ATP-induced protein Hsp70 complex dissociation requires K⁺ but not ATP hydrolysis

Abstract

THE molecular chaperone proteins, particularly Hsp60 and Hsp70, have been implicated in essential cell functions under both normal and stress conditions (reviewed in refs 1–5). Members of the family of heat-shock proteins ofM_r70K, Hsp70, bind to unfolded proteins and short peptides^6–13. Addition of Mg–ATP results in the dissociation of the substrate polypeptides from the chaperone^7–11, but as ATP-γS (an ATP analogue that is only slowly hydrolysable) cannot substitute for ATP in this reaction^7,9,11, it has been concluded that ATP hydrolysis is necessary to dissociate Hsp70–substrate protein complexes. By independently measuring the rates of ATP hydrolysis and substrate protein dissociation, we show here that Mg–ATP binding but not Mg–ATP hydrolysis is essential for substrate dissociation. We also show that there is an absolute requirement for K⁺for the effect of Mg–ATP: only the combination of K⁺ and Mg–ATP will cause the conformational change in HspTO that is necessary for substrate dissociation. Moreover, in the absence of K⁺, Mg–ATP favours complex formation. We consider these results in terms of a G-protein-like model.