Sti1 Is a Non-competitive Inhibitor of the Hsp90 ATPase

The molecular chaperone Hsp90 is known to be involved in the activation of key regulatory proteins such as kinases, steroid hormone receptors, and transcription factors in an ATP-dependent manner. During the chaperone cycle, Hsp90 has been found associated with the partner protein Hop/Sti1, which seems to be required for the progression of the cycle. However, little is known about its specific function. Here we have investigated the interaction of Sti1 from Saccharomyces cerevisiae with Hsp90 and its influence on the ATPase activity. We show that the inhibitory mechanism of Sti1 on the ATPase activity of Hsp90 is non-competitive. Sti1 binds to the N- and C-terminal part of Hsp90 and prevents the N-terminal dimerization reaction that is required for efficient ATP hydrolysis. The first 24 amino acids of Hsp90, a region shown previously to be important for the association of the N-terminal domains and stimulation of ATP hydrolysis, seems to be important for this interaction.