van Dooren Giel G, Tomova Cveta, Agrawal Swati, Humbel Bruno M, Striepen Boris
Center for Tropical and Emerging Global Diseases and Department of Cellular Biology, University of Georgia, Athens, GA 30602, USA.
Proc Natl Acad Sci U S A. 2008 Sep 9;105(36):13574-9. doi: 10.1073/pnas.0803862105. Epub 2008 Aug 29.
Apicomplexan parasites harbor a secondary plastid that has lost the ability to photosynthesize yet is essential for the parasite to multiply and cause disease. Bioinformatic analyses predict that 5-10% of all proteins encoded in the parasite genome function within this organelle. However, the mechanisms and molecules that mediate import of such large numbers of cargo proteins across the four membranes surrounding the plastid remain elusive. In this work, we identify a highly diverged member of the Tic20 protein family in Apicomplexa. We demonstrate that Tic20 of Toxoplasma gondii is an integral protein of the innermost plastid membrane. We engineer a conditional null-mutant and show that TgTic20 is essential for parasite growth. To characterize this mutant functionally, we develop several independent biochemical import assays to reveal that loss of TgTic20 leads to severe impairment of apicoplast protein import followed by organelle loss and parasite death. TgTic20 is the first experimentally validated protein import factor identified in apicoplasts. Our studies provide experimental evidence for a common evolutionary origin of import mechanisms across the innermost membranes of primary and secondary plastids.
顶复门寄生虫含有一个次生质体，该质体已失去光合作用能力，但对寄生虫的繁殖和致病至关重要。生物信息学分析预测，寄生虫基因组中编码的所有蛋白质中有5-10%在这个细胞器内发挥作用。然而，介导如此大量的货物蛋白穿过围绕质体的四层膜进行导入的机制和分子仍然不清楚。在这项工作中，我们在顶复门中鉴定出Tic20蛋白家族的一个高度分化的成员。我们证明，刚地弓形虫的Tic20是最内层质体膜的整合蛋白。我们构建了一个条件性无效突变体，并表明TgTic20对寄生虫生长至关重要。为了从功能上表征这个突变体，我们开发了几种独立的生化导入分析方法，以揭示TgTic20的缺失会导致质体蛋白导入严重受损，随后细胞器丢失和寄生虫死亡。TgTic20是在质体中鉴定出的第一个经过实验验证的蛋白导入因子。我们的研究为初级和次级质体最内层膜的导入机制的共同进化起源提供了实验证据。
