XcpR (PA3103) is the GspE-family secretion ATPase of the Xcp type II secretion system of Pseudomonas aeruginosa. It is a soluble, Walker-motif nucleotide-binding protein that has no hydrophobic anchor of its own and is recruited to the cytoplasmic face of the inner membrane by XcpY/GspL, where it joins XcpS, XcpY and XcpZ in the inner-membrane assembly platform of the secreton. Acting as a dimer, it is the motor that energises the machine - substitution of the conserved Walker A glycine abolishes export of exoproteins such as exotoxin A and acts dominant-negatively, leaving normally secreted enzymes stranded in the periplasm. It belongs to the same ATPase superfamily as the type IV pilus assembly motor PilB.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the GspE entry IPR013369 to ATP binding. Reason: The record carries an annotated Walker A P-loop and further nucleotide-binding residues, and mutating the conserved Walker A glycine abolishes secretion, so nucleotide binding is both predicted and functionally required. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt BINDING 263..270 PMID:8102361 unable to secrete exotoxin A or assemble pili, respectively. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN001250205, the broad GspE/PilB-type ATPase node, whose IBD was seeded by this gene and one other family member. Reason: XcpR is experimentally found associated with the cytoplasmic (inner) membrane, which in a Gram-negative bacterium is the plasma membrane, and that is where it works as part of the XcpRSYZ platform. The node term is generic enough for the whole ATPase clade, and this gene appearing in its own WITH/FROM reflects its experimental seed, as expected. Supporting Evidence: PMID:9882649 In a wild-type P. aeruginosa strain, XcpR was found associated with the file:PSEAE/xcpR/xcpR-uniprot.txt Cell inner membrane |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping (cell inner membrane) to plasma membrane. Reason: Reinforces the EXP and IBA rows for the same location; the curated UniProt location itself rests on the experimental localisation in PMID:9882649. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt Cell inner membrane |
| GO:0005886 plasma membrane | EXP PMID:9882649 Assembly of XcpR in the cytoplasmic membrane is required for... | ACCEPT | Summary: Experimental localisation of XcpR to the cytoplasmic membrane, dependent on XcpY. Reason: Direct subcellular localisation in wild-type P. aeruginosa. Note that the association is not intrinsic - XcpR has no hydrophobic segment and needs XcpY to be membrane-bound and stable - but the location is where the protein acts. Supporting Evidence: PMID:9882649 In a wild-type P. aeruginosa strain, XcpR was found associated with the file:PSEAE/xcpR/xcpR-uniprot.txt Membrane association is not an intrinsic property but requires the |
| GO:0008564 protein-exporting ATPase activity | IEA GO_REF:0000003 | KEEP AS NON CORE | Summary: EC2GO mapping from EC 7.4.2.8, the type II secretion ATPase class, which UniProt assigns to this entry. Reason: GO defines this class to cover the ATP-hydrolysing enzymes of the type II secretory pathway acting as components of a multi-subunit complex, so the term is the intended one for a GspE protein and the claim is not wrong. Two caveats keep it off the core-function list. First, the EC assignment is a curator inference from the Walker-site genetics (ECO:0000305 from PubMed:8102361) rather than a measured ATPase or transport activity for XcpR. Second, XcpR is the cytoplasmic motor, not the conduit - the channel is formed by XcpS and the XcpQ secretin - so a transporter-activity term is better qualified contributes_to than enables. The motor activity itself is captured by GO:0016887. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt Acts as a molecular motor to provide the energy that is required for assembly of the pseudopilus PMID:8102361 unable to secrete exotoxin A or assemble pili, respectively. |
| GO:0015627 type II protein secretion system complex | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN002016240, a Gammaproteobacterial subnode whose IBD rests solely on the PseudoCAP IDA row for this gene (PMID:16168578). Reason: The seed is sound - the native XcpRSYZ subcomplex was isolated - and confining the T2SS-specific term to a Gammaproteobacterial subnode keeps it off the PilB-type pilus assembly ATPases that share this PANTHER family. A single well-characterised seed is not weak support, and the gene appearing in its own WITH/FROM is expected. Supporting Evidence: PMID:16168578 XcpS to form a XcpRSYZ complex which was isolated under native conditions. file:PSEAE/xcpR/xcpR-uniprot.txt Forms an inner membrane platform subcomplex with |
