Folding properties of the nucleotide exchange factor GrpE from Thermus thermophilus: GrpE is a thermosensor that mediates heat shock response

Hsp70 proteins like DnaK bind unfolded polypeptides in a nucleotide-dependent manner. The switch from high-affinity ADP-state to low-affinity ATP-state with concomitant substrate release is accelerated significantly by GrpE proteins. GrpE thus fulfils an important role in regulation of the chaperone cycle. Here, we analysed the thermal stability of GrpE from Thermus thermophilus using differential scanning calorimetry and CD-spectroscopy. The protein exhibits unusual unfolding characteristics with two observable thermal transitions. The first transition is CD-spectroscopically silent with a transition midpoint at 90 °C. The second transition, mainly constituting the CD-signal, ranges between 100 and 105 °C depending on the GrpETth concentration, according to the model N29⇌I2⇌2U. Using a C-terminally truncated version of GrpETth it was possible to assign the second thermal transition to the dimerisation of GrpETth, while the first transition represents the completely reversible unfolding of the globular C-terminal domain. The unfolding of this domain is accompanied by a distinct decrease in nucleotide exchange rates and impaired binding to DnaKTth. Under heat shock conditions, the DnaK·ADP·protein-substrate complex is thus stabilised by a reversibly inactivated GrpE-protein that refolds under permissive conditions. In combination with studies on GrpE from Escherichia coli presented recently by Christen and co-workers, it thus appears that the general role of GrpE is to function as a thermosensor that modulates nucleotide exchange rates in a temperature-dependent manner to prevent substrate dissociation at non-permissive conditions.