Novel NEDD1 phosphorylation sites regulate γ-tubulin binding and mitotic spindle assembly

Structure of the microtubule-anchoring factor NEDD1 bound to the γ-tubulin ring complex

The γ-tubulin ring complex (γ-TuRC) is an essential multiprotein assembly that provides a template for microtubule nucleation. The γ-TuRC is recruited to microtubule-organizing centers (MTOCs) by the evolutionarily conserved attachment factor NEDD1. However, the structural basis of the NEDD1-γ-TuRC interaction is not known. Here, we report cryo-EM structures of NEDD1 bound to the human γ-TuRC in the absence or presence of the activating factor CDK5RAP2. We found that the C-terminus of NEDD1 forms a tetrameric α-helical assembly that contacts the lumen of the γ-TuRC cone and orients its microtubule-binding domain away from the complex. The structure of the γ-TuRC simultaneously bound to NEDD1 and CDK5RAP2 reveals that both factors can associate with the "open" conformation of the complex. Our results show that NEDD1 does not induce substantial conformational changes in the γ-TuRC but suggest that anchoring of γ-TuRC-capped microtubules by NEDD1 would be structurally compatible with the significant conformational changes experienced by the γ-TuRC during microtubule nucleation.

Structure of the microtubule anchoring factor NEDD1 bound to the γ-tubulin ring complex

The γ-tubulin ring complex (γ-TuRC) is an essential multiprotein assembly, in which γ-tubulin, GCP2-6, actin, MZT1 and MZT2 form an asymmetric cone-shaped structure that provides a template for microtubule nucleation. The γ-TuRC is recruited to microtubule organizing centers (MTOCs), such as centrosomes and pre-existing mitotic spindle microtubules, via the evolutionarily-conserved attachment factor NEDD1. NEDD1 contains an N-terminal WD40 domain that binds to microtubules, and a C-terminal domain that associates with the γ-TuRC. However, the structural basis of the NEDD1-γ-TuRC interaction is not known. Here, we report cryo-electron microscopy (cryo-EM) structures of NEDD1 bound to the human γ-TuRC in the absence or presence of the activating factor CDK5RAP2, which interacts with GCP2 to induce conformational changes in the γ-TuRC and promote its microtubule nucleating function. We found that the C-terminus of NEDD1 forms a tetrameric α-helical assembly that contacts the lumen of the γ-TuRC cone, is anchored to GCP4, 5 and 6 via protein modules consisting of MZT1 & GCP3 subcomplexes, and orients its microtubule-binding WD40 domains away from the complex. We biochemically tested our structural models by identifying NEDD1 mutants unable to pull-down γ -tubulin from cultured cells. The structure of the γ-TuRC simultaneously bound to NEDD1 and CDK5RAP2 reveals that both factors can associate with the "open" conformation of the complex. Our results show that NEDD1 does not induce conformational changes in the γ-TuRC, but suggest that anchoring of γ-TuRC-capped microtubules by NEDD1 would be structurally compatible with the significant conformational changes experienced by the γ-TuRC during microtubule nucleation.

Multifaceted modes of γ-tubulin complex recruitment and microtubule nucleation at mitotic centrosomes

Microtubule nucleation is mediated by γ-tubulin ring complexes (γ-TuRCs). In most eukaryotes, a GCP4/5/4/6 "core" complex promotes γ-tubulin small complex (γ-TuSC) association to generate cytosolic γ-TuRCs. Unlike γ-TuSCs, however, this core complex is non-essential in various species and absent from budding yeasts. In Drosophila, Spindle defective-2 (Spd-2) and Centrosomin (Cnn) redundantly recruit γ-tubulin complexes to mitotic centrosomes. Here, we show that Spd-2 recruits γ-TuRCs formed via the GCP4/5/4/6 core, but Cnn can recruit γ-TuSCs directly via its well-conserved CM1 domain, similar to its homologs in budding yeast. When centrosomes fail to recruit γ-tubulin complexes, they still nucleate microtubules via the TOG domain protein Mini-spindles (Msps), but these microtubules have different dynamic properties. Our data, therefore, help explain the dispensability of the GCP4/5/4/6 core and highlight the robustness of centrosomes as microtubule organizing centers. They also suggest that the dynamic properties of microtubules are influenced by how they are nucleated.

