knh4 (SPBC1E8.05; synonym gaz2) is a fission-yeast member of the Kre9/Knh1 family of fungal cell-surface glycoproteins (Pfam PF10342 Kre9_KNH; InterPro IPR018466 Kre9/Knh1-like_N; PANTHER PTHR40633). The protein has an N-terminal signal peptide, a single N-terminal Kre9/Knh1 fold, a long serine/threonine-rich low-complexity disordered stalk, predicted N- and O-glycosylation, and a hydrophobic C-terminus consistent with a GPI-anchor signal; it was identified in a genome-wide screen for predicted GPI-anchored proteins and localizes to the cell surface. In S. pombe, deletion of knh4 is viable but confers sensitivity to cell-wall-degrading (beta-glucanase) enzymes, and high-copy expression of knh4 suppresses the growth defect of N-glycosylation-defective och1 cells; these data indicate that Knh4 non-enzymatically supports cell-surface beta-glucan as part of the fungal cell wall. Kre9/Knh1-family proteins are non-catalytic and, in the budding yeast homologs Kre9p/Knh1p, function in cell-wall (1->6)-beta-glucan assembly. The specific molecular activity of Knh4, and its individual contribution relative to the essential Kre9 ortholog and other paralogs, have not been experimentally established.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root molecular_function annotation with the ND (no biological data) evidence code, indicating that no specific molecular function has been curated for knh4. This accurately reflects the current state of knowledge: Kre9/Knh1 family proteins are non-catalytic and knh4 has no experimentally defined molecular activity. Reason: ND on the root term is the correct GO representation when no informative molecular function is known. knh4 is a "conserved unknown" cell-surface protein with no demonstrated catalytic or binding activity; the family is non-enzymatic. Retain as-is; the genuine molecular-function gap is recorded in knowledge_gaps rather than by asserting a speculative activity. |
| GO:0008150 biological_process | ND GO_REF:0000015 | MODIFY | Summary: Root biological_process annotation with the ND evidence code. This ND annotation predates (and does not incorporate) the later functional data showing a cell-wall beta-glucan-support role. Reason: The ND root-process annotation is now superseded by knh4-specific evidence: knh4-delta is beta-glucanase-sensitive and the protein non-enzymatically supports cell-surface beta-glucan (PMID:34738170). A specific biological process can therefore be assigned. Propose replacing the ND root with a cell-wall organization / (1->6)-beta-D-glucan metabolic-process term reflecting this role. Proposed replacements: fungal-type cell wall organization (1->6)-beta-D-glucan metabolic process Supporting Evidence: PMID:34738170 non-enzymatically support Ξ²-glucan on the cell-surface of S. pombe |
| GO:0009986 cell surface | TAS PMID:12845604 Genome-wide identification of fungal GPI proteins. | ACCEPT | Summary: Cell-surface localization, curated (TAS) from the genome-wide GPI-protein prediction and consistent with the signal peptide, GPI-anchor-like C-terminus, and demonstrated GPI-anchored status of knh4. Reason: Cell surface (GO:0009986) is the correct and well-supported cellular location for this GPI-anchored, signal-peptide-bearing cell-surface glycoprotein. The localization is corroborated by the functional study (PMID:34738170), which treats SPBC1E8.05 as a GPI-anchored cell-surface protein. Retain as a core location. Note the GPI "anchored component of membrane" GO terms are obsolete, so cell surface is the appropriate CC. Supporting Evidence: PMID:12845604 only 33 GPI candidates were identified PMID:34738170 non-enzymatically support Ξ²-glucan on the cell-surface of S. pombe |
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Download this section (compressed HTML)Q: What is the molecular activity of Knh4 at the cell surface β does it bind or cross-link (1->6)-beta-glucan, and which cell-wall components are its partners?
Q: Is knh4 functionally redundant with the essential S. pombe Kre9 ortholog and the other Kre9/Knh1 paralogs, and does a higher-order deletion phenocopy the essential family member?
Q: How does high-copy knh4 suppress the growth defect of N-glycosylation-defective och1 cells β by restoring cell-wall integrity, and is the GPI anchor required?
Experiment: Express and purify recombinant Knh4 (with and without the Ser/Thr-rich stalk) and assay binding to and cross-linking of defined beta-1,6- and beta-1,3-glucan substrates in vitro; test whether the isolated Kre9/Knh1 domain is sufficient.
Hypothesis: Knh4 directly binds/cross-links cell-wall (1->6)-beta-glucan.
Type: biochemical binding/activity assay
Experiment: Construct single and combinatorial deletions of knh4 and its paralogs (and conditional alleles of the essential Kre9 ortholog) and quantify cell-wall beta-glucan content/structure and beta-glucanase/stress sensitivity to test for redundancy versus specialization.
Hypothesis: Knh4 is functionally redundant with other S. pombe Kre9/Knh1-family members.
Type: genetic epistasis / cell-wall analysis
Experiment: Compare wild-type, GPI-anchor-deleted, and mislocalized Knh4 constructs for rescue of knh4-delta beta-glucanase sensitivity and suppression of the och1 growth defect.
Hypothesis: The GPI anchor and cell-surface localization of Knh4 are required for its cell-wall beta-glucan support function.
Type: structure-function complementation
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular activity of Knh4 is unknown. It is undetermined whether Knh4 binds, cross-links, transports, or otherwise scaffolds cell-wall beta-glucan; no catalytic activity, ligand, or biochemical partner has been demonstrated, and the "support" of cell-surface beta-glucan is explicitly non-enzymatic.
OPEN BIOLOGY MF_DARK
What is known: Knh4 is firmly established as a GPI-anchored, Kre9/Knh1-family cell-surface glycoprotein whose deletion causes beta-glucanase sensitivity, and whose overexpression rescues N-glycosylation-defective och1 cells (PMID:34738170); the budding-yeast homologs Kre9p/Knh1p act in cell-wall (1->6)-beta-glucan assembly. What Knh4 does at the molecular level to support beta-glucan is not known.
Significance: Defining the molecular activity would convert a "conserved unknown" cell-surface protein into a mechanistic component of fungal cell-wall beta-glucan organization, informing antifungal cell-wall biology.
What would resolve it: Biochemical characterization of purified Knh4 for beta-glucan binding/ cross-linking activity; interactomics to identify cell-wall partners; and structural/genetic dissection of the Kre9/Knh1 domain versus the Ser/Thr-rich stalk.
Provenance (the field's own admissions):
Gap: Knh4's specific biological role and its contribution relative to the essential Kre9 ortholog and the other non-essential Kre9/Knh1 paralogs in S. pombe are undetermined. Whether knh4 acts redundantly, in a specialized condition, or in a particular cell-wall layer is not resolved.
OPEN BIOLOGY BP_DARK
What is known: knh4-delta is viable with only cell-wall-stress (beta-glucanase) sensitivity (PMID:34738170; PMID:20473289), implying substantial functional redundancy; the S. pombe genome encodes an essential Kre9 ortholog that carries the core beta-1,6-glucan-formation function, plus additional Kre9/Knh1-family paralogs. The individual and combined contributions of these genes have not been mapped.
Significance: Distinguishing redundancy from specialization would clarify how the Kre9/Knh1 family divides labor in fission-yeast cell-wall assembly.
What would resolve it: Systematic single- and combinatorial-deletion analysis of the S. pombe Kre9/Knh1 family (knh4 with its paralogs and the essential Kre9 ortholog), with cell-wall beta-glucan quantification/structural analysis and stress profiling.
Provenance (the field's own admissions):
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