Assembly mechanism of early Hsp90-Cdc37-kinase complexes.

Molecular chaperones have an essential role for the maintenance of a balanced protein homeostasis. Here, we investigate how protein kinases are recruited and loaded to the Hsp90-Cdc37 complex, the first step during Hsp90-mediated chaperoning that leads to enhanced client kinase stability and activation. We show that conformational dynamics of all partners is a critical feature of the underlying loading mechanism. The kinome co-chaperone Cdc37 exists primarily in a dynamic extended conformation but samples a low-populated, well-defined compact structure. Exchange between these two states is maintained in an assembled Hsp90-Cdc37 complex and is necessary for substrate loading. Breathing motions at the N-lobe of a free kinase domain partially expose the kinase segment trapped in the Hsp90 dimer downstream in the cycle. Thus, client dynamics poise for chaperone dependence. Hsp90 is not directly involved during loading, and Cdc37 is assigned the task of sensing clients by stabilizing the preexisting partially unfolded client state.