NEDD1-S411 phosphorylation plays a critical function in the coordination of microtubule nucleation during mitosis

During mitosis, spindle assembly relies on centrosomal and acentrosomal microtubule nucleation pathways that all require the γ-Tubulin Ring Complex (γ-TuRC) and its adaptor protein NEDD1. The activity of these different pathways needs to be coordinated to ensure bipolar spindle assembly ( Cavazza et al., 2016) but the underlying mechanism is still unclear. Previous studies have identified three sites in NEDD1 (S377, S405 and S411) that when phosphorylated drive MT nucleation at the centrosomes, around the chromosomes and on pre-existing MTs respectively ( Lüders et al., 2006; Pinyol et al., 2013; Sdelci et al., 2012). Here we aimed at getting additional insights into the mechanism that coordinates the different MT nucleation pathways in dividing cells using a collection of HeLa stable inducible cell lines expressing NEDD1 phospho-variants at these three sites and Xenopus egg extracts. Our results provide further support for the essential role of phosphorylation at the three residues. Moreover, we directly demonstrate that S411 phosphorylation is essential for MT branching using TIRF microscopy in Xenopus egg extracts and we show that it plays a crucial role in ensuring the balance between centrosome and chromosome-dependent MT nucleation required for bipolar spindle assembly in mitotic cells.

γ-Tubulin in microtubule nucleation and beyond

Microtubules composed of αβ-tubulin dimers are dynamic cytoskeletal polymers that play key roles in essential cellular processes such as cell division, organelle positioning, intracellular transport, and cell migration. γ-Tubulin is a highly conserved member of the tubulin family that is required for microtubule nucleation. γ-Tubulin, together with its associated proteins, forms the γ-tubulin ring complex (γ-TuRC), that templates microtubules. Here we review recent advances in the structure of γ-TuRC, its activation, and centrosomal recruitment. This provides new mechanistic insights into the molecular mechanism of microtubule nucleation. Accumulating data suggest that γ-tubulin also has other, less well understood functions. We discuss emerging evidence that γ-tubulin can form oligomers and filaments, has specific nuclear functions, and might be involved in centrosomal cross-talk between microtubules and microfilaments.

Self-assembly of pericentriolar material in interphase cells lacking centrioles

The major microtubule-organizing center (MTOC) in animal cells, the centrosome, comprises a pair of centrioles surrounded by pericentriolar material (PCM), which nucleates and anchors microtubules. Centrosome assembly depends on PCM binding to centrioles, PCM self-association and dynein-mediated PCM transport, but the self-assembly properties of PCM components in interphase cells are poorly understood. Here, we used experiments and modeling to study centriole-independent features of interphase PCM assembly. We showed that when centrioles are lost due to PLK4 depletion or inhibition, dynein-based transport and self-clustering of PCM proteins are sufficient to form a single compact MTOC, which generates a dense radial microtubule array. Interphase self-assembly of PCM components depends on γ-tubulin, pericentrin, CDK5RAP2 and ninein, but not NEDD1, CEP152, or CEP192. Formation of a compact acentriolar MTOC is inhibited by AKAP450-dependent PCM recruitment to the Golgi or by randomly organized CAMSAP2-stabilized microtubules, which keep PCM mobile and prevent its coalescence. Linking of CAMSAP2 to a minus-end-directed motor leads to the formation of an MTOC, but MTOC compaction requires cooperation with pericentrin-containing self-clustering PCM. Our data reveal that interphase PCM contains a set of components that can self-assemble into a compact structure and organize microtubules, but PCM self-organization is sensitive to motor- and microtubule-based rearrangement.

