The hsp70 chaperone machine performs many diverse roles in the cell, including folding of nascent proteins, translocation of polypeptides across organelle membranes, coordinating responses to stress, and targeting selected proteins for degradation. DnaJ is a member of the hsp40 family of molecular chaperones, which is also called the J-protein family, the members of which regulate the activity of hsp70s. DnaJ (hsp40) binds to dnaK (hsp70) and stimulates its ATPase activity, generating the ADP-bound state of dnaK, which interacts stably with the polypeptide substrate [1, [2].

Structurally, the DnaJ protein consists of an N-terminal conserved domain (called 'J' domain) of about 70 amino acids, a glycine-rich region ('G' domain') of about 30 residues, a central domain containing four repeats of a CXXCXGXG motif ('CRR' domain) and a C-terminal region of 120 to 170 residues.

Such a structure is shown in the following schematic representation:

 +------------+-+-------+-----+-----------+--------------------------------+ | J-domain | | Gly-R | | CXXCXGXG | C-terminal | +------------+-+-------+-----+-----------+--------------------------------+

The structure of the J-domain has been solved [3]. The J domain consists of four helices, the second of which has a charged surface that includes basic residues that are essential for interaction with the ATPase domain of hsp70 [4].

J-domains are found in many prokaryotic and eukaryotic proteins [5]. In yeast, three J-like proteins have been identified containing regions closely resembling a J-domain, but lacking the conserved HPD motif - these proteins do not appear to act as molecular chaperones [6].

1. Folding of newly translated proteins in vivo: the role of molecular chaperones. Annu. Rev. Biochem. 70, 603-47
2. The J-protein family: modulating protein assembly, disassembly and translocation. EMBO Rep. 5, 567-71
3. NMR structure of the J-domain and the Gly/Phe-rich region of the Escherichia coli DnaJ chaperone. J. Mol. Biol. 260, 236-50
4. Scanning mutagenesis identifies amino acid residues essential for the in vivo activity of the Escherichia coli DnaJ (Hsp40) J-domain. Genetics 162, 1045-53
5. A module of the DnaJ heat shock proteins found in malaria parasites. Trends Biochem. Sci. 17, 129

The hsp70 chaperone machine performs many diverse roles in the cell, including folding of nascent
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