Abstract
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.
Highlights
Compartmentalisation is an indispensable feature in biological systems, especially for eukaryotic cells
In order to address the question of molecular features that drive the choice of trafficking pathways of apicoplast proteins, the first step was to identify such features; these will be discussed individually
Most proteins destined to the apicoplast have bipartite signalling sequences at their N-termini (Waller et al, 1998b), directing them to the apicoplast, while proteins destined to the mitochondrion have mitochondrial targeting sequences mediating their uptake into the mitochondrion (Hay, Böhni & Gasser, 1984; Douglas, McCammon & Vassarotti, 1986)
Summary
Compartmentalisation is an indispensable feature in biological systems, especially for eukaryotic cells. Cellular compartments include endosymbiotic organelles, such as the mitochondria and chloroplasts, thought to have once been free-living prokaryotes that have since undergone an evolutionary transformation into the compartments that define the eukaryotic cell we know today. The apicoplast is an organelle enclosed by four distinct membranes (Ferguson et al, 2005; Lemgruber et al, 2013), which, like the mitochondrion and chloroplast, has evolved specialised mechanisms for the import of nuclearencoded proteins. In addition to the apicoplast, apicomplexans harbour another endosymbiotic organelle, the mitochondrion This organelle, like other eukaryotic mitochondria, acquires newly synthesised proteins from the cytosol through translocons of the Tim and Tom complexes, proteins that appear to be derived from the outer membrane of the free-living prokaryote that was engulfed during endosymbiosis (van Dooren et al, 2016)
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