Autosomal recessive variants in TUBGCP2 alter the γ-tubulin ring complex leading to neurodevelopmental disease

Microtubules help building the cytoskeleton of neurons and other cells. Several components of the gamma-tubulin (γ-tubulin) complex have been previously reported in human neurodevelopmental diseases. We describe two siblings from a consanguineous Turkish family with dysmorphic features, developmental delay, brain malformation, and epilepsy carrying a homozygous mutation (p.Glu311Lys) in TUBGCP2 encoding the γ-tubulin complex 2 (GCP2) protein. This variant is predicted to disrupt the electrostatic interaction of GCP2 with GCP3. In primary fibroblasts carrying the variant, we observed a faint delocalization of γ-tubulin during the cell cycle but normal GCP2 protein levels. Through mass spectrometry, we observed dysregulation of multiple proteins involved in the assembly and organization of the cytoskeleton and the extracellular matrix, controlling cellular adhesion and of proteins crucial for neuronal homeostasis including axon guidance. In summary, our functional and proteomic studies link TUBGCP2 and the γ-tubulin complex to the development of the central nervous system in humans.

Centriole-independent mitotic spindle assembly relies on the PCNT-CDK5RAP2 pericentriolar matrix

Centrosomes, composed of centrioles that recruit a pericentriolar material (PCM) matrix assembled from PCNT and CDK5RAP2, catalyze mitotic spindle assembly. Here, we inhibit centriole formation and/or remove PCNT-CDK5RAP2 in RPE1 cells to address their relative contributions to spindle formation. While CDK5RAP2 and PCNT are normally dispensable for spindle formation, they become essential when centrioles are absent. Acentriolar spindle assembly is accompanied by the formation of foci containing PCNT and CDK5RAP2 via a microtubule and Polo-like kinase 1-dependent process. Foci formation and spindle assembly require PCNT-CDK5RAP2-dependent matrix assembly and the ability of CDK5RAP2 to recruit γ-tubulin complexes. Thus, the PCM matrix can self-organize independently of centrioles to generate microtubules for spindle assembly; conversely, an alternative centriole-anchored mechanism supports spindle assembly when the PCM matrix is absent. Extension to three cancer cell lines revealed similar results in HeLa cells, whereas DLD1 and U2OS cells could assemble spindles in the absence of centrioles and PCNT-CDK5RAP2, suggesting cell type variation in spindle assembly mechanisms.

Microtubule Organization in Striated Muscle Cells

Distinctly organized microtubule networks contribute to the function of differentiated cell types such as neurons, epithelial cells, skeletal myotubes, and cardiomyocytes. In striated (i.e., skeletal and cardiac) muscle cells, the nuclear envelope acts as the dominant microtubule-organizing center (MTOC) and the function of the centrosome—the canonical MTOC of mammalian cells—is attenuated, a common feature of differentiated cell types. We summarize the mechanisms known to underlie MTOC formation at the nuclear envelope, discuss the significance of the nuclear envelope MTOC for muscle function and cell cycle progression, and outline potential mechanisms of centrosome attenuation.

PPP2R2A prostate cancer haploinsufficiency is associated with worse prognosis and a high vulnerability to B55α/PP2A reconstitution that triggers centrosome destabilization

