Augmin is a Ran-regulated spindle assembly factor

The NLS3 Motif in TPX2 Regulates Spindle Architecture in Xenopus Egg Extracts

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.

Structural mechanisms for centrosomal recruitment and organization of the microtubule nucleator γ-TuRC

The γ-tubulin ring complex (γ-TuRC) acts as a structural template for microtubule formation at centrosomes, associating with two main compartments: the pericentriolar material and the centriole lumen. In the pericentriolar material, the γ-TuRC is involved in microtubule organization, while the function of the centriole lumenal pool remains unclear. The conformational landscape of the γ-TuRC, which is crucial for its activity, and its centrosomal anchoring mechanisms, which determine γ-TuRC activity and turnover, are not understood. Using cryo-electron tomography, we analyze γ-TuRCs in human cells and purified centrosomes. Pericentriolar γ-TuRCs simultaneously associate with the essential adapter NEDD1 and the microcephaly protein CDK5RAP2. NEDD1 forms a tetrameric structure at the γ-TuRC base through interactions with four GCP3/MZT1 modules and GCP5/6-specific extensions, while multiple copies of CDK5RAP2 engage the γ-TuRC in two distinct binding patterns to promote γ-TuRC closure and activation. In the centriole lumen, the microtubule branching factor Augmin tethers a condensed cluster of γ-TuRCs to the centriole wall with defined directional orientation. Centriole-lumenal γ-TuRC-Augmin is protected from degradation during interphase and released in mitosis to aid chromosome alignment. This study provides a unique view on γ-TuRC structure and molecular organization at centrosomes and identifies an important cellular function of centriole-lumenal γ-TuRCs.

Phase separation of microtubule-binding proteins - implications for neuronal function and disease

Protein liquid-liquid phase separation (LLPS) is driven by intrinsically disordered regions and multivalent binding domains, both of which are common features of diverse microtubule (MT) regulators. Many in vitro studies have dissected the mechanisms by which MT-binding proteins (MBPs) regulate MT nucleation, stabilization and dynamics, and investigated whether LLPS plays a role in these processes. However, more recent in vivo studies have focused on how MBP LLPS affects biological functions throughout neuronal development. Dysregulation of MBP LLPS can lead to formation of aggregates - an underlying feature in many neurodegenerative diseases - such as the tau neurofibrillary tangles present in Alzheimer's disease. In this Review, we highlight progress towards understanding the regulation of MT dynamics through the lens of phase separation of MBPs and associated cytoskeletal regulators, from both in vitro and in vivo studies. We also discuss how LLPS of MBPs regulates neuronal development and maintains homeostasis in mature neurons.

The cell cycle controls spindle architecture in Arabidopsis by activating the augmin pathway

To ensure an even segregation of chromosomes during somatic cell division, eukaryotes rely on mitotic spindles. Here, we measured prime characteristics of the Arabidopsis mitotic spindle and built a three-dimensional dynamic model using Cytosim. We identified the cell-cycle regulator CYCLIN-DEPENDENT KINASE B1 (CDKB1) together with its cyclin partner CYCB3;1 as key regulators of spindle morphology in Arabidopsis. We found that the augmin component ENDOSPERM DEFECTIVE1 (EDE1) is a substrate of the CDKB1;1-CYCB3;1 complex. A non-phosphorylatable mutant rescue of ede1 resembled the spindle phenotypes of cycb3;1 and cdkb1 mutants and the protein associated less efficiently with spindle microtubules. Accordingly, reducing the level of augmin in simulations recapitulated the phenotypes observed in the mutants. Our findings emphasize the importance of cell-cycle-dependent phospho-control of the mitotic spindle in plant cells and support the validity of our model as a framework for the exploration of mechanisms controlling the organization of the eukaryotic spindle.

HURP facilitates spindle assembly by stabilizing microtubules and working synergistically with TPX2

In vertebrate spindles, most microtubules are formed via branching microtubule nucleation, whereby microtubules nucleate along the side of pre-existing microtubules. Hepatoma up-regulated protein (HURP) is a microtubule-associated protein that has been implicated in spindle assembly, but its mode of action is yet to be defined. In this study, we show that HURP is necessary for RanGTP-induced branching microtubule nucleation in Xenopus egg extract. Specifically, HURP stabilizes the microtubule lattice to promote microtubule formation from γ-TuRC. This function is shifted to promote branching microtubule nucleation through enhanced localization to TPX2 condensates, which form the core of the branch site on microtubules. Lastly, we provide a high-resolution cryo-EM structure of HURP on the microtubule, revealing how HURP binding stabilizes the microtubule lattice. We propose a model in which HURP stabilizes microtubules during their formation, and TPX2 preferentially enriches HURP to microtubules to promote branching microtubule nucleation and thus spindle assembly.

The structure of the γ-TuRC at the microtubule minus end - not just one solution

In cells, microtubules (MTs) assemble from α/β-tubulin subunits at nucleation sites containing the γ-tubulin ring complex (γ-TuRC). Within the γ-TuRC, exposed γ-tubulin molecules act as templates for MT assembly by interacting with α/β-tubulin. The vertebrate γ-TuRC is scaffolded by γ-tubulin-interacting proteins GCP2-6 arranged in a specific order. Interestingly, the γ-tubulin molecules in the γ-TuRC deviate from the cylindrical geometry of MTs, raising the question of how the γ-TuRC structure changes during MT nucleation. Recent studies on the structure of the vertebrate γ-TuRC attached to the end of MTs came to varying conclusions. In vitro assembly of MTs, facilitated by an α-tubulin mutant, resulted in a closed, cylindrical γ-TuRC showing canonical interactions between all γ-tubulin molecules and α/β-tubulin subunits. Conversely, native MTs formed in a frog extract were capped by a partially closed γ-TuRC, with some γ-tubulin molecules failing to align with α/β-tubulin. This review discusses these outcomes, along with the broader implications.

