Duy Daniela, Soll Jürgen, Philippar Katrin
Department Biologie 1, Botanik, Biochemie und Physiologie der Pflanzen, Ludwig-Maximilians-Universität München, Menzingerstrasse 67, D-80638 Munich, Germany.
Biol Chem. 2007 Sep;388(9):879-89. doi: 10.1515/BC.2007.120.
Chloroplasts, unique organelles of plants, originated from endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. It is assumed that the outer envelope membrane, which delimits the chloroplast from the surrounding cytosol, was thus inherited from its Gram-negative bacterial ancestor. This plastid-specific membrane is thus equipped with elements of prokaryotic and eukaryotic origin. In particular, the membrane-intrinsic outer envelope proteins (OEPs) form solute channels with properties reminiscent of porins and channels in the bacterial outer membrane. OEP channels are characterised by distinct specificities for metabolites and a quite peculiar expression pattern in specialised plant organs and plastids, thus disproving the assumption that the outer envelope is a non-specific molecular sieve. The same is true for the outer membrane of Gram-negative bacteria, which functions as a permeability barrier in addition to the cytoplasmic membrane, and embeds different classes of channel pores. The channels of these prokaryotic prototype proteins, ranging from unspecific porins to specific channels to ligand-gated receptors, are exclusively built of beta-barrels. Although most of the OEP channels are formed by beta-strands as well, phylogeny based on sequence homology alone is not feasible. Thus, the comparison of structural and functional properties of chloroplast outer envelope and bacterial outer membrane channels is required to pinpoint the ancestral OEP 'portrait gallery'.
叶绿体是植物特有的细胞器，起源于当今蓝细菌的一个祖先与一个含有线粒体的宿主细胞的内共生。据推测，将叶绿体与周围细胞质分隔开的外被膜是从其革兰氏阴性细菌祖先那里继承而来的。因此，这种质体特异性膜配备了原核和真核起源的成分。特别是，膜内在外被蛋白（OEPs）形成溶质通道，其特性让人联想到细菌外膜中的孔蛋白和通道。OEP通道的特点是对代谢物具有明显的特异性，并且在专门的植物器官和质体中有相当独特的表达模式，从而推翻了外被膜是一种非特异性分子筛的假设。革兰氏阴性细菌的外膜也是如此，它除了作为细胞质膜外，还起到渗透屏障的作用，并嵌入不同类别的通道孔。这些原核原型蛋白的通道，从不特异性孔蛋白到特异性通道再到配体门控受体，完全由β桶组成。尽管大多数OEP通道也是由β链形成的，但仅基于序列同源性的系统发育分析是不可行的。因此，需要比较叶绿体外被膜和细菌外膜通道的结构和功能特性，以确定祖先OEP的“画像集”。
