PROJECT SUMMARY / ABSTRACT Tetratricopeptide repeat (TPR) cochaperones are a diverse family of adaptor proteins that cooperate with the heat shock protein (Hsp) 70 and Hsp90 chaperone systems. These modular proteins are composed of their eponymous TPR domain, which mediates complex formation with the Hsps, and an enzymatic or scaffolding domain that can aid the chaperone in client recruitment or quality control. The importance of these accessory functionalities in maintaining protein homeostasis is evidenced by the implication of TPR cochaperones in a wide range of diseases from neurodegeneration to cancer. However, our understanding of the molecular mechanisms that underpin substrate recognition by these proteins is incomplete. In particular, there is increasing evidence that TPR domains can recruit proteins beyond their canonical chaperone binding partners, and the significance of this alternative pathway is currently unknown. In addition, it is generally difficult to establish substrate relationships for TPR cochaperones given the compensation that can occur within the protein homeostasis network following genetic perturbations. Development of chemical probes that can specifically inhibit TPR cochaperone complexes with high temporal resolution is therefore highly desirable. The effort to develop such tools would be greatly augmented by an understanding of which molecular features of TPR domains can be exploited to achieve high affinity and selective binding. The objective of this proposal is to develop a chemical toolkit that helps solve both of these problems by decoding the molecular logic of interactions between TPR cochaperones and their substrates. In my first aim, I will develop chemical proteomic tools to profile the binding of TPR cochaperones and measure changes in substrate association in response to different stimuli. These probes will also enable the specificity of TPR inhibitors to be assessed in a rapid fashion. In my second aim, I will refine a predictive scoring function in order to comprehensively identify substrates that are autonomously recognized by the E3 ligase CHIP. In exploring the molecular determinants of CHIP's interactions with its substrates, I will also identify features that enable an individual TPR cochaperone to bind ligands that are distinct from its related family members. This work is significant because it will provide fundamental insights into the rules governing the biology of TPR cochaperones, and will serve as the bedrock for future substrate discovery and probe development efforts across this protein family. The proposed studies will also provide a training environment that is ideally suited to my goal of becoming a translational chemical biologist, with opportunities to gain experience with state-of-the-art techniques while also cultivating my ability to conduct research independently.