Atomic structure of Hsp90:Cdc37:Cdk4 reveals Hsp90 regulates kinase via dramatic unfolding

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. Here we report a 3.9Å cryoEM structure of the Hsp90:Cdc37:Cdk4 kinase complex. 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 and extensive previous data, we propose unifying conceptual and mechanistic models of chaperone-kinase interactions. One Sentence Summary The first structure of a chaperone:kinase complex reveals that the Hsp90 system modulates and stabilizes kinases via a functionally relevant, unfolded open state.