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

描述（由申请人提供）：朊病毒是传染性蛋白质亚型，在哺乳动物中引起致命和无法治愈的神经退行性疾病，并在酵母中传播遗传性状。大多数朊病毒形成高度有序的纤维聚合物（淀粉样蛋白），类似于其他淀粉样病和神经包涵性疾病（如阿尔茨海默病、帕金森病或亨廷顿病）的聚集体。因此，朊病毒可以被认为是一种淀粉样蛋白，它可以招募相同序列的蛋白质分子，将它们转化为淀粉样蛋白状态，并传播到其他细胞。现在越来越清楚的是，除了在疾病中发挥作用外，一些朊病毒和其他淀粉样蛋白或淀粉样蛋白聚集体还具有重要的生物学功能：储存肽激素；附着：微生物细胞相互附着或附着在基质上；表观遗传开关的控制；适应紧张的环境条件；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。可能基础通路和诱导通路都有一些共同的特征。同聚体和/或异聚体复合物中蛋白质的初始结合可能导致朊病毒“种子”的形成。当淀粉样蛋白在细胞和/或局部细胞室中高浓度积累时，这种事件的可能性增加。事实上，朊病毒蛋白的过度表达和/或其他富含qn的聚集蛋白的存在促进了酵母朊病毒的从头形成。已知影响蛋白质水平和体内平衡的过程，如一些生理应激、泛素蛋白酶体系统（UPS）功能的损伤或未折叠蛋白反应（UPR）的缺陷，也会增加朊病毒的形成。相反，肌动蛋白细胞骨架的缺陷会降低朊病毒的形成速度，这表明与细胞骨架的结合对于朊病毒成核的某些步骤很重要。因此，引起朊病毒蛋白或异源朊蛋白积累并控制其定位的生理应激可能对朊病毒的形成有深远的影响。Lsb2是一种酵母蛋白，在酵母翻译终止因子Sup35过表达的情况下，其过表达会刺激朊病毒（称为[PSI+]）的形成。与其在前列腺增生中的作用一致，Lsb2受应激诱导，在K80时泛素化，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真核细胞中异源蛋白稳态参与朊病毒形成的分子机制。我们将定义Lsb2在应力诱导形成[PSI+]朊病毒中的作用。我们将探讨是否需要Lsb2的特定细胞定位才能诱导朊病毒，Lsb2的UPS和共价修饰在触发朊病毒形成中起什么作用，以及Lsb2水平的生理和应激相关变化是否涉及朊病毒诱导。

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.