Department of Biosciences & Bioengineering, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India.
Biol Cell. 2021 Jan;113(1):58-78. doi: 10.1111/boc.202000050. Epub 2020 Nov 23.
Like other apicomplexan parasites, Toxoplasma gondii harbours a four-membraned endosymbiotic organelle - the apicoplast. Apicoplast proteins are nuclear encoded and trafficked to the organelle through the endoplasmic reticulum (ER). From the ER to the apicoplast, two distinct protein trafficking pathways can be used. One such pathway is the cell's secretory pathway involving the Golgi, whereas the other is a unique Golgi-independent pathway. Using different experimental approaches, many apicoplast proteins have been shown to utilize the Golgi-independent pathway, whereas a handful of reports show that a few proteins use the Golgi-dependent pathway. This has led to an emphasis towards the unique Golgi-independent pathway when apicoplast protein trafficking is discussed in the literature. Additionally, the molecular features that drive proteins to each pathway are not known.
In this report, we systematically test eight apicoplast proteins, using a C-terminal HDEL sequence to assess the role of the Golgi in their transport. We demonstrate that dually localised proteins of the apicoplast and mitochondrion (TgSOD2, TgTPx1/2 and TgACN/IRP) are trafficked through the Golgi, whereas proteins localised exclusively to the apicoplast are trafficked independent of the Golgi. Mutants of the dually localised proteins that localised exclusively to the apicoplast also showed trafficking through the Golgi. Phylogenetic analysis of TgSOD2, TgTPx1/2 and TgACN/IRP suggested that the evolutionary origins of TgSOD2 and TgTPx1/2 lie in the mitochondrion, whereas TgACN/IRP appears to have originated from the apicoplast.
Collectively, with these results, for the first time, we establish that the driver of the Golgi-dependent trafficking route to the apicoplast is the dual localisation of the protein to the apicoplast and the mitochondrion.
与其他顶复门寄生虫一样，刚地弓形虫拥有一个四膜共生细胞器——质体。质体蛋白由核编码，并通过内质网（ER）运输到细胞器。从 ER 到质体，可以使用两种不同的蛋白运输途径。一种途径是细胞的分泌途径，涉及高尔基体，而另一种途径是独特的高尔基体独立途径。使用不同的实验方法，已经证明许多质体蛋白利用高尔基体独立途径，而少数报道表明一些蛋白利用高尔基体依赖途径。这导致在文献中讨论质体蛋白运输时，重点放在独特的高尔基体独立途径上。此外，驱动蛋白进入每条途径的分子特征尚不清楚。
在本报告中，我们使用 C 端 HDEL 序列系统地测试了 8 种质体蛋白，以评估高尔基体在其运输中的作用。我们证明了质体和线粒体双重定位的蛋白（TgSOD2、TgTPx1/2 和 TgACN/IRP）通过高尔基体运输，而仅定位于质体的蛋白则独立于高尔基体运输。仅定位于质体的双重定位蛋白的突变体也显示通过高尔基体运输。TgSOD2、TgTPx1/2 和 TgACN/IRP 的系统发育分析表明，TgSOD2 和 TgTPx1/2 的进化起源来自线粒体，而 TgACN/IRP 似乎起源于质体。
总的来说，这些结果首次确立了高尔基体依赖性运输途径到质体的驱动因素是蛋白对质体和线粒体的双重定位。
