Abstract
Hsp90 is a dimeric molecular chaperone that is important for the folding, stabilization, activation, and maturation of hundreds of protein substrates called “clients” in cells. Dozens of co‐chaperones and hundreds of post‐translational modifications (PTMs) regulate the ATP‐dependent client activation cycle. The Aha1 co‐chaperone is the most potent stimulator of the ATPase cycle of Hsp90 and phosphorylation of threonine 22 in Hsp90 can regulate the recruitment of Aha1 in cells. We report here that phosphorylation of threonine 22 regulates specific aspects of Aha1 function after recruitment occurs. The phosphomimetic substitution, T22E, neutralizes the action of the Aha1 NxNNWHW motif. Moreover, this substitution can exert this effect from only one protomer of the Hsp90 dimer. This work sheds light on how asymmetric modifications in the Hsp90 dimer can functionalize individual protomers and fine‐tune the Hsp90 cycle.