Sandbox Reserved 1472: Difference between revisions

The Hsp90-Cdc37-Cdk4 complex is made up of the Heat Shock Protein 90 chaperone molecule, Cell Division Cycle 37 co-chaperone molecule, and the Cyclin-dependent 4 Kinase client molecule. In the cryo-electron microscopy structure of the Hsp90-Cdc37-Cdk4 complex the β4-β5 sheet of Cdk4 is unfolded which separates it into two lobes, Cdc37 wedges itself between these lobes, and Hsp90 clamps around the β5 sheet of Cdk4<ref name="verba" />. While Cdk4 as the client protein of this complex is important because its proper functioning is the intended goal of the complex, the <scene name='80/800651/Hsp90_homodimer/1'>Hsp90 homodimer</scene> is the key factor in the process. The main structure of Hsp90 can be broken down into three domains; N-terminal domain (NTD), Middle domain (MD), and the C-terminal domain (CTD) see '''Figure 1'''. The NTD is the site where ATP binds closing the "clamp", the MD is primarily the site of client binding, and the CTD is responsible for dimerization of the promoters that forms the biological unit homodimer<ref name="hoter" />. Before the full complex is formed Cdc37 will bind with Cdk4 forming a <scene name='80/800651/Cdc37-cdk4_complex/4'>Cdc37-Cdk4 complex</scene> (Cdc37-Pink, Cdk4-Yellow). Cdc37 as in Hsp90 can be broken down into an CTD, MD, and NTD. In the full ternary complex Cdc37's MD binds to the ATP lid-segment, a group of residues that when ATP binds to Hsp90 they fold over to trap the nucleotides. Cdc37's NTD interacts mostly with the client protein and it's CTD has binding interactions with the Hsp90 homodimer<ref name="pearl" />. Cdk4 structure is more simple than Hsp90 and Cdc37 only consisting of an NTD and a CTD. Molecular dynamic simulations have shown that when complexed with cyclin-D3 in an inactive conformation considerable flexibility was noted in its N-lobe regions notably at its αC-helix, αC-β4 loop, and β4-β5 sheet<ref name="cze" />. The <scene name='80/800651/Hsp90-cdc37-cdk4_complex/1'>Hsp90-Cdc37-Cdk4 complex</scene> is an intricate multicomponent system made of a generalized clamping mechanism, a narrowing adaptive recruiter/tuner, and dynamic protein client molecule.  

Hsp90 and the co-chaperone Cdc37 are responsible for about 60% of kinases to achieve an active state<ref name="verba">PMID:27339980</ref>. One of the questions that researchers have had is why some kinases are clients of Hsp90 and Cdc37 and not others. There is evidence Cdc37 works by recognizing conformational instability of kinase clients and changing their folding landscapes as it binds to the client and recruits it to Hsp90<ref name="cze">PMID:29267381</ref>. In this way Cdc37 would be selecting kinase clients for Hsp90, where less thermodynamically stable clients would be delivered to Hsp90 and more stable clients would not. The open and closed conformations of Hsp90 are driven by the binding and release of adenosine triphosphate (ATP) in the N-terminal domain (NTD), where the closed state has ATP bound and the open state does not. Before Hsp90 will interact with Cdc37 and Cdk4 they will first interact with each other. Evidence points to the NTD of Cdc37 being the region that recognizes both client and non-client kinases. When the NTD of Cdc37 was removed it was found to fail in interacting with bRaf, another known kinase client of the Hsp90-Cd37 system<ref>PMID:27105117</ref>. The CTD of Cdc37 has been found to function in providing additional protection when the Cdk4 client is in a partially unfolded state during the binding process<ref name="cze" /> The action that initiates Cdc37 interaction  with kinases such as Cdk4 is its phosphorylation by casein kinase 2. Then once Cdc37 has interacted with Cdk4, opening it by a separation of it's NTD and CTD followed by binding of the two proteins together they will come into contact with an open conformation Hsp90 homodimer. ATP will then bind to the NTDs of Hsp90 and cause it to close trapping the partially closed Cdk4 molecule client. Hsp90 will open upon ATP hydrolysis resulting in the release of the client, and in the process of this Cdc37 is dephosphorylated by protein phosphatase 5 allowing its release as well<ref name="verba" />. When Hsp90 is in the chaperone-client complex it becomes more stable than when it is by its self. Hsp90 MD residues in its core as well as residues 398-453 that from a three-helix bundle both exhibit reduced thermal fluctuations as new intra- and intermolecular interactions are formed when in the chaperone-client complex. The residues 513-519 that are interfacial MD-CTD residues also show an increase in stability through reduced thermal fluctuations as does helix 15 in the CTD. While the MDs and CTDs of the Hsp90 homodimer are stabilized in the chaperone-client complex the NTDs undergo and increase in flexibility. When transitioning from an unbound state to the chaperone-client complex Hsp90 experiences a relocation of major hinge clusters shifting from the NTD and NTD-MD regions to its MD-CTD regions as this happens the residues remaining in the NTD become less stabilized<ref name="cze" />. The client binding surfaces of Hsp90 range over a large area from its NTDs to its MDs and this is indicative of Hsp90 forming lots of low-energy binding patches with clients<ref name="radli" />. It must be remembered that Cdk4 is only one of many Hsp90 client proteins and Cdc37 only one of a number of Hsp90 co-chaperones. It can be inferred that Hsp90 uses the large surface area generalized binding interfaces as an effective strategy to deal with multiple clients. Also the use of co-chaperones is another way in which Hsp90 can fine tune its interactions with the multiple clients it must interact and manipulate. Another heat shock protein named HSP70 is often involved with Hsp90 chaperoning systems. When these chaperones are involved in interacting with the same client, Hsp90 interaction will follow Hsp70 client interaction. Hsp70 identifies and is able to help with the proper folding of hydrophobic areas of proteins. The interfaces of Hsp90 are less hydrophobic with charged residues being more prevalent. The binding interfaces for Cdk4 and Tau a protein that functions in microtubule construction and stabilization interact with Hsp90 residues that have a net positive charge. However it is possible for other clients to bind to Hsp90 where it has a net negative charge such as glucocorticoid receptor (GR) a steroid hormone receptor that regulates development, metabolism, and immunes responses. Cdk4 contrast with both Tau and GR in that its binding interfaces are actually much more hydrophobic in nature that the other two<ref name="radli" />. Overall the binding complex of Hsp90-Cdc37-Cdk4 causes global conformational rigidity in the binding areas of Hsp90, and at the same time Cdk4 is rendered less stable by having its domains split apart while complexed. This leads to a reversible entropy transfer where the chaperone flexible regions are ordered and allowing the client to be staged within the complex in a less stable more dynamic state<ref name="cze" />.