Karnataki Anuradha, Derocher Amy, Coppens Isabelle, Nash Coral, Feagin Jean E, Parsons Marilyn
Seattle Biomedical Research Institute, 307 Westlake Ave. N., Seattle, WA 98109, USA.
Mol Microbiol. 2007 Mar;63(6):1653-68. doi: 10.1111/j.1365-2958.2007.05619.x.
The apicoplast is a relict plastid essential for viability of the apicomplexan parasites Toxoplasma and Plasmodium. It is surrounded by multiple membranes that proteins, substrates and metabolites must traverse. Little is known about apicoplast membrane proteins, much less their sorting mechanisms. We have identified two sets of apicomplexan proteins that are homologous to plastid membrane proteins that transport phosphosugars or their derivatives. Members of the first set bear N-terminal extensions similar to those that target proteins to the apicoplast lumen. While Toxoplasma gondii lacks this type of translocator, the N-terminal extension from the Plasmodium falciparum sequence was shown to be functional in T. gondii. The second set of translocators lacks an N-terminal targeting sequence. This translocator, TgAPT1, when tagged with HA, localized to multiple apicoplast membranes in T. gondii. Contrasting with the constitutive targeting of luminal proteins, the localization of the translocator varied during the cell cycle. Early-stage parasites showed circumplastid distribution, but as the plastid elongated in preparation for division, vesicles bearing TgAPT1 appeared adjacent to the plastid. After plastid division, the protein resumes a circumplastid colocalization. These studies demonstrate for the first time that vesicular trafficking likely plays a role in the apicoplast biogenesis.
顶质体是顶复门寄生虫弓形虫和疟原虫生存所必需的残余质体。它被多层膜包围，蛋白质、底物和代谢产物必须穿过这些膜。关于顶质体膜蛋白，人们了解甚少，其分选机制更是知之甚少。我们已经鉴定出两组与转运磷酸糖或其衍生物的质体膜蛋白同源的顶复门蛋白。第一组蛋白的成员带有与将蛋白质靶向顶质体腔的序列相似的N端延伸。虽然弓形虫缺乏这种类型的转运体，但恶性疟原虫序列的N端延伸在弓形虫中显示出功能。第二组转运体缺乏N端靶向序列。这种转运体TgAPT1，当用HA标记时，定位于弓形虫的多个顶质体膜上。与腔蛋白的组成型靶向不同，转运体的定位在细胞周期中有所变化。早期寄生虫显示出围绕质体的分布，但随着质体伸长准备分裂，携带TgAPT1的囊泡出现在质体附近。质体分裂后，该蛋白恢复围绕质体的共定位。这些研究首次证明囊泡运输可能在顶质体生物发生中起作用。
