Microtubules are dynamic polymers of α- and β-tubulin and have crucial roles in cell signalling, cell migration, intracellular transport and chromosome segregation 1 . They assemble de novo from αβ-tubulin dimers in an essential process termed microtubule nucleation. Complexes that contain the protein γ-tubulin serve as structural templates for the microtubule nucleation reaction 2 . In vertebrates, microtubules are nucleated by the 2.2-megadalton γ-tubulin ring complex (γ-TuRC), which comprises γ-tubulin, five related γ-tubulin complex proteins (GCP2–GCP6) and additional factors 3 . GCP6 is unique among the GCP proteins because it carries an extended insertion domain of unknown function. Our understanding of microtubule formation in cells and tissues is limited by a lack of high-resolution structural information on the γ-TuRC. Here we present the cryo-electron microscopy structure of γ-TuRC from Xenopus laevis at 4.8 Å global resolution, and identify a 14-spoked arrangement of GCP proteins and γ-tubulins in a partially flexible open left-handed spiral with a uniform sequence of GCP variants. By forming specific interactions with other GCP proteins, the GCP6-specific insertion domain acts as a scaffold for the assembly of the γ-TuRC. Unexpectedly, we identify actin as a bona fide structural component of the γ-TuRC with functional relevance in microtubule nucleation. The spiral geometry of γ-TuRC is suboptimal for microtubule nucleation and a controlled conformational rearrangement of the γ-TuRC is required for its activation. Collectively, our cryo-electron microscopy reconstructions provide detailed insights into the molecular organization, assembly and activation mechanism of vertebrate γ-TuRC, and will serve as a framework for the mechanistic understanding of fundamental biological processes associated with microtubule nucleation, such as meiotic and mitotic spindle formation and centriole biogenesis 4 . The cryo-EM structure of the γ-tubulin ring complex (γ-TuRC) from Xenopus laevis provides insights into the molecular organization of the complex, and shows that actin is a structural component that is functionally relevant to microtubule nucleation.

中文翻译：

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对微管成核剂γ-TuRC的组装和激活的见解

微管是 α-和 β-微管蛋白的动态聚合物，在细胞信号传导、细胞迁移、细胞内转运和染色体分离 1 中具有重要作用。他们在称为微管成核的基本过程中从 αβ-微管蛋白二聚体重新组装。含有蛋白质 γ-微管蛋白的复合物可作为微管成核反应 2 的结构模板。在脊椎动物中，微管由 2.2-兆道尔顿 γ-微管蛋白环复合物 (γ-TuRC) 成核，该复合物包括 γ-微管蛋白、五种相关的 γ-微管蛋白复合物 (GCP2–GCP6) 和其他因子 3 。GCP6 在 GCP 蛋白中是独一无二的，因为它带有一个未知功能的扩展插入结构域。由于缺乏关于 γ-TuRC 的高分辨率结构信息，我们对细胞和组织中微管形成的理解受到限制。在这里，我们以 4.8 Å 的全局分辨率展示了非洲爪蟾的 γ-TuRC 的低温电子显微镜结构，并在具有均匀序列GCP 变体。通过与其他 GCP 蛋白形成特异性相互作用，GCP6 特异性插入结构域充当组装 γ-TuRC 的支架。出乎意料的是，我们将肌动蛋白鉴定为 γ-TuRC 的真正结构成分，在微管成核中具有功能相关性。γ-TuRC 的螺旋几何形状对于微管成核来说不是最理想的，并且γ-TuRC 的受控构象重排是其激活所必需的。总的来说，我们的低温电子显微镜重建提供了对分子组织的详细见解，脊椎动物 γ-TuRC 的组装和激活机制，并将作为机械理解与微管成核相关的基本生物过程的框架，例如减数分裂和有丝分裂纺锤体形成和中心粒生物发生 4。来自非洲爪蟾的 γ-微管蛋白环复合物 (γ-TuRC) 的低温 EM 结构提供了对该复合物分子组织的深入了解，并表明肌动蛋白是一种与微管成核功能相关的结构成分。