The Plastid of Toxoplasma gondii

描述（由申请人提供）：弓形虫是一种重要的机会致病菌，影响全世界多达三分之一的未经治疗的艾滋病患者。该寄生虫拥有一个独特的和必要的细胞器称为顶质体，这是遗传和功能相关的叶绿体。由于人类宿主缺乏类似的细胞器，顶质体是开发抗寄生虫剂的重要潜在靶点。关于这个细胞器的生物起源知之甚少，它由四层膜包围。到目前为止，仅探索了靶向顶质体腔。这些蛋白质是由允许进入分泌系统的信号序列和将它们从ER路由到质体的转运肽靶向的。我们已经提出了挑衅性的证据，后一步发生在一个高尔基体独立的方式。在过去的资助期间，我们确定了两个顶质体膜蛋白（转运蛋白和蛋白酶），这两个缺乏典型的靶向序列。与腔蛋白相反，这些蛋白显示出细胞周期依赖性定位于顶质体。由于这些差异以及不同的最终目的地，介导靶向非管腔蛋白的途径和序列可能涉及新的元件。该提案概述了旨在表征一组定位于各种顶质体隔室的蛋白质的实验，并将剖析这些蛋白质的适当定位的重要机制和序列。我们还将确定是否靶向顶质体膜蛋白是由已知参与ER出口过程或ER到高尔基体运输的分子辅助的。这些研究将扩大我们对这种独特细胞器的生物起源的理解，并提供有关其功能的更多信息。公共卫生相关性本项目拟对寄生虫弓形虫体内一种名为顶质体的植物样细胞器进行研究，弓形虫是一种可导致免疫功能低下个体脑炎和妊娠期先天缺陷的病原体。由于人类缺乏顶质体，它为针对寄生虫的新干预提供了潜在的目标。

DESCRIPTION (provided by applicant): Toxoplasma gondii is an important opportunistic pathogen, affecting up to one-third of untreated AIDS patients worldwide. The parasite possesses a unique and essential organelle called the apicoplast, which is genetically and functionally related to chloroplasts. Because the human host lacks a similar organelle, the apicoplast is an important potential target for development of anti-parasitic agents. Little is known about the biogenesis of this organelle, which is bounded by four membranes. Until now, only targeting to the apicoplast lumen has been explored. Such proteins are targeted courtesy of a signal sequence that allows entry into the secretory system and a transit peptide that routes them from the ER to the plastid. We have presented provocative evidence that the latter step occurs in a Golgi- independent manner. In the past grant period we identified two apicoplast membrane proteins (a transporter and a protease), both of which lack canonical targeting sequences. In contrast to luminal proteins, these proteins show cell cycle dependent localization to the apicoplast. Because of these differences as well as the distinct ultimate destinations, the routes and sequences that mediate targeting of non-luminal proteins likely involve novel elements. This proposal outlines experiments aimed at characterizing a set of proteins localized to various apicoplast compartments, and will dissect the mechanisms and sequences important for the proper localization of these proteins. We will also determine whether targeting of apicoplast membrane proteins is assisted by molecules known to be involved in ER exit processes or ER to Golgi trafficking. These studies will expand our understanding of the biogenesis of this unique organelle and provide additional information concerning its function. PUBLIC HEALTH RELEVANCE This project proposes to examine a plant-like organelle called the apicoplast in the parasite Toxoplasma gondii, an agent which causes encephalitis in immunocompromised individuals and birth defects when acquired during pregnancy. As humans lack an apicoplast, it provides a potential target for new interventions against the parasite.