The role of non-AUG codons in translation initiation and localisation of mitochondrial proteins

人体每个细胞中的DNA序列被称为基因组，存储着制造所有蛋白质的指令，这些蛋白质是生命所需的基本构件。每种蛋白质的密码都储存在被称为基因的较短DNA片段中。将基因(只有四个字母)的DNA序列转换为相应蛋白质(由20种不同的氨基酸组成)的DNA序列需要两个主要过程：转录和翻译。转录是将DNA中的序列信息复制到一个类似的称为信使RNA(信使RNA)的分子中。然后，使用翻译的过程将信使信使解码成氨基酸序列(这样命名是因为它从DNA/RNA的语言到氨基酸的语言)。用来将核苷酸(RNA)翻译成氨基酸(蛋白质)的“遗传密码”已经建立得很好，很容易预测给定的DNA片段编码什么氨基酸。然而，这个过程是复杂的，因为在信使核糖核酸末端存在“非翻译区”，它不编码任何蛋白质序列。因此，为了正确地翻译蛋白质，重要的是知道翻译从哪里开始。翻译mRNA序列的复杂分子机器被称为核糖体，当它在mRNA中发现一个称为翻译起始密码子的特定序列时，它就开始制造蛋白质，该序列通常具有序列“Aug”。这个项目将促进我们对这些翻译起始密码子的性质的了解。许多翻译启动的规则仍然不清楚，因此我们知道的越多，我们就越能从现有的数据中理解。这一点尤其正确，因为测序技术的进步意味着我们关于蛋白质宇宙的知识中越来越多的部分纯粹是通过计算将这些规则应用到DNA序列数据中来获得的。众所周知，通过在转录过程中产生不同的mRNA序列，可以从单个基因产生多个蛋白质。一个更新的发现是，通过核糖体开始在不同的位置翻译蛋白质，翻译可以类似地从同一个mRNA产生不同的蛋白质，产生更长或更短的蛋白质版本。这个项目关注如何以及为什么使用不同的翻译起始密码子，以及这种现象有多普遍。到目前为止，只有几个例子发现了这一点，但那些已知的例子非常重要。事实上，替代起始密码子可以用来制造具有完全不同功能的新形式的蛋白质，或者进入细胞内的不同位置。此外，现在越来越清楚的是，起始密码子本身并不一定是AUG三联体，我们所描述的非规范起始密码子的使用是我们拟议工作的重点。我们已经成功地从非AUG密码子中识别出翻译起始，在这个项目中，我们将特别关注制造蛋白质的基因，这些基因在细胞的“电池”中发挥作用，即线粒体。我们认为，帮助它们定位到细胞这一部分的蛋白质中的重要信号被忽略了，因为它们是从非AUG密码子开始产生的。这意味着使用错误规则的计算方法将错过它们。我们已经在一个基因中证明了这一现象，一旦我们在其他候选基因中成功识别了新的起始密码子，我们将检查产生的蛋白质的后果是什么。我们肯定会确定将蛋白质移动到线粒体所涉及的信号，但也可能为新确定的蛋白质序列找到新的作用。

The DNA sequence in every cell of the body, termed the genome, stores the instructions to make all the proteins that are the essential building blocks needed for living. The code for each protein is stored in shorter stretches of DNA called genes. Converting the DNA sequence of a gene (which has only four "letters") to that of its corresponding protein (made up of twenty different kinds of amino acids) requires two major processes: transcription and translation. Transcription is the copying of the sequence information in the DNA into a similar molecule called messenger RNA (mRNA). The mRNA messages are then decoded into the amino acid sequence using the process of translation (so called because it goes from the language of DNA/RNA, to the language of amino acids). The "Genetic Code", which is used to translate nucleotides (RNA) into amino acids (protein), is well established and it is easy to predict what amino acids are encoded by a given stretch of DNA. The process is complicated, however, by the presence of "untranslated regions" at the ends of the mRNA, which do not encode any protein sequence. As a consequence, in order to correctly translate a protein, it is important to know where translation begins.The complex molecular machine that translates the mRNA sequence is called the ribosome, which starts making a protein when it finds a particular sequence in the mRNA called a translation initiation codon, which usually has the sequence "AUG". This project will advance our knowledge regarding the nature of these translation initiation codons. Many of the rules of translation initiation remain unclear therefore the more we know, the more we can understand from existing data. This is particularly true as improvements in sequencing technology means that an ever-increasing proportion of our knowledge about the protein universe is derived purely from applying these rules computationally to DNA sequence data.It is well established that multiple proteins can be produced from a single gene by generating different mRNA sequences during transcription. A much more recent finding is that translation can similarly produce different proteins from the same mRNA by the ribosome beginning to translate the protein at different positions, making longer or shorter versions of the protein. This project is concerned with how and why different translation initiation codons are used and how widespread this phenomenon is. So far, there are only a few examples where this has been discovered but those that are known are very important. In fact alternative initiation codons can be used to make new forms of proteins which have completely different functions or go to different places within the cell. Furthermore, it is now becoming clear that the initiation codon itself does not have to be the AUG triplet and the use of what we describe as non-canonical initiation codons is the focus of our proposed work.We have successfully identified translation initiation from non-AUG codons, and in this project we will particularly focus on genes that make proteins with roles in the "batteries" of the cell, the mitochondria. We believe that important signals within the proteins which help target them to this part of the cell have been ignored because they are made by starting from non-AUG codons. This means that computational methods using the wrong rules will have missed them. We have already proven this phenomenon in one gene, and once we have successfully identified novel initiation codons in further candidates, we will then examine what the consequences are for the proteins that are produced. We will certainly identify the signals that are involved in moving proteins to the mitochondria, but may also find new roles for the newly identified protein sequence.