Roles of Tubulin Concentration during Prometaphase and Ran-GTP during Anaphase of Caenorhabditis elegans Meiosis

In many animal species, the oocyte meiotic spindle, which is required for chromosome segregation, forms without centrosomes. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in Caenorhabditis elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly occurred after auxin-induced degradation of Ran-GEF, but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown. We found that the concentration of soluble tubulin in the metaphase spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules. Measurement of the volume occupied by yolk granules and mitochondria indicated that volume exclusion would be sufficient to explain the concentration of tubulin in the spindle volume. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes.

Roles of Tubulin Concentration during Prometaphase and Ran-GTP during Anaphase of C. elegans meiosis

In many animal species, the oocyte meiotic spindle, which is required for chromosome segregation, forms without centrosomes. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in C. elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly occurred after auxin-induced degradation of Ran-GEF but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown. We found that the concentration of soluble tubulin in the metaphase spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules. Measurement of the volume occupied by yolk granules and mitochondria indicated that volume exclusion would be sufficient to explain the concentration of tubulin in the spindle volume. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes.

γ-TuRC asymmetry induces local protofilament mismatch at the RanGTP-stimulated microtubule minus end

The γ-tubulin ring complex (γ-TuRC) is a structural template for de novo microtubule assembly from α/β-tubulin units. The isolated vertebrate γ-TuRC assumes an asymmetric, open structure deviating from microtubule geometry, suggesting that γ-TuRC closure may underlie regulation of microtubule nucleation. Here, we isolate native γ-TuRC-capped microtubules from Xenopus laevis egg extract nucleated through the RanGTP-induced pathway for spindle assembly and determine their cryo-EM structure. Intriguingly, the microtubule minus end-bound γ-TuRC is only partially closed and consequently, the emanating microtubule is locally misaligned with the γ-TuRC and asymmetric. In the partially closed conformation of the γ-TuRC, the actin-containing lumenal bridge is locally destabilised, suggesting lumenal bridge modulation in microtubule nucleation. The microtubule-binding protein CAMSAP2 specifically binds the minus end of γ-TuRC-capped microtubules, indicating that the asymmetric minus end structure may underlie recruitment of microtubule-modulating factors for γ-TuRC release. Collectively, we reveal a surprisingly asymmetric microtubule minus end protofilament organisation diverging from the regular microtubule structure, with direct implications for the kinetics and regulation of nucleation and subsequent modulation of microtubules during spindle assembly.

Identification of a motif in TPX2 that regulates spindle architecture in Xenopus egg extracts

A bipolar spindle composed of microtubules and many associated proteins functions to segregate chromosomes during cell division in all eukaryotes, yet spindle size and architecture varies 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 identify a conserved nuclear localization sequence (NLS) motif, 123 KKLK 126 in X. 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 dramatically increased spontaneous formation of microtubule asters and recruitment of phosphorylated Aurora A, pericentrin, and Eg5 to meiotic spindle poles. We propose that TPX2 is a linchpin spindle assembly factor whose regulation contributes to the recruitment and activation of multiple microtubule polymerizing and organizing proteins, generating distinct spindle architectures.

Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Despite drastic cellular changes during cleavage, a mitotic spindle assembles in each blastomere to accurately segregate duplicated chromosomes. Mechanisms of mitotic spindle assembly have been extensively studied using small somatic cells. However, mechanisms of spindle assembly in large vertebrate embryos remain little understood. Here, we establish functional assay systems in medaka (Oryzias latipes) embryos by combining CRISPR knock-in with auxin-inducible degron technology. Live imaging reveals several unexpected features of microtubule organization and centrosome positioning that achieve rapid, accurate cleavage. Importantly, Ran-GTP assembles a dense microtubule network at the metaphase spindle center that is essential for chromosome segregation in early embryos. This unique spindle structure is remodeled into a typical short, somatic-like spindle after blastula stages, when Ran-GTP becomes dispensable for chromosome segregation. We propose that despite the presence of centrosomes, the chromosome-derived Ran-GTP pathway has essential roles in functional spindle assembly in large, rapidly dividing vertebrate early embryos, similar to acentrosomal spindle assembly in oocytes.

HURP facilitates spindle assembly by stabilizing microtubules and working synergistically with TPX2

In large vertebrate spindles, the majority of microtubules are formed via branching microtubule nucleation, whereby microtubules nucleate along the side of pre-existing microtubules. Hepatoma up-regulated protein (HURP) is a microtubule-associated protein that has been implicated in spindle assembly, but its mode of action is yet to be defined. In this study, we show that HURP is necessary for RanGTP-induced branching microtubule nucleation in Xenopus egg extract. Specifically, HURP stabilizes the microtubule lattice to promote microtubule formation from γ-TuRC. This function is shifted to promote branching microtubule nucleation in the presence of TPX2, another branching-promoting factor, as HURP's localization to microtubules is enhanced by TPX2 condensation. Lastly, we provide a structure of HURP on the microtubule lattice, revealing how HURP binding stabilizes the microtubule lattice. We propose a model in which HURP stabilizes microtubules during their formation, and TPX2 preferentially enriches HURP to microtubules to promote branching microtubule nucleation and thus spindle assembly.