“Role of the two ClpP protease subunits in Mycobacterium tuberculosis”
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Caseinolytic (Clp) proteases are the most widespread energy-dependent proteases in bacteria. They are involved in protein quality control by degrading misfolded and aggregated proteins and have a role in regulatory proteolysis. The main group of substrates of the Clp proteases is the SsrA-tagged proteins, which arise in the presence of defective translation. SsrA tagging is carried out by tmRNA, encoded by ssrA, together with a protein partner SmpB. While most organisms have only one ClpP, Mycobacterium tuberculosis has two ClpP protease subunits (ClpP1 and ClpP2) with at least one of them essential for growth. Co-expression of clpP1 and clpP2 was demonstrated showing that clpP1 and clpP2 are not expressed under different conditions. The promoter region of clpP1P2 was identified, together with the potential ClgR binding site. A reporter system to assay ClpP1 and ClpP2 enzymatic activities was developed based on LacZ incorporating SsrA tag sequences. This showed that both ClpP1 and ClpP2 degrade SsrA-tagged LacZ, whilst only ClpP2 degrades untagged proteins. This suggests different pattern recognition for the two ClpP proteins with substrate recognition by ClpP1 dependent on the last three residues of the C-terminus of the tag sequence. Mutagenesis analysis of the accessory components demonstrated that ssrA is essential but SmpB deletion is viable. SmpB is not required for aerobic growth but the smpBΔ mutant strain was more sensitive to antibiotics targeting the ribosome as compared to wildtype cells.
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