SpuA (PA0297, also called PauD1) is a cytoplasmic class I glutamine amidotransferase-like hydrolase of the peptidase C26 (PuuD-like) family and the P. aeruginosa homologue of E. coli PuuD. It is encoded first in the spuABCDEFGH operon for polyamine uptake and utilization. SpuA is a gamma-glutamyl-gamma-aminobutyrate hydrolase (EC 3.5.1.94): it cleaves gamma-Glu-GABA to glutamate and GABA through a cysteine-linked glutamyl thioester intermediate. This is the last step of the gamma-glutamylation pathway that turns putrescine, and the aminobutyl moiety of spermidine, into GABA. Purified SpuA prefers gamma-Glu-GABA as substrate, and spuA mutants are defective in growth on spermidine but grow normally on putrescine, presumably because of a redundant hydrolase.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006541 L-glutamine metabolic process | IEA GO_REF:0000104 | MARK AS OVER ANNOTATED | Summary: UniRule transfer from the class I glutamine amidotransferase fold. SpuA's substrate is gamma-Glu-GABA; glutamine use is reported for the comparator GATase, not for SpuA. Reason: Fold-level inference; the characterized activity is hydrolysis of a gamma-glutamyl amide in polyamine catabolism, not glutamine metabolism. Not removed because SpuA's activity on glutamine was not reported either way in the abstract. Supporting Evidence: PMID:34605433 Activity and substrate-binding assays confirm that SpuA exhibits a preference for Ξ³-Glu-GABA as a substrate. |
| GO:0006598 polyamine catabolic process | IBA GO_REF:0000033 | TOO GENERAL | Summary: IBA from PTN000230196, seeded by E. coli PuuD and by SpuA's own PseudoCAP spermidine catabolism row (self-seed, expected). The node term suits a PuuD-like clade. Reason: True, but GO:0046203 spermidine catabolic process, accepted in this review, is_a GO:0006598. |
| GO:0016787 hydrolase activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: True but uninformative; GO:0033969 is accepted. Reason: GO:0033969 is_a GO:0016811 is_a GO:0016810 is_a GO:0016787. |
| GO:0016811 hydrolase activity, acting on carbon-nitrogen (but not peptide) bonds, in linear amides | IEA GO_REF:0000002 | TOO GENERAL | Summary: True but uninformative; GO:0033969 is accepted. Reason: GO:0033969 is_a GO:0016811. |
| GO:0033969 gamma-glutamyl-gamma-aminobutyrate hydrolase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA seeded by E. coli PuuD (IDA). SpuA is PuuD's only homologue in PAO1, and activity assays on purified SpuA confirm gamma-Glu-GABA hydrolysis. Reason: Core molecular function, now with direct support on the gene itself. Supporting Evidence: PMID:34605433 Activity and substrate-binding assays confirm that SpuA exhibits a preference for Ξ³-Glu-GABA as a substrate. |
| GO:0033969 gamma-glutamyl-gamma-aminobutyrate hydrolase activity | IEA GO_REF:0000003 | ACCEPT | Summary: EC mapping; agrees with the IBA and the enzyme assays. Reason: Consistent with the IBA row on the same term. Supporting Evidence: PMID:34605433 SpuA hydrolyzes Ξ³-glutamyl-Ξ³-aminobutyrate (Ξ³-Glu-GABA) to glutamate and Ξ³-aminobutyric acid (GABA) |
| GO:0046203 spermidine catabolic process | IMP PMID:21622750 Functional characterization of seven Ξ³-Glutamylpolyamine syn... | ACCEPT | Summary: PseudoCAP seed of PTN000230196. spuA mutants are defective in spermidine utilization, and SpuA catalyses gamma-Glu-GABA hydrolysis in the gamma-glutamylation route that degrades the aminobutyl moiety, so it does part of the work. The experiment is in PMID:12081945; the cited 2011 paper only restates it. Reason: The claim holds for the gene (genetic plus enzymatic support); only the reference is off, recorded as a source defect. Supporting Evidence: PMID:12081945 Although spuA and spuB mutants showed an apparent defect in spermidine utilization, the biochemical functions of the gene products have yet to be elucidated. PMID:34605433 SpuA hydrolyzes Ξ³-glutamyl-Ξ³-aminobutyrate (Ξ³-Glu-GABA) to glutamate and Ξ³-aminobutyric acid (GABA) |
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Download this section (compressed HTML)Q: Should SpuA also carry GO:0009447 putrescine catabolic process, as its E. coli orthologue PuuD does? It catalyses the last step of the putrescine gamma-glutamylation route, but the spuA mutant grows normally on putrescine.
Experiment: Construct spuA and pauD2 (and PA2268) single and combined mutants and test growth on putrescine and spermidine with gamma-Glu-GABA accumulation measured, to resolve which hydrolase serves each polyamine.
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