SPAPB1E7.04c (UniProt Q9C105; folder symbol cts2) is a secreted glycosyl hydrolase family 18 (GH18) chitinase-like precursor, belonging to the chitinase class III / Cts1-like subfamily. Comparative genomic analyses indicate it is the SINGLE GH18 chitinase-family gene encoded in the S. pombe genome. The protein carries an N-terminal signal peptide, an N-terminal GH18 catalytic domain (residues ~26-325) followed by a very long heavily O-glycosylated/disordered Ser/Thr-rich serine-rich stalk, and is routed through the secretory pathway. Critically, UniProt notes it LACKS the conserved catalytic Glu residue at position 166 essential for GH18 chitinase activity, so its enzymatic (chitinase/hydrolase) function is uncertain. Falcon deep research found no primary study mapping the symbol cts2 to this ORF or characterizing its activity, substrate, or localization experimentally; functional inference is therefore bounded by GH18 biochemistry and S. pombe cell-wall context. Notably, vegetative S. pombe cell walls are reported to lack chitin (chitin is restricted to the spore/conidial wall), and cell separation in fission yeast is driven by glucanases (Eng1, Agn1) rather than a chitinase, so a primary role in vegetative cytokinesis/wall remodeling is not supported. The best-supported statements are localization to the extracellular region / fungal-type cell wall and broad carbohydrate (GlcNAc/chito-oligomer) association, with catalytic chitinase activity unlikely given the missing catalytic glutamate.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IBA GO_REF:0000033 | ACCEPT | Summary: Extracellular region localization is accurate. The protein is secreted according to UniProt and has a signal peptide for extracellular targeting. Falcon deep research independently supports a secretory pathway / cell-surface / extracellular working localization for this GH18 precursor. Reason: This annotation is well-supported by the presence of a signal peptide and secreted nature documented in UniProt. The protein functions outside the cell, consistent with chitinase-like proteins. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the most plausible working localization is the **secretory pathway and cell surface/extracellular space** |
| GO:0004568 chitinase activity | IBA GO_REF:0000033 | REMOVE | Summary: This phylogenetically-propagated chitinase activity annotation is not supported. While the protein belongs to the GH18 glycosyl hydrolase family, UniProt specifically notes it lacks the conserved Glu residue at position 166 that is the essential catalytic acid/base of the GH18 mechanism. Falcon deep research confirms that GH18 catalysis depends on a conserved catalytic glutamate, and that no experimental enzymology exists for this protein; it also cautions against over-annotating a cytokinetic/cell-separation chitinase role in S. pombe, where chitin is largely absent from vegetative walls and cell separation is glucanase-driven. Reason: UniProt explicitly states the enzyme activity is unsure due to the lack of the essential catalytic glutamate residue (the GH18 catalytic acid/base), and OpenScientist independently confirmed the E-to-N loss at position 166 through catalytic-site comparison with active GH18 homologs. The fetched GOA line shows that GO:0004568 is an IBA propagation through PANTHER:PTN005237305 with active fungal chitinase source proteins; this propagation should not be retained for the catalytically deficient S. pombe cts2 lineage without direct enzymatic evidence. Propagation Review Root cause: PROPAGATION BAD Failure modes: PSEUDO OR SUBACTIVITY LOSS Sources checked: PANTHER:PTN005237305 Β· PAINT GH18 chitinase source node SUPPORTS SOURCE BUT NOT TARGET The fetched GOA line propagates GO:0004568 through this node together with active fungal chitinase source proteins; Q9C105 lacks the catalytic Glu166 needed to support transfer of catalytic chitinase activity. SGD:S000004276 Β· Saccharomyces cerevisiae CTS1 SUPPORTS SOURCE BUT NOT TARGET CTS1 supports true chitinase activity in budding yeast, but S. pombe cts2 has lost the corresponding catalytic proton-donor glutamate. Supporting Evidence: file:SCHPO/cts2/cts2-goa.tsv UniProtKB Q9C105 SPAPB1E7.04c enables GO:0004568 chitinase activity molecular_function ECO:0000318 IBA GO_REF:0000033 CGD:CAL0000194074|PANTHER:PTN005237305|SGD:S000004276|UniProtKB:Q4WB15|UniProtKB:Q4WEM4|UniProtKB:Q4WEP7|UniProtKB:Q4WGI4|UniProtKB:Q4WWU6 284812 Schizosaccharomyces pombe (strain 972 / ATCC 24843) GO_Central Chitinase-like protein PB1E7.04c 20240606 file:SCHPO/cts2/cts2-deep-research-falcon.md GH18 chitinases use a neighboring-group participation mechanism; a conserved catalytic glutamate within a DxxDxDxE-type motif functions as general acid/base file:SCHPO/cts2/cts2-hypotheses/function-hypothesis-go-0004568/openscientist.md The catalytic proton-donor glutamate residue essential for GH18 chitinase activity is replaced by asparagine (EβN at position 166) in cts2 file:SCHPO/cts2/cts2-hypotheses/function-hypothesis-go-0004568/openscientist.md The GO:0004568 (chitinase activity) annotation for Q9C105 should be **removed** or annotated with a **NOT qualifier**, pending curator verification. