Apicomplexan parasites discharge specialized organelles called rhoptries upon host cell contact to mediate invasion. The events that drive rhoptry discharge are poorly understood, yet essential to sustain the apicomplexan parasitic life cycle. Rhoptry discharge appears to depend on proteins secreted from another set of organelles called micronemes, which vary in function from allowing host cell binding to facilitation of gliding motility. Here we examine the function of the microneme protein CLAMP, which we previously found to be necessary for Toxoplasma gondii host cell invasion, and demonstrate its essential role in rhoptry discharge. CLAMP forms a distinct complex with two other microneme proteins, the invasion-associated SPATR, and a previously uncharacterized protein we name CLAMP-linked invasion protein (CLIP). CLAMP deficiency does not impact parasite adhesion or microneme protein secretion; however, knockdown of any member of the CLAMP complex affects rhoptry discharge. Phylogenetic analysis suggests orthologs of the essential complex components, CLAMP and CLIP, are ubiquitous across apicomplexans. SPATR appears to act as an accessory factor in Toxoplasma, but despite incomplete conservation is also essential for invasion during Plasmodium falciparum blood stages. Together, our results reveal a new protein complex that mediates rhoptry discharge following host-cell contact.

中文翻译：

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微线体蛋白的保守复合物介导弓形虫的棒状放电

顶复门寄生虫在与宿主细胞接触时释放出称为菱形体的特殊细胞器以介导入侵。人们对驱动棒状体放电的事件知之甚少，但对于维持顶复门寄生生命周期至关重要。棒状体放电似乎依赖于另一组称为微线体的细胞器分泌的蛋白质，这些细胞器的功能各不相同，从允许宿主细胞结合到促进滑行运动。在这里，我们检查了微线体蛋白 CLAMP 的功能，我们之前发现它对于弓形虫宿主细胞入侵是必需的，并证明了它在棒状体放电中的重要作用。CLAMP 与另外两种微线体蛋白（入侵相关的 SPATR）和一种以前未表征的蛋白质（我们将其命名为 CLAMP 相关入侵蛋白（CLIP））形成了独特的复合物。CLAMP 缺陷不会影响寄生虫粘附或微线体蛋白分泌；然而，敲低 CLAMP 复合体的任何成员都会影响棒状体放电。系统发育分析表明，重要的复杂成分 CLAMP 和 CLIP 的直向同源物在 apicomplexans 中普遍存在。SPATR 似乎是弓形虫的辅助因子，但尽管不完全保留，但它对于恶性疟原虫血液阶段的入侵也至关重要。总之，我们的结果揭示了一种新的蛋白质复合物，可在宿主细胞接触后介导菱形放电。