Control of plastid biogenesis by the ubiquitin-proteasome system

叶绿体和线粒体是许多细胞的正常组成部分--它们是亚细胞结构，称为细胞器。有趣的是，这两个细胞器是从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类似于自由生活的细菌。叶绿体存在于植物细胞中，含有绿色素，并负责光合作用(捕捉阳光能量并利用其为细胞的活动提供动力的过程)的反应。由于光合作用是向生物世界输入能量的唯一重要机制，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。实际上，叶绿体属于一个更广泛的相关细胞器家族，称为叶绿体。该家族的其他成员包括成熟果实中高度着色的有色体和深色植物中的黄体。尽管叶绿体确实含有DNA(作为自由生活的光合细菌的进化历史的遗迹)，因此可以制造自己的一些蛋白质，但形成功能叶绿体所需的大多数蛋白质都是在细胞核的DNA上编码的；这些蛋白质是在细胞基质中被称为胞质的胞质外合成的。由于每个叶绿体都被双层膜或包膜包围，不受蛋白质被动运动的影响，这是一个很大的问题。为了克服这个问题，叶绿体进化出了一种复杂的蛋白质输入装置，它使用能量(以三磷酸腺苷的形式)驱动蛋白质从细胞质输入，穿过被膜，进入叶绿体内部。这种输入装置由两个分子机器组成：一个在外被膜上称为TOC(叶绿体外被膜上转运子的缩写)，另一个在内被膜上被称为TIC。每台机器由几种不同的蛋白质组成，这些蛋白质相互协作，以确保进口的效率。我们正在研究一种名为拟南芥的模式植物，它在研究中具有许多优势，例如可以获得大量的突变体(每个突变体都有一个特定基因的突变)。其中一种突变植物ppi1的TOC基因存在缺陷，以至于叶绿体蛋白的导入不能有效地发挥作用。几年前，我们发现了另一种名为SP1的突变(这代表着“ppi1的抑制者”)，它可以抵消ppi1的负面影响。被SP1(SP1基因)破坏的基因编码一种被称为“泛素E3连接酶”的调节蛋白。这些方法的工作原理是标记不需要的蛋白质，并针对它们进行去除。由于这一控制机制此前并不为人所知，因此这一发现是生物学上的一项重大突破。SP1 E3连接酶谨慎地控制TOC机制的组成，以便总是输入正确的蛋白质(这在正常情况下是好的，但在异常的ppi1背景下显然是一个障碍)。当叶绿体需要从一种形式转换成另一种形式时，这种控制是非常重要的；例如，当暗萌发的植物出现在阳光下时，黄体必须转变为叶绿体，才能开始光合作用。在这个项目中，我们将研究SP1在番茄果实叶绿体转化为染色质过程中是否起重要作用。如果是，那么我们的工作可能会通过操纵作物(如西红柿、甜椒、柑橘)的果实成熟而具有商业和农业重要性。我们还将更详细地研究SP1和相关蛋白质是如何控制质体发育的。例如，我们的工作可能会阐明植物如何对盐分和干旱等胁迫做出反应，这些都是世界各地农作物产量的主要限制。植物的光合作用表现(以及植物生长所需的能量)受到胁迫的强烈影响，我们怀疑SP1参与了这一过程。因此，从我们的工作中获得的知识可能使作物更好地适应不利的环境条件。

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 over 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 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. Actually, chloroplasts belong to a wider family of related organelles called plastids. Other members of the family are the highly-pigmented chromoplasts in ripe fruits, and etioplasts in dark-grown plants. Although plastids do contain DNA (a relic from their evolutionary past as free-living photosynthetic bacteria), and so can make some of their own proteins, most of the proteins needed to form a functional plastid are encoded on DNA in the cell nucleus; these proteins are made outside of the plastid in the cellular matrix known as the cytosol. As plastids 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, plastids 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, to the plastid interior. This 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. Each machine is made up of several different proteins which cooperate to ensure the efficiency of import. We work on a model plant called Arabidopsis that has many advantages for research, such as an availability of numerous mutants (each one with a mutation in a specific gene). One such mutant plant, ppi1, has a defect in a TOC gene such that plastid protein import does not work efficiently. Several years ago, we identified another mutation called sp1 (this stands for "suppressor of ppi1") that counteracts the negative effects of ppi1. The gene disrupted by sp1 (the SP1 gene) encodes a type of regulatory protein called a "ubiquitin E3 ligase". These work by labelling-up unwanted proteins and targeting them for removal. Because this control mechanism was not previously known to operate in plastids, this discovery was an important breakthrough in biology. The SP1 E3 ligase carefully controls the composition of the TOC machinery so that the right proteins are always imported (this is normally good, but in the abnormal ppi1 background it is apparently a hindrance). Such control is very important when plastids need to convert from one form to another; e.g. when dark-germinated plants emerge into the light, etioplasts must change into chloroplasts so that photosynthesis can begin. In this project we will investigate whether SP1 is important for the conversion of chloroplasts into chromoplasts in tomato fruit. If it is, then our work may have commercial, agricultural importance by enabling the manipulation of fruit ripening in crops (e.g. tomato, bell pepper, citrus). We will also study in much greater detail how SP1 and related proteins control plastid development. For example, our work may elucidate how plants respond to stresses like salinity and drought, which are major limits on crop yield across the world. Photosynthetic performance (and thus the energy available to plants for growth) is strongly affected by stress, and we suspect that SP1 is involved in this process. Thus, knowledge gained from our work may enable improved adaptation of crops to adverse environmental conditions.