The solution structure of the 21 kDa substrate-binding domain of the Escherichia coli Hsp70-chaperone protein DnaK (DnaK 386-561) has been determined to a precision of 1.00 A (backbone of the beta-domain) from 1075 experimental restraints obtained from multinuclear, multidimensional NMR experiments. The domain is observed to bind to its own C-terminus and offers a preview of the interaction of this chaperone with other proteins. The bound protein region is tightly held at a single amino acid position (a leucyl residue) that is buried in a deep pocket lined with conserved hydrophobic residues. A second hydrophobic binding site was identified using paramagnetically labeled peptides. It is located in a region close to the N-terminus of the domain and may constitute the allosteric region that links substrate-binding affinity with nucleotide binding in the Hsp70 chaperones.