Department of Pharmacology, University of Michigan Medical School, Ann Arbor, 48109, USA.
Exp Biol Med (Maywood). 2010 Mar;235(3):278-89. doi: 10.1258/ebm.2009.009250.
The Hsp90/Hsp70-based chaperone machinery plays a well-established role in signaling protein function, trafficking and turnover. A number of recent observations also support the notion that Hsp90 and Hsp70 play key roles in the triage of damaged and aberrant proteins for degradation via the ubiquitin-proteasome pathway. In the mid-1990s, it was discovered that Hsp70 is required for ubiquitin-dependent degradation of short-lived and abnormal proteins, and it became clear that inhibition of Hsp90 uniformly leads to the proteasomal degradation of Hsp90 client proteins. Subsequently, CHIP and parkin were shown to be Hsp70-binding ubiquitin E3 ligases that direct ubiquitin-charged E2 enzymes to the Hsp70-bound client protein. Stabilization by Hsp90 reflects the interaction of the chaperone with the ligand binding cleft of the client protein. These hydrophobic clefts must be open to allow passage of ligands to binding sites in the protein interior, and they are inherent sites of conformational instability. Hsp90 stabilizes the open state of the cleft and prevents Hsp70-dependent ubiquitination. In the model we propose here, progressive oxidative events result in cleft opening as the initial step in protein unfolding, and as long as Hsp90 can interact to stabilize the cleft, it will buffer the effect of oxidative damage. When cleft opening is such that Hsp90 can no longer interact, Hsp70-dependent ubiquitination occurs. We summarize evidence that Hsp90 interacts very dynamically with a variety of proteins that are not classic Hsp90 clients, and we show that this dynamic cycling of Hsp90 with nitric oxide synthase protects against CHIP-mediated ubiquitination. Scientific interest to date has focused on stringent regulation of the classic client proteins, which have metastable clefts and are inherently short lived. But, the recognition that Hsp90 cycles dynamically with longer lived proteins with more stable clefts may permit extension of the triage model to the quality control of damaged proteins in general.
Hsp90/Hsp70 基伴侣蛋白机器在信号蛋白功能、运输和周转中起着既定的作用。最近的一些观察结果也支持这样的观点，即 Hsp90 和 Hsp70 在通过泛素蛋白酶体途径对受损和异常蛋白质进行分类降解方面发挥着关键作用。在 20 世纪 90 年代中期，人们发现 Hsp70 是短寿命和异常蛋白质的泛素依赖性降解所必需的，并且很明显，Hsp90 的抑制会导致 Hsp90 客户蛋白的蛋白酶体降解。随后，CHIP 和 parkin 被证明是 Hsp70 结合的泛素 E3 连接酶，它们将泛素化的 E2 酶引导到 Hsp70 结合的客户蛋白上。Hsp90 的稳定反映了伴侣蛋白与客户蛋白配体结合裂隙的相互作用。这些疏水性裂隙必须打开，以允许配体进入蛋白质内部的结合位点，并且它们是构象不稳定的固有位点。Hsp90 稳定裂隙的开放状态并防止 Hsp70 依赖性泛素化。在我们提出的模型中，渐进的氧化事件导致裂隙打开作为蛋白质展开的初始步骤，只要 Hsp90 能够相互作用以稳定裂隙，它就会缓冲氧化损伤的影响。一旦裂隙打开，Hsp90 无法再相互作用，Hsp70 依赖性泛素化就会发生。我们总结了证据，表明 Hsp90 与各种非经典 Hsp90 客户蛋白非常动态地相互作用，并且我们表明，Hsp90 与一氧化氮合酶的这种动态循环可以防止 CHIP 介导的泛素化。迄今为止，科学兴趣主要集中在严格调节经典客户蛋白上，这些蛋白具有亚稳定的裂隙，并且本身寿命较短。但是，认识到 Hsp90 与具有更稳定裂隙的寿命更长的蛋白质动态循环可能会将分类模型扩展到一般受损蛋白质的质量控制。
