The Cpx Envelope Stress Response Is Controlled by Amplification and Feedback Inhibition

Abstract

In Escherichia coli, the Cpx two-component regulatory system activates expression of protein folding and degrading factors in response to misfolded proteins in the bacterial envelope (inner membrane, periplasm, and outer membrane). It is comprised of the histidine kinase CpxA and the response regulator CpxR. This response plays a role in protection from stresses, such as elevated pH, as well as in the biogenesis of virulence factors. Here, we show that the Cpx periplasmic stress response is subject to amplification and repression through positive and negative autofeedback mechanisms. Western blot and operon fusion analyses demonstrated that the cpxRA operon is autoactivated. Conditions that lead to elevated levels of phosphorylated CpxR cause a concomitant increase in transcription ofcpxRA. Conversely, overproduction of CpxP, a small, Cpx-regulated protein of previously unknown function, represses the regulon and can block activation of the pathway. This repression is dependent on an intact CpxA sensing domain. The ability to autoactivate and then subsequently repress allows for a temporary amplification of the Cpx response that may be important in rescuing cells from transitory stresses and cueing the appropriately timed elaboration of virulence factors.

Just scratching the surface: an expanding view of the Cpx envelope stress response

“…A second auxiliary regulator of CpxA is the periplasmic protein CpxP, which inhibits Cpx pathway activity when overexpressed (Raivio et al, 1999). Although direct evidence is still lacking, it is believed that this inhibition is mediated by protein-protein interaction between CpxP and the periplasmic domain of CpxA.…”

“…Although direct evidence is still lacking, it is believed that this inhibition is mediated by protein-protein interaction between CpxP and the periplasmic domain of CpxA. In support of this hypothesis, inhibition by CpxP is lost when the periplasmic domain of CpxA is mutated (Raivio et al, 1999). Furthermore, the addition of CpxP to an in vitro reconstituted CpxA-CpxR system decreases the rate of CpxA autophosphorylation (Fleischer et al, 2007).…”

“…The HK CpxA represents a major signal integration point. The periplasmic domain of CpxA is required for both induction by NlpE (Raivio & Silhavy, 1997) and inhibition by CpxP (Raivio et al, 1999). Mutations in the periplasmic domain of CpxA also prevent detection of envelope stresses such as alkaline pH, PapE and PapG overexpression, and envelope perturbation by EDTA (DiGiuseppe & Silhavy, 2003), all of which are sensed independently of CpxP and NlpE.…”

Signal integration by the Cpx-envelope stress system

“…As an accessory protein of the TCS (Buelow & Raivio, 2010;Heermann & Jung, 2010), CpxP is also involved in the signalling process . Overproduction of periplasmic localized CpxP protein down-regulates the Cpx signalling cascade (Raivio et al, 1999). Thus, as cpxP belongs to the Cpx regulon, CpxP acts as a negative feedback regulator for the Cpx pathway (Raivio et al, 1999).…”

“…Overproduction of periplasmic localized CpxP protein down-regulates the Cpx signalling cascade (Raivio et al, 1999). Thus, as cpxP belongs to the Cpx regulon, CpxP acts as a negative feedback regulator for the Cpx pathway (Raivio et al, 1999). In addition, CpxP protein level depends not only on its transcription but also on post-transcriptional mechanisms (Buelow & Raivio, 2005;Isaac et al, 2005;Miot & Betton, 2007).…”