Hsp90 is regulated by a switch point in the C-terminal domain

Heat shock protein 90 (HSP90) is an abundant, dimeric ATP-dependent molecular chaperone, and ATPase activity is essential for its in vivo functions. S-nitrosylation of a residue located in the carboxy-terminal domain has been shown to affect HSP90 activity in vivo. To understand how variation of a specific amino acid far away from the amino-terminal ATP-binding site regulates HSP90 functions, we mutated the corresponding residue and analysed yeast and human HSP90 variants both in vivo and in vitro. Here, we show that this residue is a conserved, strong regulator of HSP90 functions, including ATP hydrolysis and chaperone activity. Unexpectedly, the variants alter both the C-terminal and N-terminal association properties of HSP90, and shift its conformational equilibrium within the ATPase cycle. Thus, S-nitrosylation of this residue allows the fast and efficient fine regulation of HSP90.