The crystal structure of the adenine nucleotide exchange factor GrpE in complex with the adenosine triphosphatase (ATPase) domain of Escherichia coli DnaK [heat shock protein 70 (Hsp70)] was determined at 2.8 angstrom resolution. A dimer of GrpE binds asymmetrically to a single molecule of DnaK. The structure of the nucleotide-free ATPase domain in complex with GrpE resembles closely that of the nucleotide-bound mammalian Hsp70 homolog, except for an outward rotation of one of the subdomains of the protein. This conformational change is not consistent with tight nucleotide binding. Two long alpha helices extend away from the GrpE dimer and suggest a role for GrpE in peptide release from DnaK.