| GO:0015627 type II protein secretion system complex | IDA PMID:16168578 Subcomplexes from the Xcp secretion system of Pseudomonas ae... | ACCEPT | Summary: PseudoCAP IDA from Robert, Filloux and Michel 2005, which co-purified the XcpRSYZ subcomplex; sole seed of the PTN002016240 IBD for this term. Reason: The abstract itself reports that the XcpYZ unit was found with XcpR and XcpS as a native XcpRSYZ complex, so the membership claim is directly supported. XcpR's XcpY-dependent membrane recruitment (PMID:9882649) and its interaction with XcpY are consistent. Supporting Evidence: PMID:16168578 XcpS to form a XcpRSYZ complex which was isolated under native conditions. file:PSEAE/xcpR/xcpR-uniprot.txt Interacts with XcpY/GspL |
| GO:0015627 type II protein secretion system complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the GspE entry IPR013369 to the T2SS complex. Reason: Reinforces the IDA and IBA rows for the same term, from the T2SS-specific GspE signature rather than the generic P-loop domain. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt NCBIfam; TIGR02533; type_II_gspE |
| GO:0015628 protein secretion by the type II secretion system | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN002016240, whose IBD for this term rests solely on the PseudoCAP row for this gene that cites the review PMID:9818381. Reason: The biological claim is well grounded for XcpR - Walker A mutants cannot secrete exotoxin A and are dominant-negative, and dimerisation-defective variants cannot secrete - so the node conclusion stands even though the seed row carries the wrong evidence code (the cited paper is a review; full text checked). The seed should be re-keyed to PMID:8102361, not the node dropped. Supporting Evidence: PMID:8102361 unable to secrete exotoxin A or assemble pili, respectively. PMID:9426126 that this dimerization is essential for extracellular protein secretion. |
| GO:0015628 protein secretion by the type II secretion system | IDA PMID:9818381 GSP-dependent protein secretion in gram-negative bacteria: t... | ACCEPT | Summary: PseudoCAP IDA citing Filloux, Michel and Bally 1998, a review of the Xcp system; sole seed of the PTN002016240 IBD for this term. Reason: The term is right for XcpR, but a review with no original secretion data (full text checked) cannot support IDA. The primary evidence is PMID:8102361 (Walker A mutants lose exotoxin A secretion; dominant-negative periplasmic accumulation), which is IMP. The same mis-coding of this same reference sits on the xcpQ and xcpX rows for this term, so it is a PseudoCAP pattern across the xcp cluster. Supporting Evidence: PMID:8102361 unable to secrete exotoxin A or assemble pili, respectively. file:PSEAE/xcpR/xcpR-uniprot.txt Mutants show complete loss of extracellular exotoxin A secretion. |
| GO:0015628 protein secretion by the type II secretion system | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the GspE entry IPR013369 to T2SS protein secretion. Reason: Reinforces the same claim from the T2SS-specific GspE signature; correct and at the right granularity. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt NCBIfam; TIGR02533; type_II_gspE |
| GO:0016887 ATP hydrolysis activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN001250205, the broad GspE/PilB-type ATPase node, seeded by a characterised family member. Reason: ATP hydrolysis is the ancestral and defining activity of this P-loop motor family, and XcpR retains the complete Walker A and metal-binding sites; mutating the Walker A glycine abolishes its function in vivo, which is what one expects if hydrolysis is what it does. No hydrolysis rate has been measured for XcpR itself, so the IBA is doing real work here rather than restating a local experiment. Supporting Evidence: file:PSEAE/xcpR/xcpR-uniprot.txt BINDING 263..270 file:PSEAE/xcpR/xcpR-uniprot.txt Acts as a molecular motor to provide the energy that is required for assembly of the pseudopilus |
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Download this section (compressed HTML)Q: Has ATP hydrolysis ever been measured directly for purified XcpR, and is it stimulated by XcpY or by assembly into the XcpRSYZ platform? Both EC 7.4.2.8 and the protein-exporting ATPase activity row currently rest on Walker-site genetics alone.
Q: Is the Zn(2+) cofactor asserted by similarity to P37093 actually bound by XcpR, and does the tetracysteine motif contribute to dimerisation or to XcpY binding?
Experiment: Measure ATPase activity of purified XcpR alone and in reconstituted XcpRSYZ, with and without the Walker A glycine substitution, to convert the inferred catalytic activity into a direct measurement and to test whether platform assembly activates the motor.
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