XcpX (PA3097) is the GspK-family minor pseudopilin of the Xcp type II secretion system of Pseudomonas aeruginosa, the machine that exports elastase, lipases and other exoproteins from the periplasm across the outer membrane. It is a bitopic inner-membrane protein whose short leader is removed by the prepilin peptidase PilD/XcpA, leaving a large periplasmic domain. XcpX binds XcpV, XcpW and XcpU to form the calcium-binding tip complex of the periplasmic pseudopilus, sits at the top of that complex, binds the major pseudopilin XcpT, and limits pseudopilus length.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping (cell inner membrane) to plasma membrane. Reason: XcpX is a bitopic cytoplasmic (inner) membrane protein with an N-terminal TM segment and a periplasmic domain; in a Gram-negative bacterium the inner membrane is the plasma membrane. Supporting Evidence: PMID:9466253 we characterized the xcpX gene product, a bitopic cytoplasmic membrane protein file:PSEAE/xcpX/xcpX-uniprot.txt TRANSMEM 8..29 |
| GO:0009306 protein secretion | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the GspK family entry IPR005628 to protein secretion. Reason: Correct but general. The specific supported term is GO:0015628 protein secretion by the type II secretion system, which the gene already carries (IBA and PseudoCAP rows). Proposed replacements: protein secretion by the type II secretion system Supporting Evidence: PMID:9466253 The xcp gene products in Pseudomonas aeruginosa are required for the secretion of proteins across the outer membrane |
| GO:0015627 type II protein secretion system complex | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN000475915, whose IBD rests solely on the PseudoCAP IDA row for this gene (PMID:9466253). Reason: GspK proteins are conserved T2SS pseudopilus tip components. The node seed is correct for XcpX (tip-complex biochemistry and structure), so the IBD and this IBA are sound. XcpX appearing in its own WITH/FROM reflects its experimental seed and is expected. Supporting Evidence: PMID:19828448 We found an XcpV(I)-W(J)-X(K) complex, which demonstrates that the crystallized trimeric complex also exists in the P. aeruginosa T2SS file:PSEAE/xcpX/xcpX-uniprot.txt Belongs to the GSP K family |
| GO:0015627 type II protein secretion system complex | IDA PMID:9466253 The secretion apparatus of Pseudomonas aeruginosa: identific... | ACCEPT | Summary: PseudoCAP IDA from Bleves et al. 1998, the paper identifying XcpX as the fifth Xcp pseudopilin; sole seed of the PTN000475915 IBD. Reason: The abstract shows XcpX is an inner-membrane pseudopilin processed by PilD and required for function; the full text is not available to check the localization assays. Membership of the T2SS complex is independently confirmed by later work showing XcpX in the XcpU-V-W-X tip complex and in the XcpVWX crystal structure. Supporting Evidence: PMID:9466253 We, therefore, classified XcpX (GspK) as the fifth pseudopilin of the system PMID:30346996 XcpX is on the top (Fig 2B, in yellow) of the tip complex |
| GO:0015628 protein secretion by the type II secretion system | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN000475915, whose IBD rests solely on the PseudoCAP IDA row citing the review PMID:9818381. Reason: The biological claim holds for XcpX and for GspK proteins generally; the seed's evidence code is wrong (IDA on a review) but primary mutant evidence exists (PMID:9466253), so the IBD remains sound. The seed should be re-keyed rather than the node removed. Supporting Evidence: PMID:9466253 the highly conserved glycine residue preceding the hydrophobic segment could not be mutated without loss of the XcpX function |
| 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 PTN000475915 IBD. Reason: The term is correct (XcpX is a component of the Xcp secreton and its conserved glycine is needed for function), but a review with no original secretion data (full text checked) cannot support IDA. The evidence should be TAS for this reference, or the row should be re-keyed to the primary paper PMID:9466253 as IMP. A 2018 transposon mutant still secreted protease (PMID:30346996), which leaves open how strictly each substrate needs XcpX, but does not refute involvement. Supporting Evidence: PMID:9818381 we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism PMID:9466253 the highly conserved glycine residue preceding the hydrophobic segment could not be mutated without loss of the XcpX function |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from IPR005628 to membrane. Reason: Correct but general; XcpX is an inner (plasma) membrane protein, and the gene already carries GO:0005886 plasma membrane. Proposed replacements: plasma membrane Supporting Evidence: PMID:9466253 a bitopic cytoplasmic membrane protein |
Loading supporting content…
Download this section (compressed HTML)Q: Is XcpX strictly required for Xcp-dependent secretion of every substrate, given that a PAO1 transposon xcpX mutant still secreted protease (PMID:30346996) while the glycine mutant lost function (PMID:9466253)?
Q: Should the XcpX calcium binding shown by ITC (PMID:30346996) be captured as a calcium ion binding annotation?
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)