The PPP2R2A gene encodes the B55α regulatory subunit of PP2A. Here, we report that PPP2R2A is hemizygously lost in ~42% of prostate adenocarcinomas, correlating with reduced expression, poorer prognosis, and an increased incidence of hemizygous loss (>75%) in metastatic disease. Of note, PPP2R2A homozygous loss is less common (5%) and not increased at later tumor stages. Reduced expression of B55α is also seen in prostate tumor tissue and cell lines. Consistent with the possibility that complete loss of PPP2R2A is detrimental in prostate tumors, PPP2R2A deletion in cells with reduced but present B55α reduces cell proliferation by slowing progression through the cell cycle. Remarkably, B55α-low cells also appear addicted to lower B55α expression, as even moderate increases in B55α expression are toxic. Reconstitution of B55α expression in prostate cancer (PCa) cell lines with low B55α expression reduces proliferation, inhibits transformation and blocks xenograft tumorigenicity. Mechanistically, we show B55α reconstitution reduces phosphorylation of proteins essential for centrosomal maintenance, and induces centrosome collapse and chromosome segregation failure; a first reported link between B55α/PP2A and the vertebrate centrosome. These effects are dependent on a prolonged metaphase/anaphase checkpoint and are lethal to PCa cells addicted to low levels of B55α. Thus, we propose the reduction in B55α levels associated with hemizygous loss is necessary for centrosomal integrity in PCa cells, leading to selective lethality of B55α reconstitution. Such a vulnerability could be targeted therapeutically in the large pool of patients with hemizygous PPP2R2A deletions, using pharmacologic approaches that enhance PP2A/B55α activity.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

HSP70 is required for the proper assembly of pericentriolar material and function of mitotic centrosomes

Background

At the onset of mitosis, the centrosome expands and matures, acquiring enhanced activities for microtubule nucleation and assembly of a functional bipolar mitotic spindle. However, the mechanisms that regulate centrosome expansion and maturation are largely unknown. Previously, we demonstrated in an immortalized human cell line CGL2 and cancer cell line HeLa that the inducible form of heat shock protein 70 (HSP70) accumulates at the mitotic centrosome and is required for centrosome maturation and bipolar spindle assembly.

Results

In this study, we further show that HSP70 accumulated at the spindle pole in a PLK1-dependent manner. HSP70 colocalized with pericentrin (PCNT), CEP215 and γ-tubulin at the spindle pole and was required for the 3D assembly of these three proteins, which supports mitotic centrosome function. Loss of HSP70 disrupted mitotic centrosome structure, reduced pericentriolar material recruitment and induced fragmentation of spindle poles. In addition, HSP70 was necessary for the interaction between PCNT and CEP215 and also facilitated PLK1 accumulation and function at the spindle pole. Furthermore, we found that HSP70 chaperone activity is required for PCNT accumulation at the mitotic centrosome and assembly of mitotic spindles.

Conclusion

Our current results demonstrate that HSP70 is required for the accurate assembly of the pericentriolar material and proper functioning of mitotic centrosomes.

Mechanism and Regulation of Centriole and Cilium Biogenesis

The centriole is an ancient microtubule-based organelle with a conserved nine-fold symmetry. Centrioles form the core of centrosomes, which organize the interphase microtubule cytoskeleton of most animal cells and form the poles of the mitotic spindle. Centrioles can also be modified to form basal bodies, which template the formation of cilia and play central roles in cellular signaling, fluid movement, and locomotion. In this review, we discuss developments in our understanding of the biogenesis of centrioles and cilia and the regulatory controls that govern their structure and number. We also discuss how defects in these processes contribute to a spectrum of human diseases and how new technologies have expanded our understanding of centriole and cilium biology, revealing exciting avenues for future exploration.

Microtubule nucleation during central spindle assembly requires NEDD1 phosphorylation on Serine 405 by Aurora A

During mitosis, the cell sequentially constructs two microtubule-based spindles to ensure faithful segregation of chromosomes. A bipolar spindle first pulls apart the sister chromatids, then a central spindle further separates them away. Although the assembly of the first spindle is well described, the assembly of the second remains poorly understood. We report here that the inhibition of Aurora A leads to an absence of the central spindle due to a lack of nucleation of microtubules in the midzone. In the absence of Aurora A, the HURP and NEDD1 proteins that are involved in nucleation of microtubules fail to concentrate in the midzone. HURP is an effector of RanGTP and NEDD1 serves as an anchor for the γTURC. Interestingly, Aurora A already phosphorylates them during assembly of the bipolar spindle. We show here that the expression of a NEDD1 isoform mimicking Aurora A phosphorylation is sufficient to restore microtubule nucleation in the midzone in a context of Aurora A inhibition. These results reveal a new control mechanism of nucleation of microtubules by Aurora A during assembly of the central spindle.

