Role of the TOC complex in plastid protein import

描述（由申请人提供）：细胞器的身份和发育依赖于一组复杂的细胞内蛋白运输系统，该系统介导核编码蛋白特异性靶向其适当的亚细胞区室。这些贩运系统不仅必须保持高度的特异性，而且往往还必须进行调整，以适应发育、压力和生理事件造成的贩运基质水平和组成的巨大变化。叶绿体已经从最初的内共生体进化到在不同的组织和细胞类型中执行专门的功能，从而产生了称为质体的各种相互关联的细胞器。质体的功能和发育依赖于~3000种不同的核编码蛋白的协调表达和翻译后输入。因此，质体是研究前蛋白识别和膜转位的理想模型，从而有助于我们了解细胞内蛋白质靶向和细胞器生物发生的基本过程。这个建议的一个主要目的是定义介导识别的靶向信号（转运肽）的叶绿体前体蛋白的TOC GTdR受体在叶绿体表面的决定因素和分子相互作用，并了解转运肽识别耦合到GTP依赖性转移的前体蛋白进入膜通道的细胞器的外膜的过程。TOC GTP酶的拟议作用使人想起信号识别颗粒（SRP/Ffh）和SRP受体（SR/FtsY）在前蛋白靶向ER和细菌细胞质膜中的作用。在这种情况下，TOC易位子代表了一个新的，但类似的例子，GTP调节开关，调节许多细胞内蛋白质靶向途径。我们的研究还表明，多种TOC途径已经进化，以平衡蛋白质靶向与伴随细胞发育、生理和应激事件的基因表达变化。我们将使用叶绿体系统来理解过程的网络，1）控制细胞器蛋白质从细胞核表达的总体水平，2）保持包含细胞器和细胞核编码亚基的多蛋白质复合物的stichiometry，3）响应细胞器状态或功能障碍，4）在从一种细胞器类型到另一种细胞器类型的发育过渡期间协调蛋白质谱的变化。

DESCRIPTION (provided by applicant): Organelle identity and development rely on a complex set of intracellular protein trafficking systems that mediate the specific targeting of nucleus-encoded proteins to their proper subcellular compartment. Not only must these trafficking systems maintain a high degree of specificity, they often must adapt to accommodate dramatic changes in the levels and composition of trafficking substrates imposed by developmental, stress and physiological events. Chloroplasts have evolved from the original endosymbiont to perform specialized functions in different tissues and cell types, giving rise to a diverse group of inter-related organelles called plastids. Plastid function and development rely on the coordinated expression and post-translational import of ~3000 different nucleus-encoded proteins. As such, plastids are an ideal model for studies of preprotein recognition and membrane translocation, and thereby, contribute to our knowledge of the basic processes of intracellular protein targeting and organelle biogenesis. A major aim of this proposal is to define the determinants and molecular interactions that mediate recognition of the targeting signals (transit peptides) of chloroplast preproteins by the TOC GTPase receptors at the chloroplast surface, and understand the process by which transit peptide recognition is coupled to GTP-dependent transfer of the preprotein into the membrane channel at the outer membrane of the organelle. The proposed roles of the TOC GTPases are reminiscent of the roles of the signal recognition particle (SRP/Ffh) and SRP receptor (SR/FtsY) in preprotein targeting to the ER and bacterial cytoplasmic membranes. In this context, the TOC translocon represent a novel, but analogous example of the GTP-regulated switches that regulate numerous intracellular protein targeting pathways. Our studies also suggest that multiple TOC pathways have evolved to balance protein targeting with the changes in gene expression that accompany developmental, physiological and stress events in cells. We will use the chloroplast system to understand the network of processes that 1) control the overall levels of organellar proteins expressed from the nucleus, 2) maintain the stichiometry of multi-protein complexes that contain both organelle and nucleus-encoded subunits, 3) respond to organelle status or dysfunction and, 4) coordinate changes in protein profiles during developmental transitions from one organelle type to another.