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Duplicate extracellular region annotation with different evidence code. The localization is accurate and provides additional computational support, consistent with the signal peptide and falcon's inference of a secreted/cell-surface working localization. Reason: This annotation is correct and provides additional computational evidence for extracellular localization, complementing the phylogenetic and experimental evidence. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the most plausible working localization is the **secretory pathway and cell surface/extracellular space** |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This broad carbohydrate metabolic process annotation (from InterPro/GH18 domain mapping) is plausible at the family level but the specific activity is uncertain because the protein lacks the essential GH18 catalytic glutamate. Falcon bounds the most-likely reaction class as hydrolysis of beta-1,4 GlcNAc linkages (chitin/chito-oligomers) for an intact GH18 enzyme, but emphasizes no direct enzymology exists for this protein and that vegetative S. pombe walls lack chitin. Retained as non-core given residual uncertainty. Reason: The protein may retain some carbohydrate binding or peripheral carbohydrate-metabolic association despite the questionable chitinase activity. This very broad grouping term is plausible from domain membership but is not a definitively supported core function. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md Most likely reaction class:** hydrolysis of Ξ²-1,4 linkages in **chitin or chitin-like (GlcNAc) polymers/oligomers**, consistent with GH18 family biochemistry |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | REMOVE | Summary: General hydrolase activity (from a UniProt keyword) is questionable given the lack of the essential catalytic residue. Without the conserved glutamate that anchors the GH18 acid/base mechanism described in the falcon deep research, hydrolytic activity is uncertain. Reason: UniProt states enzyme activity is unsure due to the missing catalytic residue. General hydrolase activity should not be annotated without evidence of actual enzymatic function. (Note GO_REF:0000043 SPKW annotations are themselves being retired by GOA.) Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md GH18 chitinases use a neighboring-group participation mechanism; a conserved catalytic glutamate within a DxxDxDxE-type motif functions as general acid/base |
| GO:0016798 hydrolase activity, acting on glycosyl bonds | IEA GO_REF:0000043 | REMOVE | Summary: This more specific glycosyl-bond hydrolase activity is likewise questionable without the essential catalytic residue. Glycosyl bond hydrolysis via the GH18 mechanism requires the conserved catalytic glutamate that this protein lacks. Reason: Even more specifically than general hydrolase activity, glycosyl bond hydrolysis requires the catalytic machinery that UniProt indicates is defective in this protein. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md GH18 chitinases use a neighboring-group participation mechanism; a conserved catalytic glutamate within a DxxDxDxE-type motif functions as general acid/base |
| GO:0005576 extracellular region | IDA PMID:39660919 Efn1 and Efn2 are extracellular 5'-nucleotidases induced dur... | ACCEPT | Summary: PomBase-assigned IDA for extracellular localization. The extracellular localization itself is consistent with the signal peptide, UniProt's Secreted designation, and the IBA/IEA extracellular annotations, so the annotation is accepted. However, the originally recorded supporting_text was the title of PMID:39660919 (an Efn1/Efn2 phosphate-starvation 5'-nucleotidase study); that paper's text does not mention SPAPB1E7.04c / Q9C105 / chitinase, so the verbatim title is not direct supporting evidence for this protein. Support is therefore anchored on the falcon deep research inference of a secreted/cell-surface localization plus the UniProt signal peptide, pending a precise primary citation. Reason: Extracellular/secreted localization is well supported by the signal peptide and UniProt Secreted annotation; this IDA is consistent with that localization even though the cited paper's narrative does not characterize the protein directly. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the most plausible working localization is the **secretory pathway and cell surface/extracellular space** |