Microtubule nucleation by γ-tubulin complexes and beyond

In this short review, we give an overview of microtubule nucleation within cells. It is nearly 30 years since the discovery of γ-tubulin, a member of the tubulin superfamily essential for proper microtubule nucleation in all eukaryotes. γ-tubulin associates with other proteins to form multiprotein γ-tubulin ring complexes (γ-TuRCs) that template and catalyse the otherwise kinetically unfavourable assembly of microtubule filaments. These filaments can be dynamic or stable and they perform diverse functions, such as chromosome separation during mitosis and intracellular transport in neurons. The field has come a long way in understanding γ-TuRC biology but several important and unanswered questions remain, and we are still far from understanding the regulation of microtubule nucleation in a multicellular context. Here, we review the current literature on γ-TuRC assembly, recruitment, and activation and discuss the potential importance of γ-TuRC heterogeneity, the role of non-γ-TuRC proteins in microtubule nucleation, and whether γ-TuRCs could serve as good drug targets for cancer therapy.

FBXL13 directs the proteolysis of CEP192 to regulate centrosome homeostasis and cell migration

Aberrant centrosome organisation with ensuing alterations of microtubule nucleation capacity enables tumour cells to proliferate and invade despite increased genomic instability. CEP192 is a key factor in the initiation process of centrosome duplication and in the control of centrosome microtubule nucleation. However, regulatory means of CEP192 have remained unknown. Here, we report that FBXL13, a binding determinant of SCF (SKP1-CUL1-F-box)-family E3 ubiquitin ligases, is enriched at centrosomes and interacts with the centrosomal proteins Centrin-2, Centrin-3, CEP152 and CEP192. Among these, CEP192 is specifically targeted for proteasomal degradation by FBXL13. Accordingly, induced FBXL13 expression downregulates centrosomal γ-tubulin and disrupts centrosomal microtubule arrays. In addition, depletion of FBXL13 induces high levels of CEP192 and γ-tubulin at the centrosomes with the consequence of defects in cell motility. Together, we characterise FBXL13 as a novel regulator of microtubule nucleation activity and highlight a role in promoting cell motility with potential tumour-promoting implications.

Recent advances in pericentriolar material organization: ordered layers and scaffolding gels

The centrosome is an unusual organelle that lacks a surrounding membrane, raising the question of what limits its size and shape. Moreover, while electron microscopy (EM) has provided a detailed view of centriole architecture, there has been limited understanding of how the second major component of centrosomes, the pericentriolar material (PCM), is organized. Here, we summarize exciting recent findings from super-resolution fluorescence imaging, structural biology, and biochemical reconstitution that together reveal the presence of ordered layers and complex gel-like scaffolds in the PCM. Moreover, we discuss how this is leading to a better understanding of the process of microtubule nucleation, how alterations in PCM size are regulated in cycling and differentiated cells, and why mutations in PCM components lead to specific human pathologies.

Divergent regulation of functionally distinct γ-tubulin complexes during differentiation

Differentiation induces the formation of noncentrosomal microtubule arrays in diverse tissues. The formation of these arrays requires loss of microtubule-organizing activity (MTOC) at the centrosome, but the mechanisms regulating this transition remain largely unexplored. Here, we use the robust loss of centrosomal MTOC activity in the epidermis to identify two pools of γ-tubulin that are biochemically and functionally distinct and differentially regulated. Nucleation-competent CDK5RAP2-γ-tubulin complexes were maintained at centrosomes upon initial epidermal differentiation. In contrast, Nedd1-γ-tubulin complexes did not promote nucleation but were required for anchoring of microtubules, a previously uncharacterized activity for this complex. Cell cycle exit specifically triggered loss of Nedd1-γ-tubulin complexes, providing a mechanistic link connecting MTOC activity and differentiation. Collectively, our studies demonstrate that distinct γ-tubulin complexes regulate different microtubule behaviors at the centrosome and show that differential regulation of these complexes drives loss of centrosomal MTOC activity.

