Bag (Bcl2-associated athanogene) domains occur in a class of cofactors of the eukaryotic chaperone 70-kilodalton heat shock protein (Hsp70) family. Binding of the Bag domain to the Hsp70 adenosine triphosphatase (ATPase) domain promotes adenosine 5'-triphosphate-dependent release of substrate from Hsp70 in vitro. In a 1.9 angstrom crystal structure of a complex with the ATPase of the 70-kilodalton heat shock cognate protein (Hsc70), the Bag domain forms a three-helix bundle. inducing a, conformational switch in the ATPase that is incompatible with nucleotide binding. The same switch is observed in the bacterial Hsp70 homolog DnaK upon;binding of the structurally unrelated nucleotide exchange factor GrpE, Thus, functional convergence has allowed proteins with different architectures to trigger a conserved conformational shift in Hsp70 that leads to nucleotide exchange.