Knockdown of the translocon protein EXP2 in Plasmodium falciparum reduces growth and protein export

Malaria parasites remodel their host erythrocytes to gain nutrients and avoid the immune system. Host erythrocytes are modified by hundreds of effector proteins exported from the parasite into the host cell. Protein export is mediated by the PTEX translocon comprising five core components of which EXP2 is considered to form the putative pore that spans the vacuole membrane enveloping the parasite within its erythrocyte. To explore the function and importance of EXP2 for parasite survival in the asexual blood stage of Plasmodium falciparum we inducibly knocked down the expression of EXP2. Reduction in EXP2 expression strongly reduced parasite growth proportional to the degree of protein knockdown and tended to stall development about half way through the asexual cell cycle. Once the knockdown inducer was removed and EXP2 expression restored, parasite growth recovered dependent upon the length and degree of knockdown. To establish EXP2 function and hence the basis for growth reduction, the trafficking of an exported protein was monitored following EXP2 knockdown. This resulted in severe attenuation of protein export and is consistent with EXP2, and PTEX in general, being the conduit for export of proteins into the host compartment.

疟原虫（Malaria parasites）会重塑宿主红细胞以获取营养并逃避免疫系统。寄生虫向宿主细胞分泌的数百种效应蛋白会修饰宿主红细胞。蛋白质输出由PTEX转运复合体（PTEX translocon）介导，该复合体包含5个核心组分，其中EXP2被认为是构成包裹红细胞内寄生虫的液泡膜的推定孔道。为探究EXP2在恶性疟原虫（Plasmodium falciparum）无性血液阶段中对寄生虫存活的功能与重要性，我们对EXP2的表达进行了诱导性敲低。EXP2表达水平的下调显著抑制了寄生虫的增殖，且抑制程度与敲低程度呈正比，同时往往会使发育停滞在无性细胞周期的约中途阶段。当移除敲低诱导剂并恢复EXP2表达后，寄生虫的增殖能力会根据敲低的持续时长与强度得到恢复。为明确EXP2的功能及增殖抑制的内在机制，我们在EXP2敲低后对一种输出型蛋白的转运过程进行了监测。结果显示蛋白质输出受到严重阻滞，这与EXP2乃至整体PTEX复合体作为蛋白质向宿主区室输出的通道这一结论相符。