Modulating substrate choice: the SspB adaptor delivers a regulator of the extracytoplasmic-stress response to the AAA+ protease ClpXP for degradation

<jats:p>Adaptor proteins help proteases modulate substrate choice, ensuring that appropriate proteins are degraded at the proper time and place. SspB is an adaptor that delivers ssrA-tagged proteins to the AAA+ protease ClpXP for degradation. To identify new SspB-regulated substrates, we examined proteins captured by ClpXP<jats:sup>trap</jats:sup> in <jats:italic>sspB</jats:italic><jats:sup>+</jats:sup> but not <jats:italic>sspB</jats:italic><jats:sup>-</jats:sup> strains. RseA<jats:sup>1-108</jats:sup>, a fragment of a transmembrane protein that regulates the extracytoplasmic-stress response, fits this criterion. In response to stress, RseA is cleaved on each side of the membrane and is released as a cytoplasmic fragment that remains bound in an inhibitory complex with the σ<jats:sup>E</jats:sup> transcription factor. Trapping experiments together with biochemical studies show that ClpXP functions in concert with SspB to efficiently recognize and degrade RseA<jats:sup>1-108</jats:sup>, and thereby releases σ<jats:sup>E</jats:sup>. Genetic studies confirm that ClpX and SspB participate in induction of the σ<jats:sup>E</jats:sup> regulon in vivo, acting at the final step of an activating proteolytic cascade. Surprisingly, the SspB-recognition sequence in RseA<jats:sup>1-108</jats:sup> is unrelated to its binding sequence in the ssrA tag. Thus, these experiments elucidate the final steps in induction of the extracytoplasmic stress response and reveal that SspB delivers a broader spectrum of substrates to ClpXP than has been recognized.</jats:p>