The Plastid of Toxoplasma gondii

简介（由申请人提供）：刚地弓形虫是一种重要的机会性病原体，感染了全世界三分之一未经治疗的艾滋病患者。这种寄生虫拥有一种独特而重要的细胞器，称为顶质体，它在遗传和功能上与叶绿体相关。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。这个由四层膜组成的细胞器的生物起源尚不清楚。到目前为止，只对顶质体管腔进行了靶向研究。这类蛋白质的目标是一个允许进入分泌系统的信号序列和一个将它们从内质网运送到质体的转运肽。我们已经提出了具有挑衅性的证据，证明后一个步骤是以高尔基独立的方式发生的。在过去的资助期内，我们鉴定了两种顶质体膜蛋白（一种转运蛋白和一种蛋白酶），它们都缺乏规范的靶向序列。与管腔蛋白相反，这些蛋白显示细胞周期依赖于顶质体的定位。由于这些差异以及不同的最终目的地，介导非腔内蛋白靶向的途径和序列可能涉及新的元件。本提案概述了旨在表征一组定位于各种顶质体室的蛋白质的实验，并将剖析这些蛋白质正确定位的重要机制和序列。我们还将确定顶质体膜蛋白的靶向是否由已知参与内质网出口过程或内质网到高尔基转运的分子辅助。这些研究将扩大我们对这种独特细胞器的生物发生的理解，并提供有关其功能的额外信息。该项目旨在研究弓形虫体内一种叫做顶质体的植物样细胞器，弓形虫是一种在免疫功能低下的个体中引起脑炎和怀孕期间获得的出生缺陷的病原体。由于人类缺乏顶质体，它为对抗寄生虫的新干预措施提供了一个潜在的目标。

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.