Durnford Dion G, Schwartzbach Steven D
Department of Biology, University of New Brunswick, 10 Bailey Drive, Fredericton, NB, Canada, E3B 5A3.
Department of Biological Sciences, University of Memphis, Memphis, TN, 38152, USA.
Adv Exp Med Biol. 2017;979:183-205. doi: 10.1007/978-3-319-54910-1_10.
The lateral transfer of photosynthesis between kingdoms through endosymbiosis is among the most spectacular examples of evolutionary innovation. Euglena, which acquired a chloroplast indirectly through an endosymbiosis with a green alga, represents such an example. As with other endosymbiont-derived plastids from eukaryotes, there are additional membranes that surround the organelle, of which Euglena has three. Thus, photosynthetic genes that were transferred from the endosymbiont to the host nucleus and whose proteins are required in the new plastid, are now faced with targeting and plastid import challenges. Early immunoelectron microscopy data suggested that the light-harvesting complexes, photosynthetic proteins in the thylakoid membrane, are post-translationally targeted to the plastid via the Golgi apparatus, an unexpected discovery at the time. Proteins targeted to the Euglena plastid have complex, bipartite presequences that direct them into the endomembrane system, through the Golgi apparatus and ultimately on to the plastid, presumably via transport vesicles. From transcriptome sequencing, dozens of plastid-targeted proteins were identified, leading to the identification of two different presequence structures. Both have an amino terminal signal peptide followed by a transit peptide for plastid import, but only one of the two classes of presequences has a third domain-the stop transfer sequence. This discovery implied two different transport mechanisms; one where the protein was fully inserted into the lumen of the ER and another where the protein remains attached to, but effectively outside, the endomembrane system. In this review, we will discuss the biochemical and bioinformatic evidence for plastid targeting, discuss the evolution of the targeting system, and ultimately provide a working model for the targeting and import of proteins into the plastid of Euglena.
通过内共生实现的光合能力在不同生物界之间的横向转移是进化创新中最引人注目的例子之一。眼虫（Euglena）通过与绿藻的内共生间接获得了叶绿体，就是这样一个例子。与其他来自真核生物的内共生体衍生的质体一样，围绕该细胞器有额外的膜，眼虫有三层这样的膜。因此，从内共生体转移到宿主细胞核的光合基因，其蛋白质在新的质体中是必需的，现在面临着靶向定位和质体导入的挑战。早期的免疫电子显微镜数据表明，捕光复合体，即类囊体膜中的光合蛋白，是通过高尔基体在翻译后靶向定位到质体的，这在当时是一个意想不到的发现。靶向定位到眼虫质体的蛋白质具有复杂的双部分前导序列，这些序列将它们引导到内膜系统中，通过高尔基体，最终可能通过运输小泡到达质体。通过转录组测序，鉴定出了数十种靶向质体的蛋白质，从而确定了两种不同的前导序列结构。两者都有一个氨基末端信号肽，后面跟着一个用于质体导入的转运肽，但两类前导序列中只有一类有第三个结构域——终止转移序列。这一发现暗示了两种不同的运输机制；一种是蛋白质完全插入内质网腔，另一种是蛋白质仍然附着在内膜系统上，但实际上位于内膜系统之外。在这篇综述中，我们将讨论质体靶向定位的生化和生物信息学证据，讨论靶向系统的进化，并最终提供一个蛋白质靶向定位和导入眼虫质体的工作模型。