MZT1 regulates microtubule nucleation by linking γTuRC assembly to adapter-mediated targeting and activation

Regulation of the γ-tubulin ring complex (γTuRC) through targeting and activation restricts nucleation of microtubules to microtubule organizing centers (MTOCs), aiding in the assembly of ordered microtubule arrays. However, the mechanistic basis of this important regulation remains poorly understood. Here we show that in human cells γTuRC integrity, determined by the presence of γ-tubulin complex proteins (GCPs) 2-6, is a prerequisite for interaction with the targeting factor NEDD1, impacting on essentially all γ-tubulin dependent functions. Recognition of γTuRC integrity is mediated by MZT1, which binds not only to the GCP3 subunit as previously shown, but cooperatively also to other GCPs through a conserved hydrophobic motif present in the N-termini of GCP2, GCP3, GCP5, and GCP6. MZT1 knockdown causes severe cellular defects under conditions that leave γTuRC intact, suggesting that the essential function of MZT1 is not in γTuRC assembly. Instead, MZT1 specifically binds fully assembled γTuRC to enable interaction with NEDD1 for targeting, and with the CM1 domain of CDK5RAP2 for stimulating nucleation activity. Thus, MZT1 is a ‘priming factor’ for the γTuRC that allows spatial regulation of nucleation.

Centrosome function and assembly in animal cells

It has become clear that the role of centrosomes extends well beyond that of important microtubule organizers. There is increasing evidence that they also function as coordination centres in eukaryotic cells, at which specific cytoplasmic proteins interact at high concentrations and important cell decisions are made. Accordingly, hundreds of proteins are concentrated at centrosomes, including cell cycle regulators, checkpoint proteins and signalling molecules. Nevertheless, several observations have raised the question of whether centrosomes are essential for many cell processes. Recent findings have shed light on the functions of centrosomes in animal cells and on the molecular mechanisms of centrosome assembly, in particular during mitosis. These advances should ultimately allow the in vitro reconstitution of functional centrosomes from their component proteins to unlock the secrets of these enigmatic organelles.

The centrosome-Golgi apparatus nexus

A shared feature among all microtubule (MT)-dependent processes is the requirement for MTs to be organized in arrays of defined geometry. At a fundamental level, this is achieved by precisely controlling the timing and localization of the nucleation events that give rise to new MTs. To this end, MT nucleation is restricted to specific subcellular sites called MT-organizing centres. The primary MT-organizing centre in proliferating animal cells is the centrosome. However, the discovery of MT nucleation capacity of the Golgi apparatus (GA) has substantially changed our understanding of MT network organization in interphase cells. Interestingly, MT nucleation at the Golgi apparently relies on multiprotein complexes, similar to those present at the centrosome, that assemble at the cis-face of the organelle. In this process, AKAP450 plays a central role, acting as a scaffold to recruit other centrosomal proteins important for MT generation. MT arrays derived from either the centrosome or the GA differ in their geometry, probably reflecting their different, yet complementary, functions. Here, I review our current understanding of the molecular mechanisms involved in MT nucleation at the GA and how Golgi- and centrosome-based MT arrays work in concert to ensure the formation of a pericentrosomal polarized continuous Golgi ribbon structure, a critical feature for cell polarity in mammalian cells. In addition, I comment on the important role of the Golgi-nucleated MTs in organizing specialized MT arrays that serve specific functions in terminally differentiated cells.

Microtubule nucleation in mitosis by a RanGTP-dependent protein complex

BACKGROUND

The γ-tubulin ring complex (γTuRC) is a multisubunit complex responsible for microtubule (MT) nucleation in eukaryotic cells. During mitosis, its spatial and temporal regulation promotes MT nucleation through different pathways. One of them is triggered around the chromosomes by RanGTP. Chromosomal MTs are essential for functional spindle assembly, but the mechanism by which RanGTP activates MT nucleation has not yet been resolved.

