A Remarkable Self-Organization Process as the Origin of Primitive Functional Cells
A Remarkable Self-Organization Process as the Origin of Primitive Functional Cells
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DOI：
10.1002/anie.201306613
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发表时间：
2013-12-09
影响因子：
16.6
通讯作者：
Luisi, Pier Luigi
中科院分区：
文献类型：
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作者：
Stano, Pasquale;D'Aguanno, Erica;Luisi, Pier Luigi
对生命起源的研究通常集中在自我复制的化学上，[1-3]而较少关注可能揭示细胞作为分隔和动态化学系统出现的实验。现代观点认为脂质囊泡（脂质体）是原始细胞系统最合理的模型;[4-6]然而，对脂质体形成和溶质截留（细胞起源的关键事件）的实验研究迄今尚未导致对脂质体内功能性遗传代谢网络组装的解释。解决细胞起源问题的一种方法是对简化的化学系统进行实验研究。这样的模型应该足够复杂，以显示类似简单细胞的行为;然而，它们应该在组成和功能方面得到很好的表征。作为一个可行的模型，研究脂质体内的“最小”代谢的发病，我们研究了蛋白质合成的无细胞转录和翻译（TX-TL）系统。这些多分子系统包含总共约80种不同的大分子组分，包括RNA聚合酶、核糖体、tRNA、氨酰-tRNA内切酶、翻译因子和生物能相关酶（以及低分子量化合物如氨基酸和核苷酸），并且能够从相应的DNA序列开始合成蛋白质。大肠杆菌细胞提取物或充分表征的PURE系统[7]（含有TX-TL活性所需的最小数量的分子的重构试剂盒），一旦向混合物提供相应的DNA序列，就产生功能性蛋白。这是因为商业TX-TL混合物是浓缩的，因为它直接来自细胞提取物。那么，真实的问题是，在生命起源前的条件下，这些成分是如何达到临界浓度的。事实上，当我们考虑益生元溶液并假设大分子成分以某种方式自行发展时，原始海洋或泻湖环境中的溶液只能高度稀释，因此不起反应。我们推断，这种组分的积累可能发生在脂质体中，因此可能在浓缩TX-TL分子中发挥积极作用，正如我们报告的单一物种截留的简单情况（铁蛋白[8]或核糖体[9]用作模型化合物）。换句话说，我们提出了一种情况，其中脂膜的形成和闭合以形成脂质体可能是稀释溶质局部高浓度（过度拥挤）的驱动力，原则上，这一事件可能导致代谢的开始。原始细胞中。在此，我们因此专注于研究脂质体内的蛋白质的合成，脂质体是在稀释的TX-TL混合物的存在下原位形成的。这种稀释的非反应性溶液模拟了海洋或淡水泻湖中可能的生命起源场景，其中生命的组成部分（DNA，RNA和蛋白质）已经独立形成，但由于稀释度极高而无法反应。特别是，我们解决了这样一个问题，即脂质体的自发形成是否会导致囊泡内混合物的所有组分自发浓缩，从而使蛋白质合成在这些细胞样隔室中有效进行，而在游离溶液中不会进行相同的反应（图1）。当用缓冲液以1：1的比例稀释时，原始TX-TL混合物已经变得不反应（对于...
Research into the origin of life has often focused on the chemistry of self-replication,[1–3] whereas less attention has been devoted to experiments that might reveal the emergence of cells as compartmentalized and dynamic chemical systems. Modern views consider lipid vesicles (liposomes) as the most plausible model of primitive cellular systems;[4–6] however, experimental investigations into liposome formation and solute entrapment, a key event for the origin of cells, has not lead to an explanation for the assembly of a functional genetic–metabolic network inside liposomes to date. One way to tackle the question about the origin of cells is to carry out experimental research on simplified chemical systems. Such models should be complex enough so as to display a simple-cell-like behavior; however, they should be well-characterized in terms of composition and function. As a viable model for investigating the onset of “minimal” metabolism inside liposomes, we investigated protein synthesis by cell-free transcription and translation (TX-TL) systems. These multimolecular systems contain a total of about 80 different macromolecular components, including RNA polymerase, ribosomes, tRNAs, aminoacyl-tRNA synthases, translation factors, and bioenergy-related enzymes (as well as low-molecular-weight compounds such as amino acids and nucleotides), and are capable of synthesizing a protein starting from the corresponding DNA sequence.In a TX-TL system, such as a commercially available E. coli cellular extract or the well-characterized PURE system [7](a reconstituted kit that contains the minimal number of molecules necessary for TX-TL activity), a functional protein is produced as soon as the corresponding DNA sequence is supplied to the mixture. This happens because the commercial TX-TL mixture is concentrated, as it comes directly from a cell extract. The real question then is, how such a critical concentration of the components might have been reached under prebiotic conditions. In fact, when we consider a prebiotic solution and assume that the macromolecular components have somehow developed by themselves, the solution in a primitive marine or lagoon environment can only be highly diluted and therefore unreactive. We reasoned that such an accumulation of components could occur within liposomes, which might thus provide an active role in concentrating the TX-TL molecules, as we reported for a simple case of single-species entrapment (ferritin [8] or ribosomes [9] were used as model compounds). In other words, we propose a scenario where the formation and closure of lipid membranes to form liposomes could have been the driving force for a high local concentration (overcrowding) of diluted solutes, an event that, in principle, may have led to the onset of metabolism in primitive cells. Herein, we therefore focus on the investigation of the synthesis of proteins inside liposomes, which are formed in situ in the presence of a diluted TX-TL mixture. Such a dilute, non-reactive solution simulates a possible origin-oflife scenario in a marine or fresh-water lagoon, where the components of life (DNA, RNA, and proteins) have formed independently, but cannot react because of the extremely high dilution. In particular, we address the question whether the spontaneous formation of liposomes might bring about a spontaneous concentration of all components of the mixture inside vesicles, so that protein synthesis proceeds efficiently within these cell-like compartments, whereas the same reaction does not proceed in free solution (Figure 1). The original TX-TL mixture already becomes unreactive when it is diluted with a buffer in a 1: 1 ratio (for …