Cytoskeleton最新文献

Visualizing human centrosomes using cryo-electron tomography revealed the native structure and molecular organization of γ-tubulin ring complexes (γ-TuRCs). γ-TuRCs localized to two distinct centrosomal pools, one in the pericentriolar material (PCM) and another in the centriole lumen, which is released during mitosis. All detected γ-TuRCs were associated with the tetrameric adaptor protein NEDD1. Within the PCM, binding to the centrosomin (CM1) motif of the microcephaly protein CDK5RAP2 in different patterns correlates with conformational changes of γ-TuRCs. In the centriole lumen, the augmin complex anchors γ-TuRCs to the inner scaffold. These observations provide key insights into how the structural organization of γ-TuRCs and regulatory factors collectively govern the spatial and temporal control of microtubule nucleation in centrosomes.

The fission yeast Schizosaccharomyces pombe divides via closed mitosis, meaning that spindle elongation and chromosome segregation transpire entirely within the closed nuclear envelope. Both the spindle and nuclear envelope must undergo shape changes and exert varying forces on each other during this process. Previous work has demonstrated that nuclear envelope expansion (Yam, He, Zhang, Chiam, & Oliferenko, 2011; Mori & Oliferenko, 2020) and spindle pole body (SPB) embedding in the nuclear envelope are required for normal S. pombe mitosis, and mechanical modeling has described potential contributions of the spindle to nuclear morphology (Fang et al., 2020; Zhu et al., 2016). However, it is not yet fully clear how and to what extent the nuclear envelope and mitotic spindle each directly shape each other during closed mitosis. Here, we investigate this relationship by observing the behaviors of spindles and nuclei in live mitotic fission yeast following laser ablation. First, we characterize these dynamics in mitotic S. pombe nuclei with increased envelope tension, finding that nuclear envelope tension can both bend the spindle and slow elongation. Next, we directly probe the mechanical connection between spindles and nuclear envelopes by ablating each structure. We demonstrate that envelope tension can be relieved by severing spindles and that spindle compression can be relieved by rupturing the envelope. We interpret our experimental data via two quantitative models that demonstrate that fission yeast spindles and nuclear envelopes are a mechanical pair that can each shape the other's morphology.

ON THE BACK COVER: PC3 cells stained for Lamin A/C (red), mVenus-HN1 (Green) and DNA (Blue).

Credit: Gülseren Özduman and Kemal Sami Korkmaz (Cancer Biology Laboratory, Department of Bioengineering, Faculty of Engineering, Ege University, Bornova, Turkey)

ON THE FRONT COVER: Composite image of in HN1 depleted PC3 cells stained for β-tubulin (green) and DNA (Blue).

Credit: Gülseren Özduman and Kemal Sami Korkmaz (Cancer Biology Laboratory, Department of Bioengineering, Faculty of Engineering, Ege University, Bornova, Turkey)

A bipolar spindle composed of microtubules and many associated proteins functions to segregate chromosomes during cell division in all eukaryotes, yet both spindle size and architecture vary dramatically across different species and cell types. Targeting protein for Xklp2 (TPX2) is one candidate factor for modulating spindle microtubule organization through its roles in branching microtubule nucleation, activation of the mitotic kinase Aurora A, and association with the kinesin-5 (Eg5) motor. Here we characterize a conserved nuclear localization sequence (NLS) motif, ¹²³KKLK¹²⁶ in Xenopus laevis TPX2, which regulates astral microtubule formation and spindle pole morphology in Xenopus egg extracts. Addition of recombinant TPX2 with this sequence mutated to AALA stimulated spontaneous formation of microtubule asters and increased recruitment of phosphorylated Aurora A, pericentrin, and Eg5 to meiotic spindle poles while still binding to the regulatory transport factor importin α. We propose that TPX2 is a linchpin spindle assembly factor whose regulation contributes to the activation of multiple microtubule polymerizing and organizing proteins, generating distinct spindle architectures.