Multi-protein bridging factor 1(Mbf1), Rps3 and Asc1 prevent stalled ribosomes from frameshifting.
Multi-protein bridging factor 1(Mbf1), Rps3 and Asc1 prevent stalled ribosomes from frameshifting.
复制标题
DOI：
10.7554/elife.39637
复制
发表时间：
2018-11-22
期刊：
影响因子：
7.7
通讯作者：
Grayhack EJ
中科院分区：
文献类型：
--
作者：
Wang J;Zhou J;Yang Q;Grayhack EJ
阅读框的维护是准确翻译的关键。我们发现，真核/古菌蛋白Mbf1与核糖体蛋白Rps3/uS3和真核Asc1/RACK1共同作用，阻止酵母CGA-CGA密码子对的移框。RPS3中允许移框的突变涉及mRNA进入位点附近的真核生物保守残基。Mbf1和Rps3合作维持停滞核糖体的阅读框，而Asc1还介导不同的事件，导致核糖体质量控制复合体的招募和mRNA的衰变。移框是通过P位点的CGA密码子发生+1移位，并涉及进入a位点的密码子之间的竞争，这意味着P位点密码子的摆动相互作用破坏了翻译延伸的稳定性。因此，真核生物进化出了独特的机制，包括普遍保守的核糖体成分和两种真核特异性蛋白质来维持核糖体位置的阅读框。蛋白质执行维持细胞存活所需的所有化学反应；因此，正确地组装它们是必要的。它们是由被称为核糖体的分子机器制造的，核糖体遵循被称为mrna的分子的遗传密码中的一系列指令。核糖体每次读取三个字母的遗传密码；每个三联体要么编码将20个构建块中的一个插入新蛋白质中，要么作为停止该过程的信号。至关重要的是，在读取一个三联体后，核糖体精确地移动三个字母来读取下一个三联体。例如，如果核糖体只移动两个字母而不是三个字母——一种被称为“移框”的现象——它将完全改变用于制造蛋白质的构建模块。这可能导致非典型或异常的蛋白质，这些蛋白质要么不起作用，要么甚至对细胞有毒。由于各种原因，核糖体在完成蛋白质的构建之前往往会停滞不前。当这种情况发生时，核糖体更有可能移码。细胞通常通过招募其他分子来应对停滞的核糖体，这些分子可以作为质量控制系统，其中一些分子可以分解核糖体并分解mRNA。在出芽酵母中，核糖体的一部分Asc1在招募这些质量控制系统和mRNA分解中起着关键作用。如果这个成分被移除，停滞的核糖体更频繁地移码，结果，异常蛋白质在细胞中积累。由于Asc1招募蛋白位于核糖体的外部，当核糖体停止移框时，它可能会通过其他因素来阻止核糖体移框。然而，目前尚不清楚这些因素是否存在，它们是什么，以及它们如何起作用。现在，Wang等人已经确定了另外两种酵母蛋白，分别命名为Mbf1和Rps3，它们在核糖体停止后合作阻止其移框。与Asc1一样，Rps3也是核糖体的组成部分，而Mbf1则不是。看起来Rps3可能通过与进入的mRNA的相互作用来阻止移框，因为Rps3靠近mRNA进入核糖体的区域对其活性很重要。进一步的实验表明，已知的asc1介导的mrna分解不依赖于Mbf1和Rps3，但也有助于阻止帧移。因此，停滞核糖体的移码是通过两种不同的方式来阻止的：一种是直接涉及Mbf1和Rps3，另一种是由Asc1促进的，它减少了核糖体移码所依赖的mrna的数量。这些新发现的因素可能为细胞中精确控制的蛋白质生产机制和质量控制系统的作用提供见解。对防止框架移位机制的进一步了解，最终可能会导致更好的治疗一些人类疾病，这些疾病是由这些过程出错引起的，包括某些神经系统疾病。
Reading frame maintenance is critical for accurate translation. We show that the conserved eukaryotic/archaeal protein Mbf1 acts with ribosomal proteins Rps3/uS3 and eukaryotic Asc1/RACK1 to prevent frameshifting at inhibitory CGA-CGA codon pairs in the yeast Saccharomyces cerevisiae. Mutations in RPS3 that allow frameshifting implicate eukaryotic conserved residues near the mRNA entry site. Mbf1 and Rps3 cooperate to maintain the reading frame of stalled ribosomes, while Asc1 also mediates distinct events that result in recruitment of the ribosome quality control complex and mRNA decay. Frameshifting occurs through a +1 shift with a CGA codon in the P site and involves competition between codons entering the A site, implying that the wobble interaction of the P site codon destabilizes translation elongation. Thus, eukaryotes have evolved unique mechanisms involving both a universally conserved ribosome component and two eukaryotic-specific proteins to maintain the reading frame at ribosome stalls. Proteins perform all the chemical reactions needed to keep a cell alive; thus, it is essential to assemble them correctly. They are made by molecular machines called ribosomes, which follow a sequence of instructions written in genetic code in molecules known as mRNAs. Ribosomes essentially read the genetic code three letters at a time; each triplet either codes for the insertion of one of 20 building blocks into the emerging protein, or serves as a signal to stop the process. It is critical that, after reading one triplet, the ribosome moves precisely three letters to read the next triplet. If, for example, the ribosome shifted just two letters instead of three – a phenomenon known as “frameshifting” – it would completely change the building blocks that were used to make the protein. This could lead to atypical or aberrant proteins that either do not work or are even toxic to the cell. For a variety of reasons, ribosomes will often stall before they have finished building a protein. When this happens, the ribosome is more likely to frameshift. Cells commonly respond to stalled ribosomes by recruiting other molecules that work as quality control systems, some of which can disassemble the ribosome and break down the mRNA. In budding yeast, one part of the ribosome – named Asc1 – plays a key role in recruiting these quality control systems and in mRNA breakdown. If this component is removed, stalled ribosomes frameshift more frequently and, as a result, aberrant proteins accumulate in the cell. Since the Asc1 recruiter protein sits on the outside of the ribosome, it seemed likely that it might act through other factors to stop the ribosome from frameshifting when it stalls. However, it was unknown if such factors exist, what they are, or how they might work. Now, Wang et al. have identified two additional yeast proteins, named Mbf1 and Rps3, which cooperate to stop the ribosome from frameshifting after it stalls. Rps3, like Asc1, is a component of the ribosome, while Mbf1 is not. It appears that Rps3 likely stops frameshifting via an interaction with the incoming mRNA, because a region of Rps3 near the mRNA entry site to the ribosome is important for its activity. Further experiments then showed that the known Asc1-mediated breakdown of mRNAs did not depend on Mbf1 and Rps3, but also assists in stopping frameshifting. Thus, frameshifting of stalled ribosomes is prevented via two distinct ways: one that directly involves Mbf1 and Rps3 and one that is promoted by Asc1, which reduces the amounts of mRNAs on which ribosomes frameshift. These newly identified factors may provide insights into the precisely controlled protein-production machinery in the cell and into roles of the quality control systems. An improved understanding of mechanisms that prevent frameshifting could eventually lead to better treatments for some human diseases that result when these processes go awry, which include certain neurological conditions.