Prof. Daniel Southworth, U-M "The Structure and Mechanism of the Hsp90 Molecular Chaperone: A Ubiquitous Facilitator of Cellular Signaling "

Assistant Professor of Biological Chemistry; Research Assistant Professor of Life Sciences Institute. ABSTRACT- The Hsp90 molecular chaperone is the central player in a multi-component complex that promotes ATP-dependent rearrangement and activation of essential substrate ‘client’ proteins including nuclear receptors and cell cycle kinases. Despite the identification of numerous clients, little is known how Hsp90 conformational changes and cochaperone interactions contribute to rearrangement and activation of substrates. Furthermore, how Hsp90 and Hsp70 function together in critical triage decisions that lead to ubiquitination and degradation of misfolded clients is unknown. Using single particle electron microscopy (EM) we have determined 3-D structures of Hsp90 in dramatically different conformational states, establishing a three-state universal nucleotide cycle. The development of key crosslinking methods to trap dynamic Hsp90:cochaperone complexes was critical in achieving a cryo-EM reconstruction of the human Hsp90:Hop complex that receives client proteins from Hsp70. In the structure Hop stabilizes a novel apo state where Hsp90 is simultaneously poised for client loading by Hsp70 and subsequent N-terminal dimerization and ATP hydrolysis. Recent cryo-EM studies of the Hsp90:Hop:Hsp70 complex further support an asymmetric mechanism for Hsp90 regulation and client loading. Finally, future work is aimed at understanding how these complexes interact with the p53 tumor suppressor client protein and ultimately how molecular chaperones function to determine the fate of protein substrates in achieving cellular homeostasis.