Molecular Mechanism for the Chaperone Activity of Hsp90

点击翻译按钮获取中文摘要

Hsp90 is a molecular chaperone that assists the folding of newly synthesized proteins, and the maintenance of the folded state of proteins within cells. Hsp90 is abundant in the cytoplasm and nucleus in many cell types, ranging from bacterial to human, and is involved in numerous biochemical processes. The mode of action for the chaperone function of Hsp90 is unique compared to typical chaperones, as it binds partially or fully folded proteins, not the hydrophobic regions of unfolded proteins. Hsp90 assists the folding of a number of “client” proteins that include mutant, overexpressed, or misregulated forms of signaling proteins such as kinases, steroid receptors, and transcription factors, underscoring the key role for Hsp90 in signalling. The current view of the molecular mechanism for Hsp90 is that client protein activation involves binding to specific Hsp90 conformations, wherein the chaperone cycles through the various conformations in an ATP-dependent manner. The relationship between the conformational cycling Hsp90, the binding of partially, or fully folded proteins, and the binding of co-chaperones is not yet fully understood. A key player in the function of Hsp90 is the co-chaperone Aha1, which interacts with Hsp90, accelerates the ATPase activity, and ultimately helps activate kinases. It remains an outstanding problem in the Hsp90 field to develop a mechanism to describe the function of the chaperone. To that end, we derived a physicochemical kinetic model to describe the time dependent changes in the concentrations of the various ligands, proteins, protein-protein, and protein-ligand complexes for the cycle. This fundamental physical model includes experimentally determined protein-protein, and protein-ligand association rates, and the rate limiting catalytic step for ATP hydrolysis. Furthermore, the model includes the interaction of Hsp90 with the ATPase activating co-chaperone Aha1, and the interaction of Hsp90 with a folding intermediate from a client protein in equilibrium between folded, intermediate, and unfolded states. We plan to blend insights derived from the kinetic model with a combination of NMR spectroscopy and biochemical assays to better understand the molecular function of Hsp90. A detailed understanding of the mechanism can yield insights into the normal and essential function of chaperones in cells. This physicochemical kinetic model represents a novel synthesis of our understanding of the chaperone Hsp90. There is considerable interest in the chaperone field regarding the function of relative levels of Hsp90 and its activation by cochaperones in cells to control the function of proteins, or to suppress the function of kinases that control signaling pathways. The physicochemical kinetic model can be applied to such problems to predict how co-chaperones regulate the biological activity of Hsp90.**