SpeE2 (polyamine aminopropyltransferase 2, PA4774) is a spermidine synthase of Pseudomonas aeruginosa (EC 2.5.1.16), a member of the spermidine/spermine synthase (PABS) family with a Rossmann-like SAM-dependent methyltransferase fold. It catalyses the irreversible transfer of an aminopropyl group from decarboxylated S-adenosylmethionine (decarboxy-AdoMet) to putrescine, producing spermidine and methylthioadenosine; it acts together with SpeD2 (the AdoMet decarboxylase) in the spermidine from putrescine step. Spermidine made via this route is exported to the cell surface, where as a polycation it binds LPS and stabilises the outer membrane; a PA4774 mutant lacks surface spermidine and is more susceptible to polymyxin B, aminoglycosides and oxidative membrane damage. The enzyme is a soluble cytoplasmic homodimer/tetramer.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO catalytic activity from the PABS signature. Reason: Entailed by the accepted GO:0004766 spermidine synthase activity. |
| GO:0004766 spermidine synthase activity | IEA GO_REF:0000120 | ACCEPT | Summary: UniRule/RHEA/EC 2.5.1.16 mapping of spermidine synthase activity, the HAMAP-curated activity of this enzyme. Reason: Core molecular function, curated by HAMAP MF_00198. Supporting Evidence: file:PSEAE/speE2/speE2-uniprot.txt Catalyzes the irreversible transfer of a propylamine group |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping; a soluble cytoplasmic enzyme. Reason: Correct location for a soluble PABS-family synthase. |
| GO:0006596 polyamine biosynthetic process | IBA GO_REF:0000033 | TOO GENERAL | Summary: IBA from the PTN000168372 IBD for polyamine biosynthetic process, seeded by this gene's own PseudoCAP row (expected self-appearance) and S. solfataricus SpeE. The broad BP term suits a node of aminopropyltransferases. Reason: True, but entailed by the accepted GO:0008295 spermidine biosynthetic process, which is_a this term and is the specific process SpeE2 performs. |
| GO:0006596 polyamine biosynthetic process | IEA GO_REF:0000117 | TOO GENERAL | Summary: ARBA mapping of polyamine biosynthetic process. Reason: Entailed by the accepted GO:0008295 spermidine biosynthetic process. |
| GO:0008295 spermidine biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: UniRule/UniPathway mapping of spermidine biosynthetic process, matching the PseudoCAP IMP row. Reason: Core process; the enzyme performs the committed putrescine-to-spermidine step. |
| GO:0008295 spermidine biosynthetic process | IMP PMID:22155771 Surface-localized spermidine protects the Pseudomonas aerugi... | ACCEPT | Summary: PseudoCAP row and the seed of the GO:0006596 IBD at PTN000168372. The cited paper shows a PA4774 mutant makes no surface spermidine, restored by complementation. For SpeE2 the phenotype coincides with its own catalytic step - it is the spermidine synthase - so this is participation, not an indirect phenotype. Reason: Sound seed; core process performed by this enzyme. Supporting Evidence: PMID:22155771 mutant did not produce spermidine on the cell surface, but genetic |
| GO:0010487 thermospermine synthase activity | IBA GO_REF:0000033 | MODIFY | Summary: IBA from the PTN000168372 IBD for thermospermine synthase activity, seeded by Saccharolobus solfataricus SpeE (Q9UXE4, a multifunctional aminopropyltransferase) and Arabidopsis ACL5 (AT5G19530, a dedicated thermospermine synthase). P. aeruginosa SpeE2 is a putrescine aminopropyltransferase (spermidine synthase, EC 2.5.1.16) by HAMAP curation and makes spermidine, not thermospermine. Thermospermine synthase activity is the activity of the archaeal and plant seeds, not of this spermidine synthase. Reason: Lateral correction to spermidine synthase activity (GO:0004766), the curated activity this protein carries by IEA. GO:0010487 and GO:0004766 are sibling aminopropyltransferase activities; the thermospermine-synthase IBD was placed at a node that also contains dedicated spermidine synthases and over-propagated to this one. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: FUNCTIONAL DIVERGENCE GRANULARITY MISMATCH Sources checked: PANTHER:PTN000168372 · PTN000168372 SUPPORTS SOURCE BUT NOT TARGET Node carries both spermidine synthases (e.g. this gene) and thermospermine synthases; a thermospermine-synthase MF IBD here is too specific for the spermidine-synthase members. UniProtKB:Q9UXE4 · S. solfataricus SpeE (multifunctional aminopropyltransferase) SUPPORTS SOURCE BUT NOT TARGET Genuinely multifunctional, including thermospermine synthase; not a model for SpeE2. AGI_LocusCode:AT5G19530 · Arabidopsis ACL5 (thermospermine synthase) SUPPORTS SOURCE BUT NOT TARGET Dedicated thermospermine synthase; its activity does not transfer to a spermidine synthase. Proposed replacements: spermidine synthase activity |
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Download this section (compressed HTML)Q: Does P. aeruginosa SpeE2 have any thermospermine or spermine synthase side activity, or is it strictly a putrescine aminopropyltransferase as its HAMAP assignment implies?
Experiment: Assay purified SpeE2 with putrescine, spermidine and other amine acceptors and decarboxy-AdoMet to determine whether it makes spermidine only or also longer polyamines such as thermospermine.
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