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-hydrolizing domain to cycles of sequestration and release of unfolded proteins by a C-terminal substrate binding domain. Dimeric GrpE is the co-chaperone for DnaK, and acts as a nucleotide exchange factor, stimulating the rate of ADP release 5000-fold [1]. DnaK is itself a weak ATPase; ATP hydrolysis by DnaK is stimulated by its interaction with another co-chaperone, DnaJ. 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.

Besides stimulating the ATPase activity of DnaK through its J-domain, DnaJ also associates with unfolded polypeptide chains and prevents their aggregation [2]. Thus, DnaK and DnaJ may bind to one and the same polypeptide chain to form a ternary complex. The formation of a ternary complex may result in cis-interaction of the J-domain of DnaJ with the ATPase domain of DnaK. An unfolded polypeptide may enter the chaperone cycle by associating first either with ATP-liganded DnaK or with DnaJ. DnaK interacts with both the backbone and side chains of a peptide substrate; it thus shows binding polarity and admits only L-peptide segments. In contrast, DnaJ has been shown to bind both L- and D-peptides and is assumed to interact only with the side chains of the substrate.

DnaJ comprises a 70-residue N-terminal domain (the J-domain); a 30-residue glycine-rich region (the G-domain); a central domain containing 4 repeats of a CxxCxGxG motif (the CRR-domain); and a 120-170 residue C-terminal region. The J- and CRR-domains are found in many prokaryotic and eukaryotic proteins [3], either together or separately.

The three components of the DnaK-DnaJ-GrpE system are typically encoded by consecutive genes. DnaJ homologues occur in many genomes, typically not encoded near DnaK and GrpE-like genes. Only some homologues are included in this family.

1. DnaJ-like proteins: molecular chaperones and specific regulators of Hsp70. Trends Biochem. Sci. 19, 176-81
2. cis-Effect of DnaJ on DnaK in ternary complexes with chimeric DnaK/DnaJ-binding peptides. FEBS Lett. 563, 146-50
3. A module of the DnaJ heat shock proteins found in malaria parasites. Trends Biochem. Sci. 17, 129

Molecular chaperones are a diverse family of proteins that function to protect proteins in the i
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