Tuning of DnaK chaperone action by nonnative protein sensor DnaJ and thermosensor GrpE.

DnaK, an Hsp70 molecular chaperone, processes its substrates in an ATP-driven cycle, which is controlled by the co-chaperones DnaJ and GrpE. The kinetic analysis of substrate binding and release has as yet been limited to fluorescence-labeled peptides. Here, we report a comprehensive kinetic analysis of the chaperone action with protein substrates. The kinetic partitioning of the (ATP x DnaK) x substrate complexes between dissociation and conversion into stable (ADP x DnaK) x substrate complexes is determined by DnaJ. In the case of substrates that allow the formation of ternary (ATP x DnaK) x substrate x DnaJ complexes, the cis-effect of DnaJ markedly accelerates ATP hydrolysis. This triage mechanism efficiently selects from the (ATP x DnaK) x substrate complexes those to be processed in the chaperone cycle; at 45 degrees C, the fraction of protein complexes fed into the cycle is 20 times higher than that of peptide complexes. The thermosensor effect of the ADP/ATP exchange factor GrpE retards the release of substrate from the cycle at higher temperatures; the fraction of total DnaK in stable (ADP x DnaK) x substrate complexes is 2 times higher at 45 degrees C than at 25 degrees C. Monitoring the cellular situation by DnaJ as nonnative protein sensor and GrpE as thermosensor thus directly adapts the operational mode of the DnaK system to heat shock conditions.