InterPro domain: IPR000740

Molecular chaperones are a diverse family of proteins that function to protect proteins in the intracellular milieu from irreversible aggregation during synthesis and in times of cellular stress. The bacterial molecular chaperone DnaK is an enzyme that couples cycles of ATP binding, hydrolysis, and ADP release by an N-terminal ATP-hydrolysing domain to cycles of sequestration and release of unfolded proteins by a C-terminal substrate binding domain. DnaK is itself a weak ATPase; ATP hydrolysis by DnaK is stimulated by its interaction with another co-chaperone, DnaJ. In prokaryotes the dimeric GrpE is the co-chaperone for DnaK, and acts as a nucleotide exchange factor, stimulating the rate of ADP release 5000-fold [ 1 ]. GrpE participates actively in response to heat shock by preventing aggregation of stress-denatured proteins: unfolded proteins initially bind to DnaJ, the J-domain ATPase-activating protein (Hsp40 family), whereupon DnaK hydrolyzes its bound ATP, resulting in a stable complex. The GrpE dimer binds to the ATPase domain of Hsp70 catalyzing the dissociation of ADP, which enables rebinding of ATP, one step in the Hsp70 reaction cycle in protein folding. Thus the co-chaperones DnaJ and GrpE are capable of tightly regulating the nucleotide-bound and substrate-bound state of DnaK in ways that are necessary for the normal housekeeping functions and stress-related functions of the DnaK molecular chaperone cycle [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ].

In eukaryotes, only the mitochondrial Hsp70, not the cytosolic form, is GrpE dependent. Over-expression of Hsp70 molecular chaperones is important in suppressing toxicity of aberrantly folded proteins that occur in Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis, as well as several polyQ-diseases such as Huntington's disease and ataxias.

The X-ray crystal structure of GrpE in complex with the ATPase domain of DnaK revealed that GrpE is an asymmetric homodimer, bent in a manner that favours extensive contacts with only one DnaK _ATPase monomer [ 11 ]. GrpE does not actively compete for the atomic positions occupied by the nucleotide. GrpE and ADP mutually reduce one another's affinity for DnaK 200-fold, and ATP instantly dissociates GrpE from DnaK.

1. Role of the major heat shock proteins as molecular chaperones. Annu. Rev. Cell Biol. 9, 601-34
2. GrpE, a nucleotide exchange factor for DnaK. Cell Stress Chaperones 8, 218-24
3. Crystal structure of a thermophilic GrpE protein: insight into thermosensing function for the DnaK chaperone system. J. Mol. Biol. 396, 1000-11
4. Crystal structure of DnaK protein complexed with nucleotide exchange factor GrpE in DnaK chaperone system: insight into intermolecular communication. J. Biol. Chem. 287, 21461-70
5. Review: mechanisms of disaggregation and refolding of stable protein aggregates by molecular chaperones. J. Struct. Biol. 135, 84-93
6. Chaperone-assisted protein folding in the cell cytoplasm. Curr. Protein Pept. Sci. 2, 227-44
7. The archaeal molecular chaperone machine: peculiarities and paradoxes. Genetics 152, 1277-83
8. Loss of the DnaK-DnaJ-GrpE chaperone system among the Aquificales. Mol. Biol. Evol. 29, 3485-95
9. Chaperone networking facilitates protein targeting to the bacterial cytoplasmic membrane. Biochim. Biophys. Acta 1843, 1442-56
10. Thermal adaptation of heat shock proteins. Curr. Protein Pept. Sci. 9, 552-66
11. Mutational analysis of the energetics of the GrpE.DnaK binding interface: equilibrium association constants by sedimentation velocity analytical ultracentrifugation. J. Mol. Biol. 339, 447-58