This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The ubiquitous molecular chaperone Hsp90 is required in the maturation and maintanence of signalling and regulatory molecules including serine/threonine kinases. Hsp90 function requires ATP hydrolysis and this hydrolysis cycle is coupled to large conformational changes in the protein. Hsp90 is further regulated by the interaction of other proteins known as cochaperones. For the activation of kinase clients, Hsp90 forms a complex with the cochaperone p50. It is now well understood how the Hsp90 complex functions to activate its client proteins. Using SAXS, we propose to gain a structural insight into the mechanism of Hsp90's interaction with p50 and how this complex then interacts with the client protein Cdk4. We also propose to use SAXS to dissect the role of Hsp90's ATPase in regulating its interaction with both p50 and Cdk4. The structural information obtained from SAXS will be included with ongoing biochemical studies, and the combination of techniques will provide a detailed mechanistic insight into the activation of kinases by Hsp90.