Oriented nucleation of hemozoin at the digestive vacuole membrane in Plasmodium falciparum
Oriented nucleation of hemozoin at the digestive vacuole membrane in Plasmodium falciparum
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DOI：
10.1073/pnas.1118120109
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发表时间：
2012-07-10
影响因子：
11.1
通讯作者：
Elbaum, Michael
中科院分区：
文献类型：
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作者：
Kapishnikov, Sergey;Weiner, Allon;Elbaum, Michael
血红素解毒是导致疟疾的寄生虫生命周期中的关键步骤，通过结晶形成生理上不溶的血球蛋白来实现。这种成核方式对于理解抗疟疾药物抑制血吸虫生长的作用机制具有深远的意义。有几条证据表明，酰甘油脂类参与了成核过程。据报道，血球蛋白晶体形成于脂质纳米球内；或者，在体外发现，它们是在酰甘油脂质-水界面成核的。我们应用低温软X射线断层扫描和三维电子显微镜来研究消化液泡(DV)内血虫晶体的位置和取向，以此作为其成核和生长过程的标志。“水窗口”中的低温软X射线层析成像特别有利，因为对比度的产生本质上是基于原子吸收的。我们发现，血球蛋白的成核发生在DV的内膜上，结晶发生在水相而不是脂相。晶体形态表明，与模板形核预期的那样，面向膜的共同的{100}取向。这与X射线荧光和衍射法在配套工作中得出的结论是一致的。在寄生虫中观察到的均匀的黑色球体被鉴定为血红蛋白运输小泡。他们的分析支持血球蛋白主要在DV中成核的模型。对DV膜对比度的模拟表明，DV膜的厚度为4纳米，斑块厚度约为DV膜的三倍，可能与成核有关。
Heme detoxification is a critical step in the life cycle of malaria-causing parasites, achieved by crystallization into physiologically insoluble hemozoin. The mode of nucleation has profound implications for understanding the mechanism of action of antimalarial drugs that inhibit hemozoin growth. Several lines of evidence point to involvement of acylglycerol lipids in the nucleation process. Hemozoin crystals have been reported to form within lipid nanospheres; alternatively, it has been found in vitro that they are nucleated at an acylglycerol lipid-water interface. We have applied cryogenic soft X-ray tomography and three-dimensional electron microscopy to address the location and orientation of hemozoin crystals within the digestive vacuole (DV), as a signature of their nucleation and growth processes. Cryogenic soft X-ray tomography in the "water window" is particularly advantageous because contrast generation is based inherently on atomic absorption. We find that hemozoin nucleation occurs at the DV inner membrane, with crystallization occurring in the aqueous rather than lipid phase. The crystal morphology indicates a common {100} orientation facing the membrane as expected of templated nucleation. This is consistent with conclusions reached by X-ray fluorescence and diffraction in a companion work. Uniform dark spheres observed in the parasite were identified as hemoglobin transport vesicles. Their analysis supports a model of hemozoin nucleation primarily in the DV. Modeling of the contrast at the DV membrane indicates a 4-nm thickness with patches about three times thicker, possibly implicated in the nucleation.