CaaX-Like Protease of Cyanobacterial Origin Is Required for Complex Plastid Biogenesis in Malaria Parasites.
CaaX-Like Protease of Cyanobacterial Origin Is Required for Complex Plastid Biogenesis in Malaria Parasites.
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作者：
Meister TR;Tang Y;Pulkoski-Gross MJ;Yeh E
引起疟疾的疟原虫和相关的顶端复合体是重要的人类和兽医病原体。这些寄生虫代表了真核生物中一个高度分化和研究不足的分支，因此经常违背模式生物设定的期望。顶生复合体生物学的一个突出例子是顶生质体，这是一种来自不寻常的次级(真核生物-真核生物)内共生的必需但非光合作用的质体。内生共生是细胞创新的主要驱动力，而质外体生物发生途径是分子进化的热点。我们之前对恶性疟原虫的质膜生物发生基因进行了无偏见的筛选，以揭示这些必要的和创新的途径。在这里，我们从我们的筛选中验证了一个新的候选基因，并表明它在质外体生物发生中的作用与模型真核生物预测的功能注释不匹配。我们的发现表明，在这一不同的病原体分支中，一条尚未确定的叶绿体维持途径已被重复用于复杂的叶绿体生物发生。疟原虫寄生虫和相关的顶端复合体含有一种重要的“复合体”细胞器，即顶生质体，这是次级内共生起源的。这种复杂的质体的生物发生构成了一个独特的挑战，需要进化出新的细胞机制。我们先前对必需的质外体生物发生基因进行了突变筛选，以发现具有进化和生物医学意义的细胞器途径。在这里，我们验证和表征了来自我们的筛选的候选基因Pf3D7_0913500。利用条件基因敲除菌株，我们发现Pf3D7_0913500的缺失导致了生长抑制，而这种抑制可以被质外体的唯一必需产物--焦磷酸异戊烯酯所拯救，并导致质外体的丧失。由于Pf3D7_0913500没有先前的功能注释，我们将其命名为apicplast--减去ipp-assued 4(AMR4)。AMR4具有一个注释的CAAX蛋白水解酶和细菌素加工(CPBP)结构域，在真核生物中通常表明在CAAX后烯基化加工中发挥作用。事实上，AMR4是疟原虫中唯一被认为需要蛋白质预烯基化的CAAX样蛋白酶，我们证实AMR4(E352)的保守催化残基是其质外体功能所必需的。然而，我们意外地发现AMR4在恶性疟原虫中并不参与CAAX的前化后处理途径。相反，我们发现AMR4被导入到顶生质体中，并来自蓝藻CPBP基因，该基因通过初级和次级内共生保留下来。我们的发现表明，AMR4不是一个真正的CAAX蛋白水解酶，而是执行一种保守的、未被描述的叶绿体功能，该功能一直被保留在复杂的叶绿体生物发生中。
Plasmodium parasites, which cause malaria, and related apicomplexans are important human and veterinary pathogens. These parasites represent a highly divergent and understudied branch of eukaryotes, and as such often defy the expectations set by model organisms. One striking example of unique apicomplexan biology is the apicoplast, an essential but nonphotosynthetic plastid derived from an unusual secondary (eukaryote-eukaryote) endosymbiosis. Endosymbioses are a major driver of cellular innovation, and apicoplast biogenesis pathways represent a hot spot for molecular evolution. We previously conducted an unbiased screen for apicoplast biogenesis genes in P. falciparum to uncover these essential and innovative pathways. Here, we validate a novel gene candidate from our screen and show that its role in apicoplast biogenesis does not match its functional annotation predicted by model eukaryotes. Our findings suggest that an uncharacterized chloroplast maintenance pathway has been reused for complex plastid biogenesis in this divergent branch of pathogens. Plasmodium parasites and related apicomplexans contain an essential “complex plastid” organelle of secondary endosymbiotic origin, the apicoplast. Biogenesis of this complex plastid poses a unique challenge requiring evolution of new cellular machinery. We previously conducted a mutagenesis screen for essential apicoplast biogenesis genes to discover organellar pathways with evolutionary and biomedical significance. Here we validate and characterize a gene candidate from our screen, Pf3D7_0913500. Using a conditional knockdown strain, we show that Pf3D7_0913500 depletion causes growth inhibition that is rescued by the sole essential product of the apicoplast, isopentenyl pyrophosphate (IPP), and results in apicoplast loss. Because Pf3D7_0913500 had no previous functional annotation, we name it apicoplast-minus IPP-rescued 4 (AMR4). AMR4 has an annotated CaaX protease and bacteriocin processing (CPBP) domain, which in eukaryotes typically indicates a role in CaaX postprenylation processing. Indeed, AMR4 is the only putative CaaX-like protease in Plasmodium parasites which are known to require protein prenylation, and we confirm that the conserved catalytic residue of AMR4 (E352) is required for its apicoplast function. However, we unexpectedly find that AMR4 does not act in a CaaX postprenylation processing pathway in Plasmodium falciparum. Instead, we find that AMR4 is imported into the apicoplast and is derived from a cyanobacterial CPBP gene which was retained through both primary and secondary endosymbiosis. Our findings suggest that AMR4 is not a true CaaX protease, but instead it performs a conserved, uncharacterized chloroplast function that has been retained for complex plastid biogenesis.