Sequestration from Protease Adaptor Confers Differential Stability to Protease Substrate

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.