Role of the TOC complex in plastid protein import

描述(由申请人提供)：细胞器的识别和发育依赖于一套复杂的细胞内蛋白质运输系统，该系统介导核编码蛋白质的特定靶向到其适当的亚细胞室。这些贩运系统不仅必须保持高度的专一性，而且往往还必须适应发展、压力和生理事件造成的贩运底物水平和组成的急剧变化。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特殊功能，产生了一组不同的相互关联的细胞器，称为叶绿体。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，叶绿体是研究前蛋白识别和膜转运的理想模型，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。这一建议的主要目的是确定决定因素和分子相互作用，介导叶绿体表面的TOC GTP酶受体识别叶绿体前蛋白的靶向信号(转运肽)，并了解转运肽识别与依赖GTP的前蛋白进入细胞器外膜通道的过程。所提出的TOC GTP酶的作用使人想起信号识别颗粒(SRP/FFH)和SRP受体(SR/FtsY)在前蛋白靶向内质网和细菌细胞膜中的作用。在这种背景下，TOC转运子代表了一个新颖的、但类似的GTP调节开关的例子，该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了解以下过程的网络：1)控制从细胞核表达的细胞器蛋白质的总体水平；2)维持同时包含细胞器和核编码亚基的多蛋白质复合体的密度计量；3)对细胞器的状态或功能障碍做出反应；4)协调从一种细胞器类型向另一种细胞器类型的发育转变过程中蛋白质谱的变化。

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.