RESULTS

We used a combination of Xenopus egg extracts and in vitro experiments to dissect the mechanism by which RanGTP triggers MT nucleation. In egg extracts, NEDD1-coated beads promote MT nucleation only in the presence of RanGTP. We show that RanGTP promotes a direct interaction between one of its targets, TPX2, and XRHAMM that defines a specific γTuRC subcomplex. Through depletion/add-back experiments using mutant forms of TPX2 and NEDD1, we show that the activation of MT nucleation by RanGTP requires both NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex through a TPX2-dependent mechanism.

CONCLUSIONS

The XRHAMM-γTuRC complex is the target for activation by RanGTP that promotes an interaction between TPX2 and XRHAMM. The resulting TPX2-RHAMM-γTuRC supracomplex fulfills the two essential requirements for the activation of MT nucleation by RanGTP: NEDD1 phosphorylation on S405 by the TPX2-activated Aurora A and the recruitment of the complex onto a TPX2-dependent scaffold. Our data identify TPX2 as the only direct RanGTP target and NEDD1 as the only Aurora A substrate essential for the activation of the RanGTP-dependent MT nucleation pathway.

The Cep192-organized aurora A-Plk1 cascade is essential for centrosome cycle and bipolar spindle assembly

As cells enter mitosis, the two centrosomes separate and grow dramatically, each forming a nascent spindle pole that nucleates a radial array of microtubules. Centrosome growth (and associated microtubule nucleation surge), termed maturation, involves the recruitment of pericentriolar material components via an as-yet unknown mechanism. Here, we show that Cep192 binds Aurora A and Plk1, targets them to centrosomes in a pericentrin-dependent manner, and promotes sequential activation of both kinases via T-loop phosphorylation. The Cep192-bound Plk1 then phosphorylates Cep192 at several residues to generate the attachment sites for the γ-tubulin ring complex and, possibly, other pericentriolar material components, thus promoting their recruitment and subsequent microtubule nucleation. We further found that the Cep192-dependent Aurora A-Plk1 activity is essential for kinesin-5-mediated centrosome separation, bipolar spindle formation, and equal centrosome/centriole segregation into daughter cells. Thus, our study identifies a Cep192-organized signaling cascade that underlies both centrosome maturation and bipolar spindle assembly.

Proximity interactions among centrosome components identify regulators of centriole duplication

The centrosome consists of a pair of centrioles and surrounding pericentriolar material (PCM). Many vertebrate cells also have an array of granules, termed centriolar satellites, that localize around the centrosome and are associated with centrosome and cilium function. Centriole duplication occurs once per cell cycle and is effected by a set of proteins including PLK4, CEP192, CEP152, CEP63, and CPAP. Information on the relationships between these components is limited due to the difficulty in assaying interactions in the context of the centrosome. Here, we used proximity-dependent biotin identification (BioID) to identify proximity interactions among centriole duplication proteins. PLK4, CEP192, and CEP152 BioID identified known physically interacting proteins and a new interaction between CEP152 and CDK5RAP2 consistent with a function of CEP152 in PCM recruitment. BioID for CEP63 and its paralog CCDC67 revealed extensive proximity interactions with centriolar satellite proteins. Focusing on these satellite proteins identified two new regulators of centriole duplication, CCDC14 and KIAA0753. Both proteins colocalize with CEP63 to satellites, bind to CEP63, and identify other satellite proteins by BioID. KIAA0753 positively regulates centriole duplication and CEP63 centrosome localization, whereas CCDC14 negatively regulates both processes. These results suggest that centriolar satellites have a previously unappreciated function in regulating centriole duplication.

