Acts as an auxiliary protein in the Cpx two-component envelope stress response system, helping modulate the Cpx response systems response to some inducers (PubMed:25207645, PubMed:16303867). Binds the periplasmic domain of sensor histidine kinase CpxA, inhibiting induction of the Cpx envelope stress response in the absence of inducer; overexpression decreases Cpx pathway activity (PubMed:16166523, PubMed:21317318). Some periplasmic stimulii (shown for P pili subunit PapE and probably 0.3 M NaCl) increase CpxP's susceptibility to DegP, leading to CpxP degradation, inducing the Cpx pathway (PubMed:16166523, PubMed:16303867). Aids in combating extracytoplasmic protein-mediated toxicity (PubMed:9473036, PubMed:16303867, PubMed:21239493). Overexpression leads to degradation by DegP of misfolded P pili subunits in the periplasm (tested using PapE) (PubMed:21239493). Inhibits autophosphorylation of CpxA in reconstituted liposomes by 50% but has no effect on phosphatase activity of CpxA (PubMed:17259177, PubMed:21239493). Has mild protein chaperone activity (PubMed:21317898, PubMed:21239493). Homodimer; it might alter shape slightly at pH 8.0 when Cpx is induced (PubMed:21317318, PubMed:21239493). Binds the periplasmic sensor domain of CpxA (PubMed:16166523, PubMed:21317318, PubMed:21239493, PubMed:25207645). Interaction with CpxA is not seen in vivo when cells are grown in 0.3 M NaCl, or if the misfolded P pili protein PapE is overexpressed (PubMed:25207645). Induced by envelope stress such as overexpression of misfolded periplasmic proteins (PubMed:9351822). Induced by alkaine pH (tested up to pH 8.4) (PubMed:9473036, PubMed:16166523). Induction is decreased in a degP deletion (PubMed:16166523). Transcription is stimulated by the Cpx two-component signal transduction pathway; sigma factor E (rpoE) is not involved (PubMed:9351822, PubMed:9473036, PubMed:10972835). Transcription induced by spheroplasting, which removes the periplasm and thus this protein; if the protein is anchored to the outer surface of the inner membrane induction does not occur (PubMed:10972835). Induced in persister cells (PubMed:16768798). A region in the N-terminus (residues 55-61) probably interacts with the periplasmic sensor domain of CpxA to inhibit its kinase activity and is also important for CpxP stability (PubMed:16166523). The homodimer has an elongated cradle shape; a hydrophobic cleft on the convex face may interact with periplasmic protein PapE (PubMed:21239493). Degraded by DegP; some CpxP mutant proteins are more susceptible to the protease than others (PubMed:16166523). Degradation probably occurs when CpxP is associated with some misfolded proteins; overexpression of PapE leads to DegP-mediated degradation of CpxP and PapE, which requires the N-terminus of PapE. Overexpression of NlpE does not induce this degradation however (PubMed:16303867). Hypersensitive to alkaline pH (greater than pH 8.8) (PubMed:9473036). Increased accumulation and toxicity of overexpressed, misfolded periplasmic proteins (PubMed:16303867). Belongs to the CpxP/Spy family. Extended N-terminus.