Schünemann Danja
Lehrstuhl für Allgemeine und Molekulare Botanik, Ruhr-Universität Bochum, 44780 Bochum, Germany.
Curr Genet. 2004 Jan;44(6):295-304. doi: 10.1007/s00294-003-0450-z. Epub 2003 Oct 21.
The targeting of proteins, including the insertion and translocation of proteins in or across membranes, is a fundamental process within a cell, and a variety of specialized mechanisms for protein transport have been developed during evolution. The signal recognition particle (SRP) is found in the cytoplasm of most, if not all, eukaryotes and prokaryotes where it plays a central role in the co-translational insertion of membrane proteins into the endoplasmic reticulum and plasma membrane, respectively. SRP is a ribonucleoprotein consisting of an RNA and at least one polypeptide of approximately 54 kDa (SRP54). Interestingly, chloroplasts contain a specialized type of signal recognition particle. Chloroplast SRP (cpSRP) contains a SRP54 homologue but differs strikingly from cytosolic SRP in various aspects of structure and function. In contrast to cytosolic SRP, it contains a novel protein subunit (cpSRP43) and lacks RNA. CpSRP is also distinctive in its ability to interact with its substrate, light-harvesting chlorophyll a/ b-binding protein, post-translationally. Furthermore, it is remarkable that the 54 kDa subunit of cpSRP is also involved in the co-translational transport of chloroplast-encoded thylakoid proteins, and is therefore able to switch between the co- and post-translational means of interaction with its respective substrate proteins.
蛋白质的靶向定位，包括蛋白质在膜内或跨膜的插入和转运，是细胞内的一个基本过程，并且在进化过程中已经发展出了多种专门的蛋白质运输机制。信号识别颗粒（SRP）存在于大多数（如果不是全部的话）真核生物和原核生物的细胞质中，它在膜蛋白共翻译插入内质网和质膜的过程中分别发挥着核心作用。SRP是一种核糖核蛋白，由一种RNA和至少一种约54 kDa的多肽（SRP54）组成。有趣的是，叶绿体含有一种特殊类型的信号识别颗粒。叶绿体SRP（cpSRP）包含一个SRP54同源物，但在结构和功能的各个方面与胞质SRP有显著差异。与胞质SRP不同，它含有一个新的蛋白质亚基（cpSRP43）并且缺乏RNA。CpSRP在翻译后与其底物捕光叶绿素a/b结合蛋白相互作用的能力方面也很独特。此外，值得注意的是，cpSRP的54 kDa亚基也参与叶绿体编码的类囊体蛋白的共翻译运输，因此能够在与其各自底物蛋白相互作用的共翻译和翻译后方式之间切换。
