Cryo-EM structure of a delivery complex containing the SspB adaptor, an ssrA-tagged substrate, and the AAA+ ClpXP protease
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ABSTRACT:
SUBMITTER: Robert Sauer
PROVIDER: EMPIAR-11349 | biostudies-other |
REPOSITORIES: biostudies-other
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Project description:Energy-dependent protein degradation by the AAA+ ClpXP protease helps maintain protein homeostasis in bacteria and eukaryotic organelles of bacterial origin. In Escherichia coli and many other proteobacteria, the SspB adaptor assists ClpXP in degrading ssrA-tagged polypeptides produced as a consequence of tmRNA-mediated ribosome rescue. By tethering these incomplete ssrA-tagged proteins to ClpXP, SspB facilitates their efficient degradation at low substrate concentrations. How this process occurs structurally is unknown. Here, we present a cryo-EM structure of the SspB adaptor bound to a GFP-ssrA substrate and to ClpXP. This structure provides evidence for simultaneous contacts of SspB and ClpX with the ssrA tag within the tethering complex, allowing direct substrate handoff concomitant with the initiation of substrate translocation. Furthermore, our structure reveals that binding of the substrate·adaptor complex induces unexpected conformational changes within the spiral structure of the AAA+ ClpX hexamer and its interaction with the ClpP tetradecamer.
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Project description:According to the N-end rule, the N-terminal residue of a protein determines its stability. In bacteria, the adaptor ClpS mediates proteolysis by delivering substrates bearing specific N-terminal residues to the protease ClpAP. We now report that the Salmonella adaptor ClpS binds to the N terminus of the regulatory protein PhoP, resulting in PhoP degradation by ClpAP. We establish that the PhoP-activated protein MgtC protects PhoP from degradation by outcompeting ClpS for binding to PhoP. MgtC appears to act exclusively on PhoP, as it did not alter the stability of a different ClpS-dependent ClpAP substrate. Removal of five N-terminal residues rendered PhoP stability independent of both the clpS and mgtC genes. By preserving PhoP protein levels, MgtC enables normal temporal transcription of PhoP-activated genes. The identified mechanism provides a simple means to spare specific substrates from an adaptor-dependent protease.
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