ABSTRACT: In Caulobacter crescentus, timely degradation of several proteins by the ClpXP protease is critical for proper cell cycle progression. During the cell cycle, the ClpXP protease, the substrate CtrA, and many other proteins are localized to the stalked pole dependent on a polar interaction hub composed of PopZ protein oligomers. Prior work suggests that the localization of ClpXP, protease substrates, and cofactors is needed for recognition of substrates, such as CtrA, by ClpXP. Here, we formally test this hypothesis by examining the role of PopZ in ClpXP activity and find, surprisingly, that CtrA degradation is enhanced in cells lacking polar localization due to loss of PopZ. The ClpXP adaptor CpdR is required for this enhanced degradation of CtrA and other adaptor-dependent substrates, but adaptor-independent substrate degradation is not affected upon loss of PopZ. We find that overexpression of PopZ also leads to faster degradation of CtrA but is likely due to nonphysiologically relevant recognition of CtrA by ClpXP alone, as loss of CpdR does not affect this enhancement. Our main conclusion is that loss of PopZ, and therefore loss of polar localization, does not result in the loss of ClpXP-regulated proteolysis, as would be predicted from a model which requires polar localization of ClpXP for its activation. Rather, our data point to a model where PopZ normally restrains ClpXP proteolysis by promoting the inactivation of the CpdR adaptor, perhaps through the activity and localization of the CckA kinase.IMPORTANCE Regulated proteolysis is critical for the cell cycle progression of bacteria, such as Caulobacter crescentus According to one model, this regulated proteolysis requires localization of the ClpXP protease at the stalked pole for its subsequent degradation of substrates, such as CtrA. This study offers evidence that supports an alternative model to explain how localization might influence protein degradation. Using a delocalized in vivo system created by the deletion of a polar organizing protein, PopZ, we show that activation of the ClpXP protease is independent of its polar localization. The data point to a role for PopZ in restraining ClpXP activity, likely by controlling the activity of upstream regulators of protease activity, such as CckA, though changes in its localization.
