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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 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 of Hsp90, while its NTD interacts mostly with the client protein and it's CTD having more interaction with Hsp90.

Hsp90 is a 90-kDa protein that acts as a molecular chaperone for a variety of client proteins the majority of which are involved in signal transduction. It belongs to a group of proteins called heat shock proteins. Heat shock proteins are named for the response that organisms exhibit due to stresses at a cellular level such as elevated temperatures beyond the normal existing environment cells usually cope with. A major feature of this response is an alteration of an organism’s gene expression through increased heat shock protein production<ref name="hoter">PMID:30158430</ref>. Cdc37 is one the the main co-chaperones of Hsp90, it recruits kinase proteins for late folding and activation by binding to the kinase and delivering it to Hsp90. Cdk4 is a part of a kinase complex that is needed for cell cycle G1 phase progression. For the activation of Cdk4 there must be proper folding, binding of the regulator cyclin D, ATP binding, a positioning of the active site near the ATP binding site and phosphorylation of the amino acid residue T172. Hsp90 and the co-chaperone Cdc37 plays a crucial role in the late folding process of Cdk4 needed for its activation of which is critical for cell development<ref name="radli">PMID:29782836</ref>. When reading the current reseach material about Hsp90 it is generally stated that Hsp90 is involved in folding activities of its clients, however the Hsp90 homodimer is a very simple physical clamping mechanism and there is little detail currently in what ways Hsp90 is folding client proteins. Another major manner in which Hsp90 may be playing in its role as a chaperone for clients is by simply staging them to be quickly activated when and where needed. Cdk4 is bound to the Hsp90-Cdc37 complex in a way that renders it completely inactive and it is released in active form at the end of the G1 cell phase when the concentration of cyclin D has increased. Cdk4 then travels to the nucleus where it is involved in phosphorylating retinoblastoma gene products<ref name="radli" />. Hsp90 may be staging Cdk4 in the vicinity of where it needs to be and releasing it with temporal management allowing Cdk4 to perform its function with efficiency. A third chaperone duty Hsp90 may be performing for clients is to protect them from degredative processed such as being targeted by ubiquitin<ref name="cze" />. This may be especially the case with a client like Cdk4 that does show a tendency to be unstable at least in part and the instabilty it shows is inherent to its functioning as a dynamic triggering molecular apparatus.