions):
Gap: The direct binding partners of ahk1 in S. pombe are unknown. It is undetermined whether it interacts with the fission-yeast stress-MAPK components (Sty1/Spc1, the MAPKK Wis1, the MAPKKKs Win1/Wis4, or membrane osmosensors such as Wsc1/Mtl2 and the Sho1-like adaptor), or with any other proteins.
OPEN BIOLOGY MF_DARK
What is known: In S. cerevisiae the orthologous Ahk1 binds a defined set of HKR1-subbranch proteins (Hkr1-cyto, Sho1, Ste11, Pbs2); the fission-yeast interactome of ahk1 has not been mapped beyond generic high-throughput interaction datasets.
Significance: Identifying partners would confirm or refute pathway membership and reveal whether ahk1 couples a membrane sensor to a specific MAPK module in fission yeast.
What would resolve it: Co-immunoprecipitation / proximity-labelling of endogenously tagged ahk1 followed by mass spectrometry, with validation of candidate interactions by reciprocal pulldown and genetic interaction analysis.
Provenance (the field's own admissions):
Gap: Whether ahk1 has a specific, non-redundant role in the S. pombe Sty1/Spc1 stress-activated MAPK pathway is undetermined. The budding-yeast function (preventing Hkr1-to-Kss1 cross-talk) may not map onto fission yeast, whose stress core (Wis1 to Sty1) lacks the multi-branch Hkr1/Kss1 architecture, so it is unclear which, if any, signalling event ahk1 controls.
OPEN BIOLOGY BP_DARK
What is known: ahk1 deletion is viable and, in genome-wide screens, alters stress responses (osmotic/salt, cell-wall, and genotoxic agents) and mating/shmoo morphology; these are consistent with a stress-signalling contribution but do not localize ahk1 to a defined step or show it is non-redundant.
Significance: Placing ahk1 (or excluding it) within the Sty1/Spc1 pathway would resolve whether the orthology-based MAPK-cascade (GO:0000165) annotation reflects a real fission-yeast pathway role.
What would resolve it: Targeted epistasis analysis of ahk1 deletion with sty1, wis1 and osmosensor mutants under defined stresses, measuring Sty1 phosphorylation and downstream (e.g. atf1/gpd1) responses.
Provenance (the field's own admissions):
Gap: The molecular activity of the DUF1765 domain that defines the UPF0592 family, and hence the biochemical basis of any ahk1 function, is entirely unknown. DUF1765 is a domain of unknown function with no assigned catalytic or binding activity.
OPEN BIOLOGYONTOLOGY MF_DARK
What is known: ahk1 is a member of the UPF0592 family (Pfam PF08578 DUF1765; PANTHER PTHR37988) and lacks any recognizable enzymatic motif, but no structure-function study has assigned an activity to this domain in any organism.
Significance: Assigning a biochemical activity to DUF1765 would provide a mechanistic basis for the family, potentially replacing the generic scaffold-by-orthology inference with a defined GO molecular-function term.
What would resolve it: Structural characterization (e.g. AlphaFold-guided experimental structure) and biochemical assays of the DUF1765 domain; if a novel activity is found, a corresponding GO molecular function term may be required.
Provenance (the field's own admissions):
Gap: The subcellular localization of ahk1 in S. pombe has not been experimentally determined. The membrane / multi-pass membrane assignment is a sequence-based prediction (three predicted transmembrane helices) with no microscopy or fractionation evidence in fission yeast.
OPEN BIOLOGY CC_DARK
What is known: UniProt predicts three transmembrane helices (residues 277-297, 374-394, 400-420) and annotates the protein as a multi-pass membrane protein by ECO:0000305 inference, but this has not been confirmed for the endogenous fission-yeast protein.
Significance: Localization would test the membrane prediction and indicate whether ahk1 acts at a specific membrane compartment (as a membrane-tethered scaffold) or elsewhere.
What would resolve it: Live-cell imaging of endogenously tagged ahk1 and membrane-fractionation/topology assays in S. pombe.
Provenance (the field's own admissions):
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