Tarr Sarah J, Osborne Andrew R
Institute of Structural and Molecular Biology, Division of Biosciences, Birkbeck and University College London, London, United Kingdom.
PLoS One. 2015 Apr 7;10(4):e0121786. doi: 10.1371/journal.pone.0121786. eCollection 2015.
The malaria parasite exports hundreds of proteins into its host cell. The majority of exported proteins contain a Host-Targeting motif (also known as a Plasmodium export element) that directs them for export. Prior to export, the Host-Targeting motif is cleaved by the endoplasmic reticulum-resident protease Plasmepsin V and the newly generated N-terminus is N-α-acetylated by an unidentified enzyme. The cleaved, N-α-acetylated protein is trafficked to the parasitophorous vacuole, where it is translocated across the vacuole membrane. It is clear that cleavage and N-α-acetylation of the Host-Targeting motif occur at the endoplasmic reticulum, and it has been proposed that Host-Targeting motif cleavage and N-α-acetylation occur either on the luminal or cytosolic side of the endoplasmic reticulum membrane. Here, we use self-associating 'split' fragments of GFP to determine the topology of Plasmepsin V in the endoplasmic reticulum membrane; we show that the catalytic protease domain of Plasmepsin V faces the endoplasmic reticulum lumen. These data support a model in which the Host-Targeting motif is cleaved and N-α-acetylated in the endoplasmic reticulum lumen. Furthermore, these findings suggest that cytosolic N-α-acetyltransferases are unlikely to be candidates for the N-α-acetyltransferase of Host-Targeting motif-containing exported proteins.
疟原虫会向其宿主细胞输出数百种蛋白质。大多数输出的蛋白质都含有一个宿主靶向基序（也称为疟原虫输出元件），该基序指导它们进行输出。在输出之前，宿主靶向基序会被内质网驻留蛋白酶Plasmepsin V切割，新生成的N端会被一种未知酶进行N-α-乙酰化修饰。被切割且N-α-乙酰化的蛋白质会被转运至寄生泡，在那里它会穿过泡膜。很明显，宿主靶向基序的切割和N-α-乙酰化在内质网发生，并且有人提出宿主靶向基序的切割和N-α-乙酰化发生在内质网膜的腔面或胞质面。在此，我们使用绿色荧光蛋白（GFP）的自缔合“分裂”片段来确定Plasmepsin V在内质网膜中的拓扑结构；我们发现Plasmepsin V的催化蛋白酶结构域面向内质网腔。这些数据支持了一个模型，即宿主靶向基序在内质网腔中被切割并进行N-α-乙酰化修饰。此外，这些发现表明，胞质N-α-乙酰转移酶不太可能是含宿主靶向基序的输出蛋白的N-α-乙酰转移酶候选者。
