cpxP Deletion Confers Resistance to Misfolded PapE-Induced Cytotoxicity Through Enhanced CpxAR Activation in Escherichia coli

CpxAR is a two-component system that is conserved among Gram-negative bacteria. The CpxAR system is thought to respond to envelope stress derived from a variety of stimuli, including pH, salt and protein misfolding. Activation of the CpxAR response results in the transcription of gene targets such as chaperone proteins and periplasmic proteases, which combat envelope stress and re-establish envelope homeostasis. The CpxAR system is negatively by regulated the periplasmic protein CpxP, which has been shown to directly inhibit CpxA. One model suggests that CpxP has an additional role of chaperoning misfolded proteins to the DegP protease for degradation, where both CpxP and the misfolded protein undergo proteolysis. To investigate the role of CpxP in chaperoning misfolded proteins and inhibiting the CpxAR response, we induced expression of misfolded PapE protein in wild type and ΔcpxP strains of E. coli. Growth rates and activation of the CpxAR system were measured through the use of culture density and a bacterial luciferase reporter driven by the promoter of a CpxAR induced gene, respectively. Our results demonstrate that deletion of cpxP confers resistance to PapE-induced cytotoxicity and is correlated with increased activation of the CpxAR response. We propose that the loss of CpxP results in higher CpxAR transcriptional activity and thus higher expression of chaperone proteins and proteases, combating the cytotoxicity associated with misfolded PapE. Together, these results suggest that the primary role of CpxP may be negative regulation of the CpxAR response, while its chaperone activity may have secondary importance, the loss of which can be compensated through redundant mechanisms.