CpxP is a homodimeric periplasmic auxiliary protein of the Escherichia coli Cpx envelope-stress system. Under basal conditions it binds the periplasmic sensor domain of CpxA and inhibits CpxA autophosphorylation. High salt or misfolded PapE disrupts this interaction, allowing Cpx signaling. CpxP also recognizes a subset of misfolded periplasmic proteins and acts as a substrate adaptor for their DegP-dependent proteolysis, during which CpxP is degraded with the client. CpxP has weak chaperone activity in vitro; its characterized client recognition is coupled to DegP-dependent degradation rather than in-situ aggregation prevention.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030288 outer membrane-bounded periplasmic space | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for periplasmic localization, consistent with the IDA annotation for the same term (PMID:9473036). CpxP is a well-established periplasmic protein with a signal peptide (residues 1-21). Reason: Periplasmic localization of CpxP is confirmed experimentally (PMID:9473036, PMID:25207645) and phylogenetically by IBA. This is a core localization annotation. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002445564 · PANTHER:PTN002445564 SUPPORTS TRANSFER Current PTHR38102 PAINT retains GO:0030288 at this ancestral node, seeded by direct localization evidence on CpxP and Spy. UniProtKB:P0AE85 · CpxP SUPPORTS TRANSFER CpxP's own experimental localization is valid descendant evidence for the node placement and is not circular propagation. UniProtKB:P77754 · Spy SUPPORTS TRANSFER Spy is a periplasmic member of PTHR38102 and supports the ancestral localization assertion. |
| GO:0051082 unfolded protein binding | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA annotation for unfolded protein binding. CpxP does interact with misfolded periplasmic proteins via its hydrophobic cleft (PMID:21239493), but this interaction is primarily in the context of its adaptor function for DegP-mediated proteolysis, not general chaperone holdase activity. UniProt explicitly describes CpxP as having only "mild protein chaperone activity" (PMID:21317898). Reason: GO:0051082 is obsolete, and the available CpxP evidence supports recognition of misfolded clients for DegP-dependent proteolysis rather than a general aggregation-preventing holdase activity. GO:0140309 does not fit because no carrier-like escort to an acceptor or location is demonstrated. The current functional replacement is GO:0140767 enzyme-substrate adaptor activity, added below from PMID:16303867; this historical IBA should not be retained as a core molecular function. Propagation Review Root cause: SOURCE STALE OR MISSING Sources checked: PANTHER:PTN002445564 · PANTHER:PTN002445564 SOURCE STALE OR MISSING Current PTHR38102 PAINT retains only GO:0030288 at this node; the historical GO:0051082 assertion is absent. UniProtKB:P0AE85 · CpxP SOURCE STALE OR MISSING P0AE85 occurs in the historical WITH/FROM, but current PAINT no longer exposes GO:0051082 at the node. UniProtKB:P77754 · Spy SOURCE STALE OR MISSING P77754 occurs in the historical WITH/FROM, but current PAINT no longer exposes GO:0051082 at the node. Direct Spy biology does not restore a missing current propagation claim for CpxP. Supporting Evidence: PMID:16303867 CpxP functions as a periplasmic adaptor protein that is required for the effective proteolysis of a subset of misfolded substrates by the DegP protease. file:ECOLI/CpxP/CpxP-deep-research-falcon.md it promotes DegP-dependent degradation of certain misfolded pilus subunits PMID:21239493 an extended hydrophobic cleft on the convex surface suggests a potent substrate recognition site for misfolded pilus subunits |
| GO:0042597 periplasmic space | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation for periplasmic space localization. This is a broader parent of GO:0030288 (outer membrane-bounded periplasmic space) which is annotated with IDA and IBA evidence. Reason: Periplasmic localization is well established. This broader IEA is acceptable alongside the more specific IDA annotation. |
