ATP-independent substrate recruitment to proteasomal degradation in mycobacteria

Mycobacteria and other actinobacteria possess proteasomal degradation pathways in addition to the common bacterial compartmentalizing protease systems. Proteasomal degradation plays a crucial role in the survival of these bacteria in adverse environments. The mycobacterial proteasome interacts with several ring-shaped activators, including the bacterial proteasome activator (Bpa), which enables energy-independent degradation of heat shock repressor HspR. However, the mechanism of substrate selection and processing by the Bpa-proteasome complex remains unclear. In this study, we present evidence that disorder in substrates is required but not sufficient for recruitment to Bpamediated proteasomal degradation. We demonstrate that Bpa binds to the folded N-terminal helix-turn-helix domain of HspR, whereas the unstructured C-terminal tail of the substrate acts as a sequence-specific threading handle to promote efficient proteasomal degradation. In addition, we establish that the heat shock chaperone DnaK, which interacts with and co-regulates HspR, stabilizes HspR against Bpa-mediated proteasomal degradation. By phenotypical characterization of Mycobacterium smegmatis parent and bpa deletion mutant strains, we show that Bpadependent proteasomal degradation supports the survival of the bacterium under stress conditions by degrading HspR that regulates vital chaperones.

Automated summary

Mycobacteria and other actinobacteria possess proteasomal degradation pathways. This study reveals the crucial role of proteasomal degradation in the survival of bacteria in adverse environments. The mechanism of substrate selection and processing by the Bpa-proteasome complex is investigated, and it is found that the disorder in substrates and the unstructured tail of the substrate play important roles. The heat shock chaperone DnaK stabilizes the substrate against Bpa-mediated proteasomal degradation. Phenotypical characterization supports the importance of Bpa-dependent proteasomal degradation in the survival of the bacterium under stress conditions.