spa1 (SPA) is the Schizosaccharomyces pombe ornithine decarboxylase antizyme (ODC-Az), the fungal member of the ODC antizyme family (Pfam PF02100 / InterPro IPR002993). It is a negative regulator of intracellular polyamine levels: it binds and inhibits ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine biosynthesis, thereby lowering putrescine, spermidine and cadaverine pools. Loss of spa1 causes accumulation of these polyamines, especially in stationary phase, while overexpression depletes them, indicating that spa1 is the principal negative regulator of ODC activity in fission yeast. Like metazoan antizymes, expression of full-length spa1 requires a polyamine-stimulated +1 programmed ribosomal frameshift between two overlapping open reading frames, forming an autoregulatory feedback loop in which rising polyamine levels increase antizyme production. By homology to mammalian antizyme, spa1 is thought to inhibit ODC by blocking assembly of the active ODC homodimer and to promote ubiquitin-independent proteasomal degradation of ODC monomers, although the degradation-targeting step has not been directly demonstrated in S. pombe. spa1 is dispensable for viability under standard growth conditions. It is unrelated to the mammalian Rap GTPase-activating protein SIPA1/SPA1.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic (IBA) propagation from the antizyme tree, reflecting the cytoplasm-nucleus shuttling of mammalian antizymes. There is no experimental evidence that S. pombe spa1 localizes to or acts in the nucleus, and the only characterization paper (PMID:10775274) does not localize the protein. Reason: Nuclear localization/action is an over-propagation from mammalian antizyme biology; it is not supported by any S. pombe data. spa1's demonstrated activity (ODC inhibition) and the polyamine biosynthetic machinery it regulates are cytoplasmic, so a nuclear location is not the core function and is likely an over-annotation. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN001616256 Antizyme family node (PTHR10279). The nucleus term reflects mammalian antizyme cytoplasm-nucleus shuttling; localization does not transfer to the S. pombe member. UniProtKB:Q9UMX2 Mammalian antizyme with/from source; nuclear localization is a mammalian property not established for spa1. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) propagation. A cytoplasmic location is consistent with spa1's role in inhibiting cytoplasmic ODC, but no experimental localization exists for the S. pombe protein; this term is broad and uninformative. Reason: Plausible and not contradicted (ODC and polyamine biosynthesis are cytoplasmic), but purely inferred and non-specific. Retain as non-core context rather than as an established localization. |
| GO:0008073 ornithine decarboxylase inhibitor activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the core molecular function of spa1 and is directly demonstrated experimentally: recombinant spa1 protein inhibits S. pombe ODC in vitro (PMID:10775274, Fig. 2). The IBA propagation from the antizyme family is therefore corroborated by direct evidence for this gene. Reason: Correct and experimentally supported core function; ODC inhibition is the defining, assayed activity of spa1. Supporting Evidence: PMID:10775274 the recombinant protein can inhibit S.pombe ODC |
| GO:0008073 ornithine decarboxylase inhibitor activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the ODC antizyme domain (IPR002993 / Pfam PF02100), which spans essentially the whole protein. This assigns the same correct molecular function as the IBA/experimental annotations. Reason: Domain-based assignment of the correct core molecular function; redundant with the experimentally supported IBA annotation but not wrong. Supporting Evidence: PMID:10775274 the recombinant protein can inhibit S.pombe ODC |
| GO:0005829 cytosol | NAS GO_REF:0000051 | KEEP AS NON CORE | Summary: Cytosol assignment from S. pombe keyword mapping (NAS, GO_REF:0000051), i.e. inferred from the antizyme annotation rather than from microscopy. It is a reasonable location for spa1 given that its substrate ODC and the polyamine biosynthetic pathway are cytosolic, but there is no experimental localization for the S. pombe protein. Reason: Plausible, more specific than the IBA cytoplasm term, and not contradicted, but keyword-inferred rather than experimentally determined. Keep as non-core localization context. |
| GO:1901305 negative regulation of spermidine biosynthetic process | NAS GO_REF:0000051 | MODIFY | Summary: Captures spa1's role in lowering polyamine levels; Ξspa cells accumulate spermidine (along with putrescine and cadaverine) (PMID:10775274). However the effect is not spermidine-specific: spa1 inhibits ODC, the entry enzyme, reducing the whole polyamine pool. The broader term negative regulation of polyamine biosynthetic process (GO:0170066) β already used by PomBase/UniProt as TAS β more accurately describes the function. Reason: The essence (spa1 negatively regulates polyamine biosynthesis) is correct, but the spermidine-specific term is too narrow for an ODC inhibitor that reduces putrescine, spermidine and cadaverine. Generalize to the polyamine biosynthetic process term. Proposed replacements: negative regulation of polyamine biosynthetic process Supporting Evidence: PMID:10775274 The cellular concentrations of putrescine, spermidine and cadaverine (but not spermine) were higher in the knockout strains than in wild-type cells. |
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Download this section (compressed HTML)Q: Does S. pombe spa1 target ODC for ubiquitin-independent proteasomal degradation, or does it act purely as a stoichiometric inhibitor of ODC dimerization?