| GO:0042802 identical protein binding | IPI PMID:21317318 Structure of the periplasmic stress response protein CpxP. | KEEP AS NON CORE | Summary: IPI annotation for identical protein binding. CpxP forms a homodimer, as demonstrated by crystal structure (PMID:21317318, PMID:21239493). The homodimer has an intertwined antiparallel alpha-helical structure. Reason: The IPI assertion is consistent with the structurally demonstrated antiparallel CpxP homodimer and there is no contradictory evidence. It is retained as structural context rather than a core molecular function; the informative core functions are CpxA inhibition and DegP substrate-adaptor activity. Supporting Evidence: PMID:21317318 The structure revealed an antiparallel dimer of intertwined alpha-helices with a highly basic concave surface. |
| GO:0005515 protein binding | IDA PMID:17259177 Purification, reconstitution, and characterization of the Cp... | MODIFY | Summary: IDA annotation for protein binding from Fleischer et al. (2007), which demonstrated direct protein-protein interaction between CpxP and CpxA in reconstituted proteoliposomes. CpxP inhibited CpxA autophosphorylation by 50%. Reason: The "protein binding" term is too generic. CpxP binds CpxA specifically to inhibit its sensor kinase activity. A more informative term would be GO:0030547 "signaling receptor inhibitor activity" which captures the functional consequence of the binding -- inhibition of CpxA signaling. Proposed replacements: signaling receptor inhibitor activity Supporting Evidence: PMID:17259177 Purified tagless CpxP protein reduced the phosphorylation status of CpxA to 50% but had no effect on CpxA phosphotransfer or phosphatase activities. |
| GO:0030162 regulation of proteolysis | EXP PMID:16303867 The extracytoplasmic adaptor protein CpxP is degraded with s... | MODIFY | Summary: EXP annotation for regulation of proteolysis. Isaac et al. (2005) demonstrated that CpxP acts as a periplasmic adaptor protein required for the effective DegP-mediated degradation of misfolded P pilus subunits. The presence of misfolded substrate enhances CpxP proteolysis by DegP. Reason: CpxP serves as an adaptor for DegP protease, and cpxP mutations prevent effective degradation of the substrates tested in PMID:16303867. The evidence therefore supports the more informative child GO:0045862 positive regulation of proteolysis rather than the direction-neutral parent. Proposed replacements: positive regulation of proteolysis Supporting Evidence: PMID:16303867 CpxP functions as a periplasmic adaptor protein that is required for the effective proteolysis of a subset of misfolded substrates by the DegP protease. |
| GO:0030288 outer membrane-bounded periplasmic space | IDA PMID:9473036 CpxP, a stress-combative member of the Cpx regulon. | ACCEPT | Summary: IDA annotation for periplasmic localization based on Danese and Silhavy (1998). CpxP is a periplasmic protein induced by the Cpx system. Reason: Direct experimental evidence confirms CpxP periplasmic localization (PMID:9473036). The protein has a signal peptide and is found in the periplasm. Supporting Evidence: PMID:9473036 cpxP specifies a periplasmic protein that can combat the lethal phenotype associated with the synthesis of a toxic envelope protein. |
| GO:0051082 unfolded protein binding | ISM PMID:21239493 Structural basis for two-component system inhibition and pil... | MARK AS OVER ANNOTATED | Summary: ISM (sequence model) annotation for unfolded protein binding based on Zhou et al. (2011). The crystal structure revealed a hydrophobic cleft on the convex surface that may serve as a substrate recognition site for misfolded proteins. Reason: GO:0051082 is obsolete. The structural model supports a hydrophobic misfolded-substrate recognition cleft, but the functional evidence identifies CpxP as a DegP proteolysis adaptor rather than a general holdase. GO:0140309 is carrier-specific and is not supported because no escort to an acceptor or location is shown. GO:0140767 captures the demonstrated adaptor activity. Supporting Evidence: PMID:21239493 an extended hydrophobic cleft on the convex surface suggests a potent substrate recognition site for misfolded pilus subunits |
| GO:0051082 unfolded protein binding | IDA PMID:21239493 Structural basis for two-component system inhibition and pil... | MARK AS OVER ANNOTATED | Summary: IDA annotation for unfolded protein binding from Zhou et al. (2011). This study showed CpxP binds misfolded PapE pilus subunits and promotes their degradation by DegP. The study also confirmed mild chaperone activity for CpxP. Reason: The experimental study supports recognition of misfolded pilus subunits in the Cpx/DegP quality-control pathway, but it does not demonstrate general aggregation prevention. Because GO:0051082 is obsolete, and neither the carrier-specific GO:0140309 nor a general holdase NTR is evidence-matched here, the annotation is over-annotated. The demonstrated molecular function is enzyme-substrate adaptor activity (GO:0140767). Proposed replacements: enzyme-substrate adaptor activity Supporting Evidence: PMID:21239493 CpxP both inhibits activation of CpxA and is indispensable for the quality control system of P pili |
