Molecular Mechanism for the Chaperone Activity of Hsp90

Hsp90是一种分子伴侣，帮助新合成的蛋白质折叠，并维持细胞内蛋白质的折叠状态。从细菌到人类，许多细胞类型的细胞质和细胞核中都含有丰富的Hsp90，并参与许多生物化学过程。与典型的伴侣蛋白相比，Hsp90的伴侣蛋白功能的作用模式是独特的，因为它结合部分或完全折叠的蛋白质，而不是未折叠蛋白质的疏水区域。Hsp90帮助折叠一些“客户”蛋白，包括突变、过表达或错误调节的信号蛋白，如激酶、类固醇受体和转录因子，强调了Hsp90在信号传导中的关键作用。目前关于Hsp90分子机制的观点是，客户蛋白的激活涉及与特定Hsp90构象的结合，其中伴侣蛋白以atp依赖的方式在各种构象中循环。构象循环Hsp90与部分折叠或完全折叠蛋白的结合以及共伴侣蛋白的结合之间的关系尚不完全清楚。Hsp90功能的一个关键参与者是协同伴侣Aha1，它与Hsp90相互作用，加速atp酶活性，最终帮助激活激酶。建立一种描述伴侣蛋白功能的机制是目前Hsp90领域的一个突出问题。为此，我们推导了一个物理化学动力学模型来描述各种配体、蛋白质、蛋白质-蛋白质和蛋白质-配体复合物在循环中的浓度随时间的变化。这个基本的物理模型包括实验确定的蛋白质-蛋白质和蛋白质-配体结合速率，以及ATP水解的限速催化步骤。此外，该模型还包括Hsp90与激活atp酶的共伴侣Aha1的相互作用，以及Hsp90与来自客户蛋白的折叠中间体在折叠、中间和未折叠状态之间平衡的相互作用。我们计划将从动力学模型中获得的见解与核磁共振光谱和生化分析相结合，以更好地了解Hsp90的分子功能。对这一机制的详细了解可以深入了解细胞中伴侣蛋白的正常和基本功能。这种物理化学动力学模型代表了我们对伴侣蛋白Hsp90的理解的一种新的综合。在伴侣蛋白领域，人们对Hsp90相对水平的功能及其在细胞中被伴侣蛋白激活来控制蛋白质的功能或抑制控制信号通路的激酶的功能非常感兴趣。物理化学动力学模型可以应用于此类问题，以预测协同伴侣如何调节Hsp90的生物活性。

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.