UCSF

The Hsp90 molecular chaperone and its Cdc37 co-chaperone help stabilize and activate over half of the human kinome. However, neither the mechanism by which these chaperones assist their client kinases nor why some kinases are addicted to Hsp90 while closely related family members are independent is known. Missing has been any structural understanding of these interactions, with no full-length structures of human Hsp90, Cdc37 or either of these proteins with a kinase. My thesis work focused on characterizing these interactions on a molecular level. I started with in vitro investigation of interactions between Hsp90/Cdc37 and Her2 kinase domain, only to find that under most conditions these proteins didn’t form a complex, although there was a subtle difference in kinase activity in response to Hsp90. I also was able to form in vitro complex between Hsp90/Cdc37 and bRaf kinase and followed this up by some preliminary biochemical characterization and EM. However, the main focus of my work was cryoEM work on Hsp90/Cdc37/Cdk4 kinase complex, where I was able to attain a 3.9Å reconstruction of this complex. In this structure, Cdk4 is in a novel conformation, with its two lobes completely separated. Cdc37 mimics part of the kinase N-lobe, stabilizing an open kinase conformation by wedging itself between the two lobes. Finally, Hsp90 clamps around the unfolded kinase β5 strand and interacts with exposed N- and C- lobe interfaces, safely trapping the kinase in an unfolded state. Based on this novel structure, extensive previous data, and also recent advancements from other groups I propose a new model of kinase activity and regulation.