Arrest of Nuclear Division in Plasmodium through Blockage of Erythrocyte Surface Exposed Ribosomal Protein P2

Malaria parasites reside inside erythrocytes and the disease manifestations are linked to the growth inside infected erythrocytes (IE). The growth of the parasite is mostly confined to the trophozoite stage during which nuclear division occurs followed by the formation of cell bodies (schizogony). The mechanism and regulation of schizogony are poorly understood. Here we show a novel role for a Plasmodium falciparum 60S stalk ribosomal acidic protein P2 (PfP2) (PFC0400w), which gets exported to the IE surface for 6–8 hrs during early schizogony, starting around 26–28 hrs post-merozoite invasion. The surface exposure is demonstrated using multiple PfP2-specific monoclonal antibodies, and is confirmed through transfection using PfP2-GFP. The IE surface-exposed PfP2-protein occurs mainly as SDS-resistant P2-homo-tetramers. Treatment with anti-PfP2 monoclonals causes arrest of IEs at the first nuclear division. Upon removal of the antibodies, about 80–85% of synchronized parasites can be released even after 24 hrs of antibody treatment. It has been reported that a tubovesicular network (TVN) is set up in early trophozoites which is used for nutrient import. Anti-P2 monoclonal antibodies cause a complete fragmentation of TVN by 36 hrs, and impairs lipid import in IEs. These may be downstream causes for the cell-cycle arrest. Upon antibody removal, the TVN is reconstituted, and the cell division progresses. Each of the above properties is observed in the rodent malaria parasite species P. yoelii and P. berghei. The translocation of the P2 protein to the IE surface is therefore likely to be of fundamental importance in Plasmodium cell division.疟原虫寄生于红细胞（erythrocyte）内，该病的临床症状与寄生虫在感染红细胞（infected erythrocytes, IE）内的增殖密切相关。该寄生虫的增殖主要局限于滋养体阶段（trophozoite stage），此阶段先发生核分裂，随后形成细胞体，即裂殖生殖（schizogony）。目前学界对裂殖生殖的调控机制仍知之甚少。本研究揭示了恶性疟原虫（Plasmodium falciparum）60S茎状核糖体酸性蛋白P2（PfP2，基因编号PFC0400w）的全新功能：在裂殖生殖早期，即裂殖子入侵红细胞后约26~28小时开始，该蛋白会被转运至感染红细胞表面，持续时长约6~8小时。研究团队通过多种PfP2特异性单克隆抗体（monoclonal antibody）验证了该蛋白的膜表面暴露特性，并通过PfP2-GFP转染实验进一步证实了这一结果。感染红细胞表面的PfP2蛋白主要以抗十二烷基硫酸钠（sodium dodecyl sulfate, SDS）的P2同源四聚体形式存在。使用抗PfP2单克隆抗体处理后，感染红细胞会在首次核分裂阶段发生增殖阻滞。即便经过24小时的抗体处理，移除抗体后仍有约80%~85%的同步化疟原虫可恢复增殖。已有研究表明，早期滋养体阶段会形成管状泡状网络（tubovesicular network, TVN），用于摄取营养物质。抗P2单克隆抗体可在感染后36小时内完全破坏管状泡状网络，并损伤感染红细胞的脂质摄取能力。上述现象或为细胞周期阻滞的下游诱因。移除抗体后，管状泡状网络可重新形成，细胞分裂过程得以恢复。上述所有特性在啮齿类疟原虫——约氏疟原虫（P. yoelii）和伯氏疟原虫（P. berghei）中均得到了验证。由此可见，P2蛋白向感染红细胞表面的转运过程，可能对疟原虫的细胞分裂具有核心意义。