Mechanisms of regulation of ribosome biogenesis and stress resistance in yeast

在真核细胞中，近三分之一的新合成蛋白质进入内质网(ER)，内质网是一个含有特殊质量控制机制的细胞器，确保新生多肽的正确折叠。不能折叠成正确的构象会导致蛋白质功能丧失和细胞死亡。因此，细胞已经进化出信号通路，对由突变或环境侮辱等各种因素引起的内质网错折叠蛋白负荷(ER Stress)的突然增加做出反应。作为对内质网应激的响应，细胞激活未折叠蛋白反应(UPR)。这种进化上保守的应对机制增加了编码蛋白质的基因的表达，这些蛋白质通过改善蛋白质折叠和错误折叠的蛋白质降解来帮助恢复内质网的动态平衡。内质网应激分解和UPR的机制已经用发芽酵母酿酒酵母来广泛表征，这使得快速和广泛的遗传操作成为可能。酵母还拥有一个简单得多的后生动物UPR版本，使我们能够破译基本的机制。 有趣的是，内质网应激激活了UPR以外的其他通路。在酵母中，长时间的内质网应激阻止正确折叠的蛋白质输送到细胞壁，激活另一种应对机制，即细胞壁完整性途径(CWI)。在内质网应激过程中，CWI途径的不同分支有两个主要作用：增加细胞壁成分的合成以修复缺陷和抑制核糖体的生物合成。酵母细胞的分裂将其大部分资源分配给核糖体的生物发生，因此，抑制核糖体的生产对于将资源重新分配到逆境反应途径如CWI和UPR是至关重要的。然而，来自我的实验室的新数据表明，虽然对急性内质网应激有利，但长期抑制核糖体的生物发生对细胞有害，并阻止它们在内质网应激消失后重新进入生长阶段。我的中心假设是，在慢性内质网应激过程中，细胞壁成分的合成增加将减弱核糖体生物发生的抑制，允许蛋白质合成，并最终提高细胞存活率。我的研究计划的长期目标是了解CWI途径如何将内质网应激耐受与核糖体生物发生和细胞生长联系起来。具体地说，我的短期目标是： (1)阐明CWI如何调控细胞对内质网应激的反应。 (2)确定在长时间的内质网应激过程中，CWI如何调控核糖体的生物发生。 (3)确定控制CWI依赖的ER胁迫耐受性的遗传网络。 该计划将揭示适应内质网应激的新的、基本的调节机制，这些机制独立于UPR激活，稍后可以在其他系统中进行研究。

In eukaryotic cells, nearly one third of all newly synthetized proteins enter the endoplasmic reticulum (ER), an organelle containing specialized quality control machinery that ensures the proper folding of the nascent peptides. Failure to fold into their proper conformation leads to loss of protein function and cell death. Thus, cells have evolved signaling pathways that respond to sudden increases in the ER misfolded protein burden (ER stress) caused by various factors such as mutation or environmental insults. In response to ER stress, cells activate the unfolded protein response (UPR). This evolutionarily conserved coping mechanism increases the expression of genes encoding proteins that help restore ER homeostasis by improving protein folding and misfolded protein degradation. The mechanisms of ER stress resolution and UPR have been extensively characterized using the budding yeast Saccharomyces cerevisiae, which allows rapid and extensive genetic manipulations. Yeast also possess a much simpler version of the metazoan UPR, allowing us to decipher basic mechanisms.  Interestingly, ER stress activates other pathways beyond the UPR. In yeast, prolonged ER stress prevents delivery of properly folded proteins to the cell wall, activating another coping mechanism, the cell wall integrity pathway (CWI). During ER stress, distinct branches of the CWI pathway have two major roles: to increase synthesis of cell wall components to repair defects and to repress ribosome biogenesis. Dividing yeast cells allocate much of their resources to ribosome biogenesis, thus, repression of ribosome production is essential to reallocate resources to stress response pathways such as the CWI and the UPR. However, new data from my laboratory indicate that, while beneficial for acute ER stress, prolonged repression of ribosome biogenesis is detrimental to cells and prevents their re-entry into the growth phase upon resolution of ER stress. My central hypothesis is that increased synthesis of cell wall components during chronic ER stress will attenuate the repression of ribosome biogenesis, allow protein synthesis, and ultimately increase cell survival. The long-term goal of my research program is to understand how the CWI pathway links ER stress tolerance to ribosome biogenesis and cell growth. Specifically, my short-term objectives are to: (1) Elucidate how CWI regulates cellular responses to ER stress. (2) Define how CWI regulates ribosome biogenesis during prolonged ER stress. (3) Identify the genetic networks controlling CWI-dependent tolerance to ER stress. This program will uncover new, fundamental regulatory mechanisms for adaptation to ER stress that are independent of UPR activation and could later be investigated in other systems.