C.Prodromou and B.Panaretou contributed equally to this workHow the ATPase activity of Heat shock protein 90 (Hsp90) is coupled to client protein activation remains obscure. Using truncation and missense mutants of Hsp90, we analysed the structural implications of its ATPase cycle. C-terminal truncation mutants lacking inherent dimerization displayed reduced ATPase activity, but dimerized in the presence of 5¢-adenylamido-diphosphate (AMP-PNP), and AMP-PNPpromoted association of N-termini in intact Hsp90 dimers was demonstrated. Recruitment of p23/Sba1 to C-terminal truncation mutants also required AMP-PNP-dependent dimerization. The temperaturesensitive (ts) mutant T101I had normal ATP af®nity but reduced ATPase activity and AMP-PNP-dependent N-terminal association, whereas the ts mutant T22I displayed enhanced ATPase activity and AMP-PNP-dependent N-terminal dimerization, indicating a close correlation between these properties. The locations of these residues suggest that the conformation of the`lid' segment (residues 100±121) couples ATP binding to N-terminal association. Consistent with this, a mutation designed to favour`lid' closure (A107N) substantially enhanced ATPase activity and N-terminal dimerization. These data show that Hsp90 has a molecular`clamp' mechanism, similar to DNA gyrase and MutL, whose opening and closing by transient N-terminal dimerization are directly coupled to the ATPase cycle.