The γ-tubulin complex protein GCP6 is crucial for spindle morphogenesis but not essential for microtubule reorganization in Arabidopsis

First author: Huiying Miao; Affiliations: University of California, Davis (加州大学戴维斯分校): Davis, US

Corresponding author: Bo Liu

γ-Tubulin typically forms a ring-shaped complex with 5 related γ-tubulin complex proteins (GCP2 to GCP6), and this γ-tubulin ring complex (γTuRC) serves as a template for microtubule (MT) nucleation in plants and animals. While the γTuRC takes part in MT nucleation in most eukaryotes, in fungi such events take place robustly with just the γ-tubulin small complex (γTuSC) assembled by γ-tubulin plus GCP2 and GCP3. To explore whether the γTuRC is the sole functional γ-tubulin complex in plants, we generated 2 mutants of the GCP6 gene encoding the largest subunit of the γTuRC in Arabidopsis thaliana. Both mutants showed similar phenotypes of dwarfed vegetative growth and reduced fertility. The gcp6 mutant assembled the γTuSC, while the wild-type cells had GCP6 join other GCPs to produce the γTuRC. Although the gcp6 cells had greatly diminished γ-tubulin localization on spindle MTs, the protein was still detected there. The gcp6 cells formed spindles that lacked MT convergence (收敛) and discernable (可辨别的) poles; however, they managed to cope with the challenge of MT disorganization and were able to complete mitosis and cytokinesis. Our results reveal that the γTuRC is not the only functional form of the γ-tubulin complex for MT nucleation in plant cells, and that γ-tubulin-dependent, but γTuRC-independent, mechanisms meet the basal need of MT nucleation. Moreover, we show that the γTuRC function is more critical for the assembly of spindle MT array than for the phragmoplast. Thus, our findings provide insight into acentrosomal MT nucleation and organization.

γ-微管蛋白通常会与5个相关的γ-微管蛋白复合蛋白GCP2-6形成一个环状复合物γTuRC，并且该环状复合物在动植物中作为微管成核的模版发挥作用。尽管在大多数真核中γTuRC参与微管成核，但真菌中该过程仅需要由γ-微管蛋白和2个γ-微管蛋白复合蛋白GCP2/3组成的γ-微管蛋白小复合物γTuSC的参与。为了研究γTuRC是否是植物中唯一的功能性γ-微管蛋白复合物，作者构建了两个拟南芥GCP6基因的突变体，而该基因负责编码了γTuRC中最大的亚基蛋白。两个突变体均显示出了相似的表型，即较矮的营养生长和较低的繁殖能力。gcp6突变体中仅组装形成γTuSC，而野生型的细胞中则是GCP6联合其它的GCPs形成γTuRC。尽管gcp6突变体细胞中定位于纺锤体微管的γ-微管蛋白大大地减少，但仍然可以在此处检测到γ-微管蛋白。尽管gcp6细胞中缺乏微管收敛性和可辨别的极点， 但这些细胞仍然成功避免了微管乱序，并且能够成功完成有丝分裂和胞质分裂。本文的结果揭示了γTuRC并不是植物细胞中作用于微管成核的唯一功能性γ-微管蛋白复合体，并且γ-微管蛋白依赖型，而非独立于γ-微管蛋白的分子机制满足了微管成核的基本需求。此外，作者还显示了相比于原生质体，γTuRC的功能对于纺锤体微管阵列的组装要更加重要。因此，本文的研究为植物中非中心化的微管成核和组织提供了新的视野。

通讯：Bo Liu (https://biology.ucdavis.edu/people/bo-liu)

个人简介：1985年，北京大学，学士；1988年，北京大学，硕士；1995年，佐治亚大学，博士。

研究方向：植物细胞分裂和生长过程中微管和肌动蛋白微丝的动态；驱动蛋白在有丝分裂和胞质分裂中的功能； 丝状真菌中细胞骨架介导的菌丝生长的分子机制。

doi: https://doi.org/10.1073/pnas.1912240116

Journal: PNAS

First Published: December 09, 2019