Centrioles are polarized microtubule‐based organelles that seed the formation of cilia, and which assemble from a cartwheel containing stacked ring oligomers of SAS‐6 proteins. A cryo‐tomography map of centrioles from the termite flagellate Trichonympha spp. was obtained previously, but higher resolution analysis is likely to reveal novel features. Using sub‐tomogram averaging (STA) in T. spp. and Trichonympha agilis, we delineate the architecture of centriolar microtubules, pinhead, and A‐C linker. Moreover, we report ~25 Å resolution maps of the central cartwheel, revealing notably polarized cartwheel inner densities (CID). Furthermore, STA of centrioles from the distant flagellate Teranympha mirabilis uncovers similar cartwheel architecture and a distinct filamentous CID. Fitting the CrSAS‐6 crystal structure into the flagellate maps and analyzing cartwheels generated in vitro indicate that SAS‐6 rings can directly stack onto one another in two alternating configurations: with a slight rotational offset and in register. Overall, improved STA maps in three flagellates enabled us to unravel novel architectural features, including of centriole polarity and cartwheel stacking, thus setting the stage for an accelerated elucidation of underlying assembly mechanisms.

中文翻译：

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冷冻断层扫描显示中心粒极性和车轮堆积的新特征。

质心是基于极化的基于微管的细胞器，可形成纤毛的种子，并从包含SAS-6蛋白的堆叠环寡聚物的车轮上组装。来自白蚁鞭毛Trichonympha spp的中心粒的低温层析成像图。是以前获得的，但更高分辨率的分析可能会揭示出新颖的功能。在T.  spp中使用子断层图平均（STA）。和Trichonympha agilis，我们描述了中心微管，针头和A-C连接子的体系结构。此外，我们报告了约25Å的中心车轮分辨率图，揭示了极化的车轮内部密度（CID）。此外，来自遥远的鞭毛Teranympha mirabilis的中心粒STA发现类似的车轮结构和独特的丝状CID。将CrSAS-6晶体结构拟合到鞭毛图上并分析体外产生的车轮，表明SAS-6环可以以两种交替的配置直接彼此堆叠：有轻微的旋转偏移并对齐。总体而言，在三个鞭毛虫中改进的STA映射使我们能够揭示新颖的建筑特征，包括中心极极性和车轮堆叠，从而为加速阐明基础组装机制奠定了基础。