Involvement of Bacillus subtilis ClpE in CtsR degradation and protein quality control

The heat-inducible CtsR regulon of Bacillus subtilis codes for three Clp proteins with chaperone or protease activity. While the importance of ClpC and ClpP has been elucidated for a wide range of cellular adaptation processes, this study deals with the physiological role of B. subtilis ClpE. Northern experiments and reporter gene analyses revealed that ClpE is essential both for efficient CtsR-dependent gene derepression and for rerepression during heat stress. ClpEP was found to destabilize the global regulator CtsR after heat shock in vivo with different kinetics than ClpCP, which is known to degrade CtsR in vitro and in vivo upon heat stress. Furthermore, ClpE was localized at heat-generated inclusion bodies by electron microscopy. The comparison of radiolabeled aggregated protein fractions of wild-type and clpE mutant cells during heat stress displayed a significant delay of protein disaggregation in the absence of ClpE. A kinetic Western blotting approach confirmed the long-term residence of ClpE in the insoluble cell fraction rather than in the cytoplasmic fraction. These observations indicate the involvement of ClpE in global protein disaggregation. As a characteristic structural element of ClpE, the N-terminal zinc finger domain was proven to be essential for basal in vitro ATPase activity.