Ubiquitin and regulation of prion induction by a short-lived protein

描述（由申请人提供）：朊病毒是感染性蛋白质同种型，可在哺乳动物中引起致命和不可治愈的神经退行性疾病，并在酵母中传递可遗传性状。大多数朊病毒形成高度有序的纤维状聚合物（淀粉样蛋白），类似于其他淀粉样变性和神经包涵体疾病（如阿尔茨海默病、帕金森病或亨廷顿病）中涉及的聚集体。因此，朊病毒可以被认为是一种淀粉样蛋白，它可以募集相同序列的蛋白质分子，将它们转化为淀粉样蛋白状态，并被传递到其他细胞。现在越来越清楚的是，除了它们在疾病中的作用外，一些朊病毒和其他淀粉样蛋白或淀粉样蛋白样聚集体还执行重要的生物学功能：储存肽激素;微生物细胞相互附着或附着在基质上;控制表观遗传开关;适应压力环境条件;以及与记忆相关的长期突触变化。虽然许多蛋白质在体外形成淀粉样蛋白，但对淀粉样蛋白或朊病毒在体内形成的机制知之甚少。可能存在共享一些共同特征的基础途径和诱导途径。同源多聚体和/或异源多聚体复合物中蛋白质的初始缔合可导致朊病毒“种子”的形成。当淀粉样蛋白在细胞和/或局部隔室中以高浓度积累时，这种事件的概率增加。事实上，酵母朊病毒的从头形成是由朊病毒蛋白的过表达和/或其他聚集的富含QN的蛋白的存在促进的。已知影响蛋白质水平和稳态的过程，如一些生理应激、泛素蛋白酶体系统（UPS）功能受损或未折叠蛋白反应（UPR）缺陷，也会增加朊病毒的形成。相反，肌动蛋白细胞骨架的缺陷降低了朊病毒形成的速率，这表明与细胞骨架的结合对于朊病毒成核的某些步骤是重要的。因此，引起朊病毒蛋白或异源性朊病毒原蛋白积累并控制其定位的生理应激可能对朊病毒的形成具有深远影响。Lsb 2是一种酵母蛋白，其过表达在酵母翻译终止因子Sup 35过表达的情况下刺激朊病毒（命名为[PSI+]）的形成。与其在朊病毒发生中的作用一致，Lsb 2是由应激诱导的，在K80处泛素化，以蛋白酶体依赖的方式降解，并定位于肌动蛋白斑块。本研究的总体目标是揭示异源蛋白质稳态参与真核细胞朊病毒形成的分子机制。我们将定义Lsb 2在应激诱导的[PSI+]朊病毒形成中的作用。我们将询问朊病毒诱导能力是否需要Lsb 2的特定细胞定位，Lsb 2的UPS和共价修饰在触发朊病毒形成中发挥什么作用，以及Lsb 2水平的生理和应激相关变化是否参与朊病毒诱导。

DESCRIPTION (provided by applicant): Prions are infectious protein isoforms that cause fatal and incurable neurodegenerative diseases in mammals and transmit heritable traits in yeast. Most prions form highly ordered fibrous polymers (amyloids), resembling the aggregates involved in other amyloidoses and neural inclusion diseases, such as Alzheimer, Parkinson, or Huntington diseases. Thus, a prion can be thought of as an amyloid that can recruit protein molecules of the same sequence, convert them into an amyloid state, and be transmitted to other cells. It is now becoming clear that in addition to their role in disease, some prions and other amyloids or amyloid-like aggregates perform important biological functions: storage of peptide hormones; attachment of microbial cells to each other or to substrates; control of epigenetic switches; adaptation to stressful environmental conditions; and long-term synaptic changes associated with memory. While many proteins form amyloids in vitro, little is known about the mechanisms of amyloid or prion formation in vivo. There are likely both basal and induced pathways that share some common features. An initial association of proteins in homomultimeric and/or heteromultimeric complexes may lead to formation of the prion "seed". The probability of such an event is increased when the amyloidogenic protein is accumulated at high concentrations in the cell and/or in a local compartment. Indeed, de novo formation of a yeast prion is promoted by overexpression of the prion protein and/or the presence of other aggregated QN-rich proteins. Processes known to affect protein levels and homeostasis, such as some physiological stresses, impairment of ubiquitin proteasome system (UPS) function, or defects in the unfolded protein response (UPR), also increase prion formation. Conversely, defects in the actin cytoskeleton diminish the rate of prion formation, suggesting that binding to cytoskeleton is important for some step(s) involved in prion nucleation. Thus, physiological stresses that cause the accumulation of prion proteins or heterologous prionogenic proteins, and that control their localization may have profound effects on the formation of prions. Lsb2 is a yeast protein whose overexpression stimulates the formation of a prion (designated [PSI+]) in the presence of overexpression of the yeast translational termination factor Sup35. Consistent with its role in prionogenesis, Lsb2 is induced by stress, ubiquitinated at K80, degraded in a proteasome-dependent fashion, and localized to actin patches. The overall goal of this research is to uncover the molecular mechanisms by which heterologous protein homeostasis participates in prion formation in eukaryotic cells. We will define the role of Lsb2 in the stress-induced formation of the [PSI+] prion. We will ask if a specific cellular localization of Lsb2 is required for prion-inducing ability, what role UPS and covalent modifications of Lsb2 play in triggering the formation of prions, and if physiological and stress- related variations in Lsb2 levels are involved in prion induction.