Schwartzbach S D, Osafune T, Löffelhardt W
School of Biological Sciences, University of Nebraska, Lincoln 68588, USA.
Plant Mol Biol. 1998 Sep;38(1-2):247-63.
Higher-plant, green and red algal chloroplasts are surrounded by a double membrane envelope. The glaucocystophyte plastid (cyanelle) has retained a prokaryotic cell wall between the two envelope membranes. The complex chloroplasts of Euglena and dinoflagellates are surrounded by three membranes while the complex chloroplasts of chlorarachniophytes, cryptomonads, brown algae, diatoms and other chromophytes, are surrounded by 4 membranes. The peptidoglycan layer of the cyanelle envelope and the additional membranes of complex chloroplasts provide barriers to chloroplast protein import not present in the simpler double membrane chloroplast envelope. Analysis of presequence structure and in vitro import experiments indicate that proteins are imported directly from the cytoplasm across the two envelope membranes and peptidoglycan layer into cyanelles. Protein import into complex chloroplasts is however fundamentally different. Analysis of presequence structure and in vitro import into microsomal membranes has shown that translocation into the ER is the first step for protein import into complex chloroplasts enclosed by three or four membranes. In vivo pulse chase experiments and immunoelectronmicroscopy have shown that in Euglena, proteins are transported from the ER to the Golgi apparatus prior to import across the three chloroplast membranes. Ultrastructural studies and the presence of ribosomes on the outermost of the four envelope membranes suggests protein import into 4 membrane-bounded complex chloroplasts is directly from the ER like outermost membrane into the chloroplast. The fundamental difference in import mechanisms, posttranslational direct chloroplast import or co-translational translocation into the ER prior to chloroplast import, appears to reflect the evolutionary origin of the different chloroplast types. Chloroplasts with a two-membrane envelope are thought to have evolved through the primary endosymbiotic association between a eukaryotic host and a photosynthetic prokaryote while complex chloroplasts are believed to have evolved through a secondary endosymbiotic association between a heterotrophic or possibly phototrophic eukaryotic host and a photosynthetic eukaryote.
高等植物、绿藻和红藻的叶绿体被双层膜包被。蓝隐藻的质体（蓝小体）在两层包膜之间保留了原核细胞壁。眼虫和甲藻的复杂叶绿体被三层膜包围，而绿藻门、隐藻、褐藻、硅藻和其他色素体植物的复杂叶绿体则被四层膜包围。蓝小体包膜的肽聚糖层和复杂叶绿体的额外膜为叶绿体蛋白质输入提供了屏障，而简单的双层膜叶绿体包膜则不存在这种屏障。前导序列结构分析和体外输入实验表明，蛋白质直接从细胞质穿过两层包膜和肽聚糖层进入蓝小体。然而，蛋白质输入复杂叶绿体的过程却有根本不同。前导序列结构分析和体外输入微粒体膜的实验表明，转运到内质网是蛋白质输入被三层或四层膜包围的复杂叶绿体的第一步。体内脉冲追踪实验和免疫电子显微镜显示，在眼虫中，蛋白质在穿过三层叶绿体膜输入之前，先从内质网运输到高尔基体。超微结构研究以及四层包膜最外层存在核糖体表明，蛋白质输入四层膜包围的复杂叶绿体是直接从内质网样的最外层膜进入叶绿体。输入机制的根本差异，即翻译后直接输入叶绿体或在叶绿体输入之前共翻译转运到内质网，似乎反映了不同类型叶绿体的进化起源。具有双层包膜的叶绿体被认为是通过真核宿主与光合原核生物之间的初级内共生结合进化而来的，而复杂叶绿体则被认为是通过异养或可能是光合真核宿主与光合真核生物之间的次级内共生结合进化而来的。
