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

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(trap) in sspB(+) but not sspB(-) strains. RseA(1-108), 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 sigma(E) transcription factor. Trapping experiments together with biochemical studies show that ClpXP functions in concert with SspB to efficiently recognize and degrade RseA(1-108), and thereby releases sigma(E). Genetic studies confirm that ClpX and SspB participate in induction of the sigma(E) regulon in vivo, acting at the final step of an activating proteolytic cascade. Surprisingly, the SspB-recognition sequence in RseA(1-108) 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.