| GO:0000324 fungal-type vacuole | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | REMOVE | Summary: High-throughput (HDA) vacuolar localization from a genome-wide ORFeome localization screen. This conflicts with the strong evidence for a secreted, extracellular protein (signal peptide, UniProt Secreted, and the extracellular IDA/IBA/IEA annotations), and falcon likewise infers a secretory-pathway/cell-surface working localization with no microscopy or localization assay specifically validating a vacuolar pool. The cited PMID:16823372 is a global dataset paper whose narrative text does not describe this ORF individually, so it provides no direct supporting statement. Most consistent interpretation: false-positive / transit signal from the high-throughput screen. Reason: This high-throughput annotation contradicts the strong, convergent evidence for secreted/extracellular localization (signal peptide, UniProt Secreted, multiple extracellular GO annotations). Likely an HTP false positive. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the most plausible working localization is the **secretory pathway and cell surface/extracellular space** |
| GO:0006032 chitin catabolic process | IC GO_REF:0000111 | REMOVE | Summary: Chitin catabolic process is a curator inference (IC) from the chitinase activity annotation, which is itself unsupported. Without the essential catalytic glutamate the protein cannot catabolize chitin, and falcon notes that vegetative S. pombe walls lack chitin and that this single GH18 enzyme should not be assigned a principal septum-dissolving / chitin-degrading role in vegetative growth (cell separation is glucanase-driven via Eng1/Agn1). Reason: This annotation is a downstream inference from the (removed) chitinase activity and contradicts both the biochemical evidence (missing catalytic residue) and the organism-level context (vegetative walls lack chitin; cell separation is glucanase-driven). Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md cts2/Q9C105 should not be annotated as the principal septum-dissolving enzyme |
| GO:0009277 fungal-type cell wall | ISO GO_REF:0000024 | ACCEPT | Summary: Fungal-type cell wall localization (orthology-transferred, ISO) is plausible for a secreted GH18 chitinase-like protein even without catalytic activity, since such proteins associate with the cell-wall / cell-surface compartment. Falcon supports a secreted/cell-surface working localization but cautions that the protein is unlikely to be a bulk vegetative wall-remodeling enzyme. Retained as a localization (component) annotation, distinct from any wall-remodeling process claim. Reason: Even without enzymatic activity, secreted chitinase-like proteins can associate with cell-wall components; this component localization is consistent with the protein's secreted nature and is independent of the questionable catalytic process annotations. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the most plausible working localization is the **secretory pathway and cell surface/extracellular space** |
| GO:0030246 carbohydrate binding | NAS | NEW | Summary: Carbohydrate (GlcNAc/chito-oligosaccharide) binding is the most defensible residual molecular function for this catalytically-deficient GH18 protein: the GH18 fold provides a substrate-binding cleft even when the catalytic glutamate is absent. Falcon explicitly references the GH18 binding-cleft architecture, and bounds the protein's likely substrate as chitin/chitin-like GlcNAc polymers/oligomers. Added to capture the core molecular function in the absence of demonstrable catalytic activity. (Note: chitin binding, GO:0008061, sits under carbohydrate derivative binding GO:0097367 rather than under carbohydrate binding GO:0030246; GO:0030246 is retained here as the better-supported, appropriately general term given that the substrate is inferred, not experimentally demonstrated, for this protein.) Reason: Core molecular function not present in existing_annotations. A carbohydrate-binding (lectin-like) role is the best-supported residual MF for a GH18 protein lacking the catalytic glutamate, consistent with falcon's GlcNAc/chito-oligomer substrate inference and its statement that GH18 substrate engagement depends on binding-cleft architecture. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md The best-supported statement is that GH18 enzymes can span endo- and exo-acting modes depending on binding cleft architecture. |
| GO:0071555 cell wall organization | NAS | NEW | Summary: Broad cell wall organization process, consistent with the protein's secreted/cell-wall-associated localization. Falcon cautions that this single S. pombe GH18 enzyme is unlikely to act in bulk vegetative wall remodeling (vegetative walls lack chitin; cell separation is glucanase-driven), so any wall-organization role is most plausibly stage-specific (e.g. spore/conidial wall) rather than a core vegetative function. Kept as a broad, non-core process annotation. Reason: Broad process term consistent with cell-wall-associated localization; retained as non-core because falcon argues against a primary vegetative wall-remodeling / cell-separation role for this catalytically-uncertain GH18 protein. Supporting Evidence: file:SCHPO/cts2/cts2-deep-research-falcon.md the organismβs single GH18 enzyme is more plausibly involved in **developmental stages (e.g., spores/conidia) or environmental chitin processing** than in routine vegetative wall turnover |
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