Q: Does spa1 regulate a polyamine transporter in S. pombe, as suggested by the incomplete rescue of the overexpression phenotype by exogenous putrescine?
Experiment: Compare endogenous ODC protein levels and turnover (cycloheximide chase, with and without proteasome inhibition) in wild-type versus Ξspa cells, and test whether spa1 blocks ODC homodimer formation in vitro.
Hypothesis: spa1 destabilizes S. pombe ODC by targeting it for ubiquitin-independent proteasomal degradation.
Type: protein stability / biochemistry
Experiment: Determine the subcellular localization of endogenously tagged spa1 by live-cell fluorescence microscopy, accounting for the +1 frameshift required for full-length expression.
Hypothesis: spa1 localizes to the cytosol, where ODC and polyamine biosynthesis occur.
Type: fluorescence microscopy / localization
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The subcellular localization of spa1 in S. pombe is undetermined; no microscopy or fractionation has placed the protein in a compartment.
OPEN BIOLOGYCURATION CC_DARK
What is known: All cellular-component annotations for spa1 are inferences: cytosol from S. pombe keyword mapping (NAS) and cytoplasm/nucleus from phylogenetic (IBA) propagation of mammalian antizyme localization. A cytosolic site is plausible because spa1's substrate ODC and the polyamine biosynthetic pathway are cytosolic, but this has not been shown for the fission-yeast protein.
Significance: Whether spa1 accesses the nucleus (as mammalian antizymes do during shuttling) or is strictly cytosolic bears on whether it has functions beyond cytosolic ODC control.
What would resolve it: Endogenous-tagging (e.g. GFP knock-in) live-cell microscopy, or the S. pombe ORFeome localization datasets, would directly establish spa1 localization.
Provenance (the field's own admissions):
Gap: Whether S. pombe spa1 promotes ubiquitin-independent proteasomal degradation of ODC (the mammalian antizyme mechanism), or only sterically inhibits ODC dimer assembly, has not been tested in fission yeast.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Direct ODC inhibition by spa1 is experimentally demonstrated (PMID:10775274). The monomer-binding / anti-dimerization and degradation-targeting steps are asserted only by orthology to mammalian antizyme 1 (UniProt Q02803, ISS), not assayed for spa1.
Significance: The degradation-targeting step defines the difference between a simple competitive inhibitor and the antizyme's characteristic role in ubiquitin-independent ODC turnover, and determines whether fission-yeast ODC is regulated by the full conserved mechanism.
What would resolve it: Test ODC protein stability and proteasome dependence in wild-type vs Ξspa cells, and assay spa1-ODC binding / inhibition of ODC dimerization.
Provenance (the field's own admissions):
Gap: The physiological purpose of the deeply conserved antizyme/frameshift feedback circuit is unknown, given that spa1 is dispensable for growth and its loss produces only condition-dependent polyamine accumulation and stress phenotypes.
OPEN BIOLOGY BP_DARK
What is known: spa1 controls polyamine pools (largest effect in stationary phase) and is the principal negative regulator of ODC, yet Ξspa cells are viable with no overt defect; the paper's authors explicitly note the conservation is unexplained.
Significance: Understanding why polyamine feedback by antizyme is preserved from fission yeast to human, despite being non-essential, could reveal a still-undetected aspect of polyamine biology.
What would resolve it: Systematic characterization of Ξspa phenotypes across stresses, growth states and polyamine-limiting conditions, and dissection of a possible second target (polyamine transporter).
Provenance (the field's own admissions):
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