Characterization of the TOC complexes which define distinct client-specific chloroplast protein import pathways in Arabidopsis

叶绿体如何有效地输入数千种不同的蛋白质。叶绿体和线粒体是许多细胞的正常组成部分——它们是被称为细胞器的亚细胞结构。有趣的是，这两种细胞器是从10亿多年前被其他细胞吞噬的细菌进化而来的，在许多方面它们仍然像自由生活的细菌。叶绿体存在于植物细胞中，含有绿色色素叶绿素，并专门负责光合作用反应（捕获阳光能量并利用它为细胞活动提供动力的过程）。由于光合作用是生物世界唯一重要的能量输入机制，叶绿体不仅对植物，而且对地球上的所有生命都具有不可估量的重要性。叶绿体在许多其他方面也很重要，因为它们在油、蛋白质和淀粉的生物合成中起着至关重要的作用。虽然叶绿体确实含有DNA （DNA是它们作为自由生活的光合细菌进化而来的古老遗迹），因此能够制造一些自己的蛋白质，但构建一个功能齐全的叶绿体所需的3000多种蛋白质中，有90%以上是在细胞核内的DNA上编码的。因此，大多数叶绿体蛋白质是在叶绿体外的细胞质基质中产生的。由于叶绿体都被一层双层膜或包膜包围着，这对蛋白质的被动运动是不渗透的，这就提出了一个重要的问题。为了克服这个问题，叶绿体进化出了一种复杂的蛋白质进口装置，它利用能量（以ATP的形式）驱动蛋白质从细胞质中进口，穿过包膜，进入叶绿体内部。这种蛋白质输入装置包括两个分子机器：一个在外包膜上称为TOC（“叶绿体外包膜上的转座子”的缩写），另一个在内包膜上称为TIC。这个项目的重点是TOC机器，它执行两个基本功能。首先，它识别那些到达叶绿体表面时需要输入的蛋白质。其次，它通过外膜形成一个通道，这样一旦被识别，蛋白质就可以通过。我们将学习其中的第一个功能：识别。为了实现这一功能，TOC机器使用一种叫做受体的特殊分子，当蛋白质到达叶绿体包膜时，受体会与蛋白质结合。最近，我们发现实际上有几种不同类型的受体，我们认为它们的存在可能是为了让叶绿体能够有效地识别它们需要输入的所有不同的蛋白质。换句话说，我们认为每个受体对必须输入的特定蛋白质子集都有一定程度的特异性。这个项目将测试这些想法，因为我们认为不同类型的受体非常重要，有助于确保形成功能齐全的叶绿体。由于叶绿体具有基本的功能，而蛋白质的输入对叶绿体的发育至关重要，因此，如果没有叶绿体蛋白质输入机制的植物无法存活（事实上，它们在胚胎阶段就死亡了），这应该不足为奇。因此，叶绿体蛋白的输入是植物的一个重要过程。同样，由于我们最终都依赖植物产品来生存，因此叶绿体蛋白质的进口在全球范围内是必不可少的。更重要的是，由于叶绿体在许多经济上重要产品（如油和淀粉）的合成中起着重要作用，对这些细胞器如何发育的更全面的了解可能使我们能够提高作物的生产力，或者以其他方式操纵它们的产品。

How chloroplasts efficiently import thousands of different proteins. Chloroplasts and mitochondria are normal components of many cells - they are sub-cellular structures called organelles. Interestingly, these two organelles evolved from bacteria that were engulfed by other cells more than a billion years ago, and in many ways they still resemble free-living bacteria. Chloroplasts are found in plant cells, contain the green pigment chlorophyll, and are exclusively responsible for the reactions of photosynthesis (the process that captures sunlight energy and uses it to power the activities of the cell). Since photosynthesis is the only significant mechanism of energy-input into the living world, chloroplasts are of inestimable importance, not just to plants but to all life on Earth. Chloroplasts are also important in many other ways, since they play essential roles in the biosynthesis of oils, proteins and starch. Although chloroplasts do contain DNA (which is a relic from their ancient, evolutionary past as free-living photosynthetic bacteria), and are therefore able to make some of their own proteins, over 90% of the 3000 or so proteins required to build a fully functional chloroplast are encoded on DNA within the cell nucleus. The majority of chloroplast proteins are therefore made outside of the chloroplast, in the cellular matrix known as the cytosol. Since chloroplasts are each surrounded by a double membrane, or envelope, that is impervious to the passive movement of proteins, this presents a significant problem. To overcome the problem, chloroplasts have evolved a sophisticated protein import apparatus, which uses energy (in the form of ATP) to drive the import of proteins from the cytosol, across the envelope, and into the chloroplast interior. This protein import apparatus comprises two molecular machines: one in the outer envelope membrane called TOC (an abbreviation of 'Translocon at the Outer envelope membrane of Chloroplasts'), and another in the inner envelope membrane called TIC. This project is focused on the TOC machine, which performs two essential functions. Firstly, it recognizes those proteins that need to be imported as they arrive at the chloroplast surface. Secondly, it forms a channel through the outer membrane so that the proteins can pass across, once recognized. We will study the first of these functions: recognition. To carry out this function, the TOC machine uses special molecules called receptors, which bind to the proteins as they arrive at the chloroplast envelope. Quite recently, we found that there are actually several different types of receptor, and we think that they probably exist so that chloroplasts can efficiently recognize all of the many different proteins they need to import. In other words, we think that each of the receptors has a degree of specificity for a particular subset of the proteins that must be imported. This project will test these ideas, since we think that the different types of receptor are very important, helping to ensure the formation of fully functional chloroplasts. Because chloroplasts carry out essential functions, and because protein import is essential for chloroplast development, it should come as no surprise to learn that plants without a functional chloroplast protein import machinery are unable to survive (in fact, they die at the embryo stage). Thus, chloroplast protein import is an essential process for plants. Similarly, since we are all ultimately dependent upon plant products for survival, it follows that chloroplast protein import is essential on a global scale. What is more, since chloroplasts play a major role in the synthesis of many economically important products (such as oils and starch), a more complete understanding of how these organelles develop may enable us to enhance the productivity of crop plants, or otherwise manipulate their products.