Department of Plant Biology, Cornell University, Ithaca, New York 14853, USA.
Plant Physiol. 2012 Mar;158(3):1172-92. doi: 10.1104/pp.111.193144. Epub 2012 Jan 24.
Plastoglobules (PGs) in chloroplasts are thylakoid-associated monolayer lipoprotein particles containing prenyl and neutral lipids and several dozen proteins mostly with unknown functions. An integrated view of the role of the PG is lacking. Here, we better define the PG proteome and provide a conceptual framework for further studies. The PG proteome from Arabidopsis (Arabidopsis thaliana) leaf chloroplasts was determined by mass spectrometry of isolated PGs and quantitative comparison with the proteomes of unfractionated leaves, thylakoids, and stroma. Scanning electron microscopy showed the purity and size distribution of the isolated PGs. Compared with previous PG proteome analyses, we excluded several proteins and identified six new PG proteins, including an M48 metallopeptidase and two Absence of bc1 complex (ABC1) atypical kinases, confirmed by immunoblotting. This refined PG proteome consisted of 30 proteins, including six ABC1 kinases and seven fibrillins together comprising more than 70% of the PG protein mass. Other fibrillins were located predominantly in the stroma or thylakoid and not in PGs; we discovered that this partitioning can be predicted by their isoelectric point and hydrophobicity. A genome-wide coexpression network for the PG genes was then constructed from mRNA expression data. This revealed a modular network with four distinct modules that each contained at least one ABC1K and/or fibrillin gene. Each module showed clear enrichment in specific functions, including chlorophyll degradation/senescence, isoprenoid biosynthesis, plastid proteolysis, and redox regulators and phosphoregulators of electron flow. We propose a new testable model for the PGs, in which sets of genes are associated with specific PG functions.
质体小球（PGs）是叶绿体中与类囊体相关的单层脂蛋白颗粒，含有 prenyl 和中性脂质以及几十种蛋白质，其中大多数蛋白质的功能未知。目前缺乏对 PG 作用的综合认识。本文通过对分离的 PGs 进行质谱分析，并与未分级叶片、类囊体和基质的蛋白质组进行定量比较，更好地定义了 PG 蛋白质组，并为进一步的研究提供了概念框架。通过对拟南芥（Arabidopsis thaliana）叶片叶绿体 PGs 的质谱分析和与未分级叶片、类囊体和基质的蛋白质组的定量比较，确定了 PG 蛋白质组。扫描电子显微镜显示了分离的 PGs 的纯度和大小分布。与之前的 PG 蛋白质组分析相比，我们排除了几种蛋白质，并鉴定了六个新的 PG 蛋白质，包括一个 M48 金属肽酶和两个 ABC1 非典型激酶，通过免疫印迹得到了证实。这个经过改进的 PG 蛋白质组由 30 种蛋白质组成，包括 6 种 ABC1 激酶和 7 种原纤维蛋白，它们共同占 PG 蛋白质质量的 70%以上。其他原纤维蛋白主要位于基质或类囊体中，而不在 PG 中；我们发现，这种分区可以通过它们的等电点和疏水性来预测。然后，根据 mRNA 表达数据构建了 PG 基因的全基因组共表达网络。结果显示，该网络由四个具有不同功能模块的模块组成，每个模块至少包含一个 ABC1K 和/或原纤维蛋白基因。每个模块都显示出对特定功能的明显富集，包括叶绿素降解/衰老、异戊二烯生物合成、质体蛋白水解、以及电子流的氧化还原调节剂和磷酸化调节剂。我们提出了一个新的、可测试的 PG 模型，其中一组基因与特定的 PG 功能相关。