| GO:0005515 protein binding | IDA PMID:25207645 Dynamic interaction between the CpxA sensor kinase and the p... | MODIFY | Summary: IDA annotation for protein binding from Tschauner et al. (2014), which demonstrated direct physical interaction between CpxP and CpxA using bacterial two-hybrid and membrane-Strep-tagged protein interaction experiments. The interaction is dynamic and modulated by stress signals. Reason: Same as the other protein binding annotation -- "protein binding" is too vague. This study specifically demonstrates CpxP-CpxA interaction that inhibits Cpx signaling. GO:0030547 "signaling receptor inhibitor activity" is more appropriate. Proposed replacements: signaling receptor inhibitor activity Supporting Evidence: PMID:25207645 CpxP modulates the activity of the Cpx system by dynamic interaction with CpxA in response to specific stresses. |
| GO:0006950 response to stress | IDA PMID:9473036 CpxP, a stress-combative member of the Cpx regulon. | KEEP AS NON CORE | Summary: IDA annotation for response to stress based on Danese and Silhavy (1998). CpxP combats extracytoplasmic protein-mediated toxicity and cpxP mutants are hypersensitive to alkaline pH. Reason: The experimental stress-combative and alkaline-sensitivity phenotypes support this broad process annotation, so it should be retained. It is non-core because GO:0006950 does not identify either of CpxP's direct mechanisms, CpxA inhibition or DegP substrate-adaptor activity. Supporting Evidence: PMID:9473036 cpxP specifies a periplasmic protein that can combat the lethal phenotype associated with the synthesis of a toxic envelope protein. |
| GO:0140767 enzyme-substrate adaptor activity | EXP PMID:16303867 The extracytoplasmic adaptor protein CpxP is degraded with s... | NEW | Summary: Proposed annotation for CpxP's demonstrated role as a periplasmic adaptor that recruits misfolded substrates to DegP for proteolysis. Reason: PMID:16303867 explicitly identifies CpxP as an adaptor required for effective DegP proteolysis of a subset of misfolded substrates. This is the evidence-matched current molecular-function term and is more informative than obsolete GO:0051082. Supporting Evidence: PMID:16303867 CpxP functions as a periplasmic adaptor protein that is required for the effective proteolysis of a subset of misfolded substrates by the DegP protease. |
| GO:0070298 negative regulation of phosphorelay signal transduction system | IDA PMID:17259177 Purification, reconstitution, and characterization of the Cp... | NEW | Summary: Proposed process annotation for direct inhibition of the CpxA sensor kinase by CpxP. Reason: Purified CpxP directly reduces the phosphorylation state of CpxA without altering phosphotransfer or phosphatase activity. This supports negative regulation of the Cpx phosphorelay and supplies the process represented in the core-function synthesis. Supporting Evidence: PMID:17259177 Purified tagless CpxP protein reduced the phosphorylation status of CpxA to 50% but had no effect on CpxA phosphotransfer or phosphatase activities. |
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Download this section (compressed HTML)Q: Which endogenous misfolded periplasmic proteins are direct CpxP clients beyond P-pilus subunits, and how broad is this substrate set under native stress conditions?
Q: Does CpxP simultaneously contact DegP and a misfolded client, and which surfaces determine substrate selection versus CpxA inhibition?
Experiment: Use stress-resolved crosslinking proteomics with separation-of-function CpxP mutants to identify ternary CpxP-DegP-client complexes and distinguish direct adaptor substrates from downstream stress phenotypes.
Type: in vivo adaptor-complex mapping
Experiment: Compare client aggregation, DegP-dependent degradation, and CpxA signaling in matched wild-type, cpxP-null, and adaptor-defective strains to test whether weak chaperone activity has an independent physiological contribution.
Type: aggregation and degradation kinetics
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