UCSF

The molecular chaperone Hsp90 collaborates with the phosphorylated Cdc37 cochaperone for the folding and activation of its many client kinases. Reconstitution of kinase loading onto Hsp90 has proven challenging. In this work we describe our efforts towards isolating a complex that captures kinase loading onto Hsp90, and we study the constituents involved in initial Hsp90-Kinase interactions. Kinases are highly regulated by chaperones like Hsp90, but also through phosphorylation at specific regulatory sites. In the second half of this work, we explore how the cochaperone phosphatase PP5 dephosphorylates the kinase CRaf and the Hsp90 cochaperone Cdc37 in an Hsp90-dependent manner. Although dephosphorylating Cdc37 has been proposed as a mechanism for releasing Hsp90-bound kinases, here we show that Cdc37 dephosphorylation does not occur if a kinase is simultaneously bound to Hsp90. Our cryoEM structure of PP5 in complex with Hsp90:Cdc37:CRaf reveals how Hsp90 both activates PP5 and scaffolds its dephosphorylation of the bound CRaf phosphorylation sites. From this work we begin to get insight into the structure of an Hsp90:Cdc37 complex, and how PP5 might dephosphorylate Cdc37’s key kinase-recruiting phosphorylation site.