InterPro: IPR015609 Molecular chaperone, heat shock protein, Hsp40, DnaJ

The Escherichia coli Hsp40 DnaJ and Hsp70 DnaK cooperate in the binding of proteins at intermediate stages of folding, assembly, and translocation across membranes [1]. Binding of protein substrates to the DnaK C-terminal domain is controlled by ATP-binding and hydrolysis in the N-terminal ATPase domain. The interaction of DnaJ with DnaK is mediated at least in part by the highly conserved N-terminal J-domain of DnaJ. The J-domain interaction is localised to the ATPase domain of DnaK and is likely to be dominated by electrostatic interactions. J-domain may tether DnaK to DnaJ-bound substrates, which DnaK then binds with its C-terminal peptide-binding domain. The peptide-binding domain of DnaJ is comprised of a beta sandwich made up of 6 beta-strands divided into 2 sheets.

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. Dimeric GrpE is the co-chaperone for DnaK, and acts as a nucleotide exchange factor, stimulating the rate of ADP release 5000-fold [2]. 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 [3]. 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.