Biocenter Oulu and Faculty of Biochemistry and Molecular Medicine, University of Oulu, Oulu, Finland.
Department of Biomedicine, University of Bergen, Bergen, Norway.
PLoS Biol. 2019 Jun 14;17(6):e3000315. doi: 10.1371/journal.pbio.3000315. eCollection 2019 Jun.
Plasmodium actins form very short filaments and have a noncanonical link between ATP hydrolysis and polymerization. Long filaments are detrimental to the parasites, but the structural factors constraining Plasmodium microfilament lengths have remained unknown. Using high-resolution crystallography, we show that magnesium binding causes a slight flattening of the Plasmodium actin I monomer, and subsequent phosphate release results in a more twisted conformation. Thus, the Mg-bound monomer is closer in conformation to filamentous (F) actin than the Ca form, and this likely facilitates polymerization. A coordinated potassium ion resides in the active site during hydrolysis and leaves together with the phosphate, a process governed by the position of the Arg178/Asp180-containing A loop. Asp180 interacts with either Lys270 or His74, depending on the protonation state of the histidine, while Arg178 links the inner and outer domains (ID and OD) of the actin protomer. Hence, the A loop acts as a switch between stable and unstable filament conformations, the latter leading to fragmentation. Our data provide a comprehensive model for polymerization, ATP hydrolysis and phosphate release, and fragmentation of parasite microfilaments. Similar mechanisms may well exist in canonical actins, although fragmentation is much less favorable due to several subtle sequence differences as well as the methylation of His73, which is absent on the corresponding His74 in Plasmodium actin I.
疟原虫肌动蛋白形成非常短的纤维，并且在 ATP 水解和聚合之间存在非典型的连接。长纤维对寄生虫有害，但限制疟原虫微丝长度的结构因素仍然未知。使用高分辨率晶体学，我们表明镁结合导致疟原虫肌动蛋白 I 单体轻微变平，随后磷酸盐释放导致更扭曲的构象。因此，与 Ca 形式相比，Mg 结合的单体在构象上更接近丝状（F）肌动蛋白，这可能促进聚合。一个协调的钾离子在水解过程中位于活性位点，并与磷酸盐一起离开，这一过程受包含 Arg178/Asp180 的 A 环的位置控制。Asp180 与 Lys270 或 His74 相互作用，具体取决于组氨酸的质子化状态，而 Arg178 将肌动蛋白原的内域（ID）和外域（OD）连接起来。因此，A 环充当稳定和不稳定纤维构象之间的开关，后者导致碎片化。我们的数据提供了一个关于聚合、ATP 水解和磷酸盐释放以及寄生虫微丝碎片化的综合模型。类似的机制可能存在于典型肌动蛋白中，尽管由于几个微妙的序列差异以及 His73 的甲基化，碎片化的可能性要小得多，而 His73 在疟原虫肌动蛋白 I 中不存在。
