Trends in Biochemical Sciences
ReviewHow Hsp90 and Cdc37 Lubricate Kinase Molecular Switches
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Although Initially Observed Decades Ago, Details of Hsp90–Kinase Interactions Are Only Now Emerging
Protein kinases are widely recognized to be key players in regulating the cell cycle, cell signaling, and early organism development 1, 2. Dysregulation of their normal function leads to a variety of human disease including cancer, and consequently protein kinases have become key therapeutic targets over the past decade [3]. During early work on the remarkable transforming power of v-Src kinase in chick embryos, two additional proteins were discovered that closely associated with v-Src,
What Is a Client Kinase?
Although historically kinases have been broken into binary client and non-client categories, recent results affirm the view that most if not all kinases depend on and interact with the Hsp90/Cdc37 system, at least during initial folding. Thus, instead of asking if a kinase is a client, the more appropriate question is where it lies in a continuum of chaperone-dependence. Before discussing this more fully, it is useful to consider the two main strategies used when seeking Hsp90/Cdc37 kinase
Structural Hallmarks of Client Kinases
The kinase domain has been definitively shown to be both necessary and sufficient for interaction with Hsp90/Cdc37 [9]. This is curious because, structurally, eukaryotic kinase domains are all very similar with the exception of an order/disorder transition at the activation loop or movement of the αC helix (Figure 1A). The most recent data from multiple groups, together with the failure to find a general client sequence motif, argues that the relevant interaction surfaces must be present in
How Does Cdc37 Interact with Client Kinases and Promote Hsp90 Function?
Cdc37 binds to kinase clients in the absence of Hsp90, whereas Hsp90 interacts only weakly without Cdc37, therefore Cdc37 constitutes an independent kinase-binding unit 8, 22. Early analysis of protein fragments identified the Cdc37 N-terminal domain as the kinase-interacting domain, the middle domain (residues 148–276) as a proteolytically stable Hsp90-interacting domain, and a C-terminal domain of unclear function (Figure 2A) [26]. In the cryoEM structure a conserved His20 Pro21 Asn22 motif
Cdc37 Interactions with Hsp90
The first Hsp90–Cdc37 interaction interface was revealed in 2004 [34] via the crystal structure of a fragment of human Cdc37 containing the middle and C-terminal domains (residues 148–348; Figure 2A) interacting with the N-terminal domain (NTD) of yeast Hsp90. That structure is consistent with prior fragment-binding assays and also explains how Cdc37 can inhibit Hsp90 ATPase activity. Cdc37 interactions displace the catalytic water, prevent the region called the ‘lid’ on Hsp90 NTD from coming
Regulation of Hsp90/Cdc37/Kinase Interactions
Both Hsp90 and Cdc37 are intricately regulated by phosphorylation. Cdc37 can be phosphorylated on Tyr4 and Tyr298 by Yes kinase [37] and on Ser13 by CKII kinase [38]. Notably, both these kinases are also Hsp90 clients. While the effects of Tyr4 phosphorylation are unclear, it was shown that Tyr298 phosphomimics lead to impaired Cdc37–kinase binding. This site is in the Cdc37 C-terminal domain, corroborating recent NMR results of the involvement of this domain in some step of kinase binding.
The
What Are the Functional Consequences of Kinase–Hsp90/Cdc37 Interactions?
As discussed previously, the canonical interpretation of the Hsp90/Cdc37 system is that it helps fold, stabilize and activate its kinase clients. However, clear-cut evidence for activation decoupled from simple stabilization is missing. In several reports on ErbB2 41, 42 and Ire1 [43], where authors carefully and promptly monitored signaling activity in response to an Hsp90 inhibitor, the kinase was actually initially activated, followed by a later degradation. In addition, signaling-driven
Concluding Remarks
Advances in two different areas have allowed rapid progress in the structural understanding of Hsp90–Cdc37–kinase interactions. First, construction of a solubilized mutant of b-Raf kinase that is marginally stable but still purifiable from E. coli allowed rigorous in vitro biochemical and NMR investigations of Cdc37–kinase interactions. Second, advances in cryoEM have allowed direct structural investigation of Hsp90–Cdc37–kinase complexes isolated intact from eukaryotic cells. These advances
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