Regulatory mechanisms of the Hsp90 chaperone machinery

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PROJECT SUMMARY The molecular chaperone Heat Shock Protein-90 (Hsp90) is essential for the folding and activity of an array of `client' proteins involved in signal transduction pathways. They are also responsible for many maladies including cancer, neurodegenerative, autoimmune and inflammatory diseases. Hsp90 inhibition strategies are currently being explored in these diseases in pre-clinical studies and clinical trials, however the optimal use of Hsp90-targeted therapeutics remains unknown. This is partly due to our limited knowledge of Hsp90 regulation in cells. Unraveling the detailed regulatory mechanisms of Hsp90 function in cells can provide new strategies to increase the cellular potency of Hsp90 inhibitors. Hsp90 chaperone function is coupled to its ATPase activity, which is regulated by co-chaperones and posttranslational modifications (PTMs). However, it is unclear how these regulatory components work together to fine tune Hsp90 function and also contribute towards drug sensitivity. During the past five years we have made major contributions towards the understanding of Hsp90 regulation by co-chaperones and PTMs. i) New co-chaperones: We have identified three new co-chaperones, FNIP1, 2 (collectively FNIPs) and Tsc1, that decelerate the chaperone cycle and facilitate chaper- oning of both kinase and non-kinase clients. They are regulated by PTMs (phosphoryla- tion, O-GlcNAcylation, SUMOlyation and ubiquitination). Their expression also enhances Hsp90 binding to drugs and consequently sensitizes cells to Hsp90 inhibitors. ii) Post- translational modification of Hsp90: Our work during the past decade on Hsp90 PTMs has redefined the regulation of its chaperone activity and revealed the reciprocal regula- tory mechanisms between client proteins, co-chaperones, and Hsp90. We have recently shown that loss of TSC1 co-chaperone leads to hypoacetylation of Hsp90 and elevated its ATPase activity. It also subsequently decreased Hsp90 binding to its inhibitors. Our long-term goal is to unravel the molecular mechanism of Hsp90 chaperone regulation in cells and regulatory factors enhancing cellular potency of Hsp90 inhibitors. Our strategy is to use biochemical, biophysical and cell-based assays to decipher the interconnectivi- ty and compensatory mechanisms between the co-chaperones and PTMs. Our vision is to utilize this information to dissect the intricate network of regulatory signals involved in fine tuning Hsp90 function and their impact towards cellular sensitivity to Hsp90 inhibi- tors.