ATPase Activity and Molecular Chaperone Function of the Stress70 Proteins

The codons for the amino acid residues making up the proposed ATP-binding sites of the maize (Zea mays L.) endoplasmic reticulum and tomato (Lycopersicon esculentum) cytoplasmic Stress70 proteins were deleted from their respective cDNAs. The deletions had little effect on the predicted secondary structure characteristics of the encoded proteins. Both wild-type and mutant proteins were expressed in Escherichia coli and purified to electrophoretic homogeneity. The mutant recombinant proteins did not bind to immobilized ATP columns, had no detectable ATPase activity, and were unable to function in vitro as molecular chaperones. Additionally, the inability to bind ATP was associated with changes in the oligomerization state of the Stress70 proteins.