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——它们是被称为细胞器的亚细胞结构。有趣的是，这两种细胞器是从十亿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，负责光合作用（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。实际上，叶绿体属于一个更广泛的细胞器家族，叫做质体。该家族的其他成员是成熟果实中的高度着色的色质体，以及深色植物中的腐殖体。虽然质体确实含有DNA（这是它们作为自由生活的光合细菌进化过程中的遗物），因此可以制造一些自己的蛋白质，但形成功能性质体所需的大多数蛋白质都编码在细胞核中的DNA上；这些蛋白质是在被称为细胞质的细胞质基质中形成的。由于每个质体都被一层双层膜或包膜所包围，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，质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，进入质体内部。这种进口仪器包括两个分子机器：一个在外包膜上称为TOC（“叶绿体外包膜上的Translocon”的缩写），另一个在内包膜上称为TIC。每台机器由几种不同的蛋白质组成，这些蛋白质相互配合以确保进口效率。我们研究的是一种叫做拟南芥的模式植物，它有很多研究优势，比如大量的突变体（每一个都有一个特定基因的突变）。一种这样的突变植物ppi1在TOC基因上有缺陷，使得质体蛋白的输入不能有效地工作。几年前，我们发现了另一种被称为sp1的突变（它代表“ppi1的抑制因子”），它抵消了ppi1的负面影响。被sp1破坏的基因（sp1基因）编码一种称为“泛素E3连接酶”的调节蛋白。它们的工作原理是标记不需要的蛋白质，并针对它们进行清除。由于这种控制机制以前不知道在质体中起作用，因此这一发现是生物学上的一个重要突破。SP1 E3连接酶小心地控制TOC机制的组成，以便总是输入正确的蛋白质（这通常是好的，但在异常的ppi1背景下，它显然是一个障碍）。当质体需要从一种形式转化为另一种形式时，这种控制非常重要；例如，当黑暗发芽的植物出现在阳光下时，腐殖体必须转变成叶绿体，这样光合作用才能开始。在这个项目中，我们将研究SP1是否在番茄果实叶绿体向色质体转化中起重要作用。如果是这样，那么我们的工作可能具有商业和农业意义，可以操纵作物（例如番茄、甜椒、柑橘）的果实成熟。我们还将更详细地研究SP1和相关蛋白如何控制质体发育。例如，我们的工作可能阐明植物如何应对盐度和干旱等压力，这是世界各地作物产量的主要限制。光合作用性能（以及植物生长所需的能量）受到压力的强烈影响，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以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.