ENDOMEMBRANE STRUCTURE AND THE CHLOROPLAST PROTEIN TARGETING PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE, CHROMISTA)
ENDOMEMBRANE STRUCTURE AND THE CHLOROPLAST PROTEIN TARGETING PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE, CHROMISTA)
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赤潮异形藻（针藻科，CHROMISTA）的内膜结构和叶绿体蛋白靶向途径
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发表时间：
2000
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通讯作者：
B. Green
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作者：
K. Ishida;T. Cavalier;B. Green
异质藻类中的叶绿体被四层膜包围，可能起源于被真核宿主吞噬并保留的红藻内共生体。了解核编码的叶绿体蛋白如何穿过这些膜从细胞质转移到叶绿体中，可以让我们了解内共生体如何在次级共生过程中整合到宿主细胞中。在多质体异质藻类（如针藻类）中，尚不清楚叶绿体周围四层膜（叶绿体内质网 [CER] 膜）的最外层是否与核膜和粗面内质网 (ER) 连续。在这里，我们报告了针叶植物 Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara 的详细超微结构观察，结果表明 CER 膜与附着核糖体的 ER 膜连续，这意味着具有三个包膜膜的叶绿体位于 ER 腔内，即拓扑结构与单质体异质藻的结构相同。然而，与其他异质藻类的 CER 膜不同，赤潮藻的 CER 膜上附着的核糖体即使有，也非常少。为了验证蛋白质首先靶向内质网，我们使用核编码叶绿体蛋白（赤潮藻的岩藻黄素-叶绿素 a/c 蛋白）的前体测定了犬微粒体中的蛋白质输入。这证明前体具有用于 ER 靶向的功能性信号序列，并且共翻译易位到 ER 中，其中约 17 个氨基酸的信号序列被去除。基于这些数据，我们假设在赤潮人中，已共翻译转运至内质网腔的核编码叶绿体蛋白前体在内质网中进行分类，并通过内质网腔转运至叶绿体。
Chloroplasts in heterokont algae are surrounded by four membranes and probably originated from a red algal endosymbiont that was engulfed and retained by eukaryotic host. Understanding how nuclear‐encoded chloroplast proteins are translocated from the cytoplasm into the chloroplast across these membranes could give us some insights about how the endosymbiont was integrated into the host cell in the process of secondary symbiogenesis. In multiplastid heterokont algae such as raphidophytes, it has been unclear if the outermost of the four membranes surrounding the chloroplast (the chloroplast endoplasmic reticulum [CER] membrane) is continuous with the nuclear envelope and rough endoplasmic reticulum (ER). Here, we report detailed ultrastructural observations of the raphidophyte Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara that show that the CER membranes were continuous with ER membranes that had attached ribosomes, implying that the chloroplast with three envelope membranes is located within the ER lumen, that is, topologically the same structure as that of monoplastid heterokont algae. However, the CER membrane of H. akashiwo had very few, if any, ribosomes attached, unlike the CER membranes in other heterokont algae. To verify that proteins are first targeted to the ER, we assayed protein import into canine microsomes using a precursor for a nuclear‐encoded chloroplast protein, the fucoxanthin‐chlorophyll a/c protein of H. akashiwo. This demonstrated that the precursor has a functional signal sequence for ER targeting and is cotranslationally translocated into the ER, where a signal sequence of about 17 amino acids is removed. Based on these data, we hypothesize that in H. akashiwo, nuclear‐encoded chloroplast protein precursors that have been cotranslationally transported into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER lumen.