Amorphous no more: subdiffraction view of the pericentriolar material architecture

The centrosome influences the shape, orientation and activity of the microtubule cytoskeleton. The pericentriolar material (PCM), determines this functionality by providing a dynamic platform for nucleating microtubules and acts as a nexus for molecular signaling. Although great strides have been made in understanding PCM activity, its diffraction-limited size and amorphous appearance on electron microscopy (EM) have limited analysis of its high-order organization. Here, we outline current knowledge of PCM architecture and assembly, emphasizing recent super-resolution imaging studies that revealed the PCM has a layered structure made of fibers and matrices conserved from flies to humans. Notably, these studies debunk the long-standing view of an amorphous PCM and provide a paradigm to dissect the supramolecular organization of organelles in cells.

The role of NEDD1 phosphorylation by Aurora A in chromosomal microtubule nucleation and spindle function

Chromatin directs de novo microtubule (MT) nucleation in dividing cells by generating a gradient of GTP-bound Ran protein (RanGTP) that controls the activity of a number of spindle assembly factors (SAFs). It is now well established that these MTs are essential for the assembly of a functional bipolar spindle. Although it has been shown that RanGTP-dependent MT nucleation requires γ-tubulin and a number of RanGTP-regulated proteins, the mechanism involved is still poorly understood. We previously showed that the mitotic kinase Aurora A, which is activated in a RanGTP-dependent manner in mitotic cells, has a role in this pathway. Here we show that Aurora A interacts with and phosphorylates the γTURC adaptor protein NEDD1 at a single residue, Ser405. Ser405 phosphorylation is not required for centrosomal MT nucleation but is critical for MT nucleation in the vicinity of the chromosomes in mitotic cells. Moreover, it is essential for RanGTP aster formation and chromatin-driven MT assembly in Xenopus egg extracts. Our data suggest that one important function of Aurora A in mitotic cells is to promote MT nucleation around the chromatin by phosphorylating NEDD1, and thereby to promote functional spindle assembly.

Subdiffraction imaging of centrosomes reveals higher-order organizational features of pericentriolar material

The centrosome is the main microtubule organization centre of animal cells. It is composed of a centriole pair surrounded by pericentriolar material (PCM). Traditionally described as amorphous, the architecture of the PCM is not known, although its intricate mode of assembly alludes to the presence of a functional, hierarchical structure. Here we used subdiffraction imaging to reveal organizational features of the PCM. Interphase PCM components adopt a concentric toroidal distribution of discrete diameter around centrioles. Positional mapping of multiple non-overlapping epitopes revealed that pericentrin (PCNT) is an elongated molecule extending away from the centriole. We find that PCM components occupy separable spatial domains within mitotic PCM that are maintained in the absence of microtubule nucleation complexes and further implicate PCNT and CDK5RAP2 in the organization and assembly of PCM. Globally, this work highlights the role of higher-order PCM organization in the regulation of centrosome assembly and function.

CEP192 interacts physically and functionally with the K63-deubiquitinase CYLD to promote mitotic spindle assembly

CEP192 is a centrosome protein that plays a critical role in centrosome biogenesis and function in mammals, Drosophila and C. elegans. Moreover, CEP192-depleted cells arrest in mitosis with disorganized microtubules, suggesting that CEP192's function in spindle assembly goes beyond its role in centrosome activity and pointing to a potentially more direct role in the regulation of the mitotic microtubule landscape. To better understand CEP192 function in mitosis, we used mass spectrometry to identify CEP192-interacting proteins. We previously reported that CEP192 interacts with NEDD1, a protein that associates with the γ-tubulin ring complex (γ-TuRC) and regulates its phosphorylation status during mitosis. Additionally, within the array of proteins that interact with CEP192, we identified the microtubule binding K63-deubiquitinase CYLD. Further analyses show that co-depletion of CYLD alleviates the bipolar spindle assembly defects observed in CEP192-depleted cells. This functional relationship exposes an intriguing role for CYLD in spindle formation and raises the tantalizing possibility that CEP192 promotes robust mitotic spindle assembly by regulating K63-polyubiquitin-mediated signaling through CYLD.