Apicomplexan parasites secrete contents of the rhoptries, club-shaped organelles in the apical region, into host cells to permit their invasion and establishment of infection. The rhoptry secretory apparatus (RSA), which is critical for rhoptry secretion, was recently discovered in Toxoplasma and Cryptosporidium. It is unknown whether a similar molecular machinery exists in the malaria parasite Plasmodium. In this study, we use in situ cryo-electron tomography to investigate the rhoptry secretion system in P. falciparum merozoites. We identify the presence of an RSA at the cell apex and a morphologically distinct apical vesicle docking the tips of the two rhoptries to the RSA. We also discover two additional rhoptry organizations that lack the apical vesicle. Using subtomogram averaging, we reveal different conformations of the RSA structure corresponding to different rhoptry organizations. Our results highlight previously unknown steps in the process of rhoptry secretion and indicate a regulatory role for the conserved apical vesicle in host invasion by apicomplexan parasites.

Apicomplexan 寄生虫将顶端区域的棒状细胞器 rhoptries 的内容物分泌到宿主细胞中，以允许它们入侵和建立感染。最近在弓形虫和隐孢子虫中发现了对菱形分泌至关重要的菱形分泌器 (RSA) 。疟疾寄生虫疟原虫中是否存在类似的分子机制尚不清楚。在这项研究中，我们使用原位低温电子断层扫描来研究恶性疟原虫的螺旋体分泌系统裂殖子。我们确定了 RSA 在细胞顶端的存在以及形态上不同的顶端囊泡将两个菱形的尖端与 RSA 对接。我们还发现了另外两个缺少顶端囊泡的菱形组织。使用 subtomogram 平均，我们揭示了 RSA 结构的不同构象对应于不同的 rhoptry 组织。我们的结果突出了之前未知的菱形分泌过程中的步骤，并表明保守的顶端囊泡在顶端复合门寄生虫入侵宿主中的调节作用。