OMP Peptide Signals Initiate the Envelope-Stress Response by Activating DegS Protease via Relief of Inhibition Mediated by Its PDZ Domain

Abstract

Transmembrane signaling between intracellular compartments is often controlled by regulated proteolysis. Escherichia coli respond to misfolded or unfolded outer-membrane porins (OMPs) in the periplasm by inducing sigma(E)-dependent transcription of stress genes in the cytoplasm. This process requires a proteolytic cascade initiated by the DegS protease, which destroys a transmembrane protein (RseA) that normally binds to and inhibits sigma(E). Here, we show that peptides ending with OMP-like C-terminal sequences bind the DegS PDZ domain, activate DegS cleavage of RseA, and induce sigma(E)-dependent transcription. These results suggest that DegS acts as a sensor of envelope stress by binding unassembled OMPs. DegS activation involves relief of inhibitory interactions between its PDZ and protease domains. Peptide binding to inhibitory PDZ domains in proteases related to DegS, including DegP/HtrA, may also regulate the degradation of specific substrates by these enzymes.

A membrane metalloprotease participates in the sequential degradation of a Caulobacter polarity determinant

“…DegS is activated by cell envelope stresses, it makes the first cleavage that removes the C-terminal domain of RseA (Ades et al, 1999;Alba et al, 2001;Walsh et al, 2003;Wilken et al, 2004). The truncated RseA fragment, RseA(DP), remains membrane-bound and continues to inhibit s E until the membrane metalloprotease YaeL makes the second cleavage; the N-terminal domain is then released into the cytoplasm for complete degradation, possibly by the ClpXP protease, relieving repression of s E (Alba et al, 2002;Kanehara et al, 2002;Flynn et al, 2003).…”

Structural basis for the negative regulation of bacterial stress response by RseB

“…DegS, which is activated when its PDZ domain is bound to the C-terminal peptide of unfolded outer membrane porins (OMPs), cleaves the C-terminal periplasmic domain of RseA. 4 Subsequently, RseP cleavage within the membrane domain of RseA releases the cytoplasmic domain of RseA (associated with the rE) from the membrane. In the final step, the cytoplasmic domain of RseA is degraded such that the released rE can interact with RNA polymerase.…”

Unfolded protein responses in bacteria and mitochondria: A central role for the ClpXP machine

“…These misfolded and/or unassembled OMPs expose a conserved C-terminal motif (YxF, where x 5 any residue), which is normally buried within the assembled porin trimer (52,53). The exposed C-termini then bind to the PDZ domains of DegS (an inner membrane-bound serine protease) causing a conformational change in DegS that results in its proteolytic activation (reviewed in Fig.…”

“…2b). Once activated, DegS is able to cleave RseA, on the periplasmic side of its transmembrane domain (36,48,49,51,53). Another periplasmic sensor protein (RseB) is involved in the pathway, which interacts with, and inhibits the processing of RseA thereby dampening the signal transduction pathway (36,46,54,55).…”

“…Thus, it appears that full activation of the envelope stress response requires two inducers one to activate DegS and another to inhibit RseB. Nevertheless, the cleavage of RseA by DegS is essential for the next step in the cascade, which is mediated by RseP (48,51,53).…”