PppA (PA0075) is a 242-aa PP2C-family (PPM-type) serine/threonine protein phosphatase encoded in the Hcp secretion island I (H1-T6SS) cluster of Pseudomonas aeruginosa. Its crystal structure shows the bacterial PPM catalytic core with a dinuclear manganese centre and a flexible flap subdomain. PppA opposes the Ser/Thr kinase PpkA: PpkA phosphorylates the FHA-domain scaffold Fha1 to trigger assembly of the H1 type VI secretion apparatus and secretion of Hcp1, and PppA reverses this signal, so that PpkA and PppA have reciprocal effects on H1-T6SS activity. PppA thereby acts as a post-translational brake on type VI protein secretion.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004722 protein serine/threonine phosphatase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from the 35-seed PP2C node PTN000352745. PppA appears among its own sources because of its own PseudoCAP IDA row, which is expected. Reason: Core activity. The node is seeded by many experimentally characterised PP2C phosphatases from yeasts, plants, mammals and M. tuberculosis PstP (P9WHW5), so the IBD stands without this seed; the PPM domain and dinuclear Mn site are conserved in PppA. Supporting Evidence: PMID:17558395 PpkA activity is antagonized by PppA, a Ser-Thr phosphatase. PMID:31208722 PppA harbors a catalytic pocket containing two manganese ions which correspond to the canonical dinuclear metal center of Ser/Thr protein phosphatases including the bacterial PPM phosphatases and human PP2C. |
| GO:0004722 protein serine/threonine phosphatase activity | IDA PMID:17558395 Threonine phosphorylation post-translationally regulates pro... | ACCEPT | Summary: PseudoCAP IDA and one of 35 seeds of PTN000352745. The abstract identifies PppA as the Ser-Thr phosphatase that antagonises PpkA on Fha1. Reason: Seed verdict SOUND. The paper names PppA as the phosphatase acting on the PpkA substrate Fha1, and the PPM fold with a dinuclear Mn centre (crystal structure) fits the activity. The assay details were not read (abstract only), but nothing contradicts the curator's call. Supporting Evidence: PMID:17558395 These proteins exhibit reciprocal effects on the H-T6SS by acting on an FHA domain-containing protein, termed Fha1. PMID:31208722 PppA is a PP2C-family protein phosphatase and plays a key role in the disassembly and reassembly of T6SS organelles. |
| GO:0004722 protein serine/threonine phosphatase activity | IEA GO_REF:0000120 | ACCEPT | Summary: InterPro PP2C / EC 3.1.3.16 mapping. Reason: Same claim as the IBA and IDA rows. Supporting Evidence: file:PSEAE/pppA/pppA-uniprot.txt InterPro; IPR015655; PP2C. |
| GO:0007165 signal transduction | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000352745. PppA terminates the PpkA-Fha1 threonine phosphorylation signal that activates the H1-T6SS. Reason: PppA is a component of a phosphorylation-based signalling pathway; GO has no more specific signalling term that fits a bacterial Ser/Thr phospho-switch controlling secretion, so the broad term is as precise as warranted. Supporting Evidence: PMID:17558395 Here, we identify a threonine phosphorylation event that post-translationally regulates the Hcp secretion island-I-encoded type VI secretion system of Pseudomonas aeruginosa (H-T6SS). |
| GO:0046872 metal ion binding | IEA GO_REF:0000104 | MODIFY | Summary: UniRule metal-binding annotation; the crystal structure has two Mn(2+) ions. Reason: Correct but general; the structure resolves a dinuclear manganese centre, so GO:0030145 manganese ion binding is the supported specific term. In vivo the PPM site may also use Mg(2+), so the metal binding is a cofactor feature, not a core function. Proposed replacements: manganese ion binding Supporting Evidence: PMID:31208722 PppA harbors a catalytic pocket containing two manganese ions which correspond to the canonical dinuclear metal center of Ser/Thr protein phosphatases including the bacterial PPM phosphatases and human PP2C. file:PSEAE/pppA/pppA-uniprot.txt FT BINDING 41 FT /ligand="Mn(2+)" FT /ligand_id="ChEBI:CHEBI:29035" FT /ligand_label="1" FT /evidence="ECO:0007829|PDB:6JKV" |
| GO:0050709 negative regulation of protein secretion | IMP PMID:17558395 Threonine phosphorylation post-translationally regulates pro... | ACCEPT | Summary: PseudoCAP IMP. PppA antagonises the PpkA signal that is required for H1-T6SS assembly and Hcp1 secretion. Reason: PppA's dephosphorylation of the PpkA-Fha1 pathway restrains type VI secretion, which is the physiological role of this phosphatase. GO has no term for regulation of type VI secretion specifically, so this is the most specific available process. Supporting Evidence: PMID:17558395 We show that a serine-threonine kinase, PpkA, is required for assembly of the H-T6SS and for secretion of Hcp1. PpkA activity is antagonized by PppA, a Ser-Thr phosphatase. |
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Download this section (compressed HTML)Q: Is Fha1 phospho-Thr362 the only physiological PppA substrate, or does PppA also dephosphorylate PpkA itself or other H1-T6SS components?
Experiment: Measure dephosphorylation of purified PpkA-phosphorylated Fha1 (and autophosphorylated PpkA) by purified PppA and an active-site metal-ligand mutant, with Mn(2+) versus Mg(2+).
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