Wunder Tobias, Martin Roman, Löffelhardt Wolfgang, Schleiff Enrico, Steiner Jürgen M
Ludwig-Maximilians-Universität Munich, Department of Biology I, VW-Research Group, Menzinger Str, 67, 80638 Munich, Germany.
BMC Evol Biol. 2007 Nov 28;7:236. doi: 10.1186/1471-2148-7-236.
Today it is widely accepted that plastids are of cyanobacterial origin. During their evolutionary integration into the metabolic and regulatory networks of the host cell the engulfed cyanobacteria lost their independency. This process was paralleled by a massive gene transfer from symbiont to the host nucleus challenging the development of a retrograde protein translocation system to ensure plastid functionality. Such a system includes specific targeting signals of the proteins needed for the function of the plastid and membrane-bound machineries performing the transfer of these proteins across the envelope membranes. At present, most information on protein translocation is obtained by the analysis of land plants. However, the analysis of protein import into the primitive plastids of glaucocystophyte algae, revealed distinct features placing this system as a tool to understand the evolutionary development of translocation systems. Here, bacterial outer membrane proteins of the Omp85 family have recently been discussed as evolutionary seeds for the development of translocation systems.
To further explore the initial mode of protein translocation, the observed phenylalanine dependence for protein translocation into glaucophyte plastids was pursued in detail. We document that indeed the phenylalanine has an impact on both, lipid binding and binding to proteoliposomes hosting an Omp85 homologue. Comparison to established import experiments, however, unveiled a major importance of the phenylalanine for recognition by Omp85. This finding is placed into the context of the evolutionary development of the plastid translocon.
The phenylalanine in the N-terminal domain signs as a prerequisite for protein translocation across the outer membrane assisted by a "primitive" translocon. This amino acid appears to be optimized for specifically targeting the Omp85 protein without enforcing aggregation on the membrane surface. The phenylalanine has subsequently been lost in the transit sequence, but can be found at the C-terminal position of the translocating pore. Thereby, the current hypothesis of Omp85 being the prokaryotic contribution to the ancestral Toc translocon can be supported.
如今，质体起源于蓝细菌这一观点已被广泛接受。在它们进化融入宿主细胞的代谢和调控网络过程中，被吞噬的蓝细菌失去了独立性。这一过程伴随着大量基因从共生体转移到宿主细胞核，这对逆行蛋白转运系统的发展提出了挑战，以确保质体的功能。这样一个系统包括质体功能所需蛋白质的特定靶向信号，以及负责将这些蛋白质转运过包膜的膜结合机制。目前，关于蛋白质转运的大多数信息是通过对陆地植物的分析获得的。然而，对蓝隐藻藻类原始质体中蛋白质导入的分析揭示了一些独特特征，使这个系统成为理解转运系统进化发展的工具。最近，Omp85家族的细菌外膜蛋白被认为是转运系统发展的进化起源。
为了进一步探索蛋白质转运的初始模式，详细研究了观察到的蛋白质转运到蓝藻质体中对苯丙氨酸的依赖性。我们证明，苯丙氨酸确实对脂质结合以及与含有Omp85同源物的蛋白脂质体的结合都有影响。然而，与已有的导入实验相比，揭示了苯丙氨酸对Omp85识别的重要性。这一发现被置于质体转运体进化发展的背景中。
N端结构域中的苯丙氨酸是由“原始”转运体协助蛋白质穿过外膜的先决条件。这种氨基酸似乎经过优化，可特异性靶向Omp85蛋白，而不会在膜表面导致聚集。苯丙氨酸随后在转运序列中消失，但可以在转运孔的C端位置找到。因此，可以支持目前关于Omp85是对祖先Toc转运体的原核贡献的假设。
