GCP6 is a substrate of Plk4 and required for centriole duplication

Centriole duplication: when PLK4 meets Ana2/STIL

Polo-like kinase 4 is known to drive centriole duplication, but the relevant substrate remains elusive. A new study shows that PLK4 phosphorylates a key centriolar component, Ana2/STIL, to initiate centriole assembly.

Mutations in PLK4, encoding a master regulator of centriole biogenesis, cause microcephaly, growth failure and retinopathy

Centrioles are essential for ciliogenesis. However, mutations in centriole biogenesis genes have been reported in primary microcephaly and Seckel syndrome, disorders without the hallmark clinical features of ciliopathies. Here we identify mutations in the master regulator of centriole duplication, the PLK4 kinase, and its substrate TUBGCP6 in patients with microcephalic primordial dwarfism and additional congenital anomalies including retinopathy, extending the human phenotype spectrum associated with centriole dysfunction. Furthermore, we establish that different levels of impaired PLK4 activity result in growth and cilia phenoptyes, providing a mechanism by which microcephaly disorders can occur with or without ciliopathic features.

Plk4 phosphorylates Ana2 to trigger Sas6 recruitment and procentriole formation

Centrioles are 9-fold symmetrical structures at the core of centrosomes and base of cilia whose dysfunction has been linked to a wide range of inherited diseases and cancer [1]. Their duplication is regulated by a protein kinase of conserved structure, the C. elegans ZYG-1 or its Polo-like kinase 4 (Plk4) counterpart in other organisms [2, 3, 4]. Although Plk4’s centriolar partners and mechanisms that regulate its stability are known, its crucial substrates for centriole duplication have never been identified. Here we show that Drosophila Plk4 phosphorylates four conserved serines in the STAN motif of the core centriole protein Ana2 to enable it to bind and recruit its Sas6 partner. Ana2 and Sas6 normally load onto both mother and daughter centrioles immediately after their disengagement toward the end of mitosis to seed procentriole formation. Nonphosphorylatable Ana2 still localizes to the centriole but can no longer recruit Sas6 and centriole duplication fails. Thus, following centriole disengagement, recruitment of Ana2 and its phosphorylation by Plk4 are the earliest known events in centriole duplication to recruit Sas6 and thereby establish the architecture of the new procentriole engaged with its parent.

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Plk4 phosphorylates Ana2 at essential residues in its conserved STAN motif

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Plk4 phosphorylation triggers the direct interaction of Ana2 with Sas6

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Ana2 phosphorylated by Plk4 recruits Sas6 to centrioles at the end of mitosis

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A phospho-null Ana2 mutant fails to recruit Sas6 and duplicate centrioles

A novel role for Plk4 in regulating cell spreading and motility

Polo family kinase 4 (Plk4) is required for mitotic progression, and is haploinsufficient for tumor suppression and timely hepatocyte polarization in regenerating liver. At the same time, recent evidence suggests that Plk4 expression may have a role in clinical cancer progression, although the mechanisms are not clear. Here we identify a gene expression pattern predictive of reduced motility in Plk4(+/-) murine embryonic fibroblasts (MEFs) and validate this prediction with functional assays of cell spreading, migration and invasion. Increased Plk4 expression enhances cell spreading in Plk4(+/-) MEFs and migration in human embryonic kidney 293T cells, and increases invasion by DLD-1 colon cancer cells. Plk4 depletion impairs invasion of wild-type MEFs and suppresses invasion by MDA-MB231 breast cancer cells. Cytoskeletal reorganization and development of polarity are impaired in Plk4-deficient cells that have been stimulated to migrate. Endogenous Plk4 phosphorylated at the autophosphorylation site S305 localizes to the protrusions of motile cells, coincident with the RhoA GEF Ect2, GTP-bound RhoA and the RhoA effector mDia. Taken together, our findings reveal an unexpected activity of Plk4 that promotes cell migration and may underlie an association between increased Plk4 expression, cancer progression and death from metastasis in solid tumor patients.

Polo-like kinases: structural variations lead to multiple functions

Members of the polo-like kinase (PLK) family are crucial regulators of cell cycle progression, centriole duplication, mitosis, cytokinesis and the DNA damage response. PLKs undergo major changes in abundance, activity, localization and structure at different stages of the cell cycle. They interact with other proteins in a tightly controlled spatiotemporal manner as part of a network that coordinates key cell cycle events. Their essential roles are highlighted by the fact that alterations in PLK function are associated with cancers and other diseases. Recent knowledge gained from PLK crystal structures, evolution and interacting molecules offers important insights into the mechanisms that underlie their regulation and activity, and suggests novel functions unrelated to cell cycle control for this family of kinases.

NME7 is a functional component of the γ-tubulin ring complex

The γ-tubulin ring complex (γTuRC) is the primary microtubule nucleator in animal cells. NME7 possesses an intrinsic kinase activity that is involved in the stimulation of the γTuRC.

As the primary microtubule nucleator in animal cells, the γ-tubulin ring complex (γTuRC) plays a crucial role in microtubule organization, but little is known about how the activity of the γTuRC is regulated. Recently, isolated γTuRC was found to contain NME7, a poorly characterized member of the NME family. Here we report that NME7 is a γTuRC component that regulates the microtubule-nucleating activity of the γTuRC. NME7 contains two putative kinase domains, A and B, and shows autophosphorylating activity. Whereas domain A is involved in the autophosphorylation, domain B is inactive. NME7 interacts with the γTuRC through both A and B domains, with Arg-322 in domain B being crucial to the binding. In association with the γTuRC, NME7 localizes to centrosomes throughout the cell cycle and to mitotic spindles during mitosis. Suppression of NME7 expression does not affect γTuRC assembly or localization to centrosomes, but it does impair centrosome-based microtubule nucleation. Of importance, wild-type NME7 promotes γTuRC-dependent nucleation of microtubules, but kinase-deficient NME7 does so only poorly. These results suggest that NME7 functions in the γTuRC in a kinase-dependent manner to facilitate microtubule nucleation.

γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning

γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examine a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, we propose a role for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.

The role of mitotic kinases in coupling the centrosome cycle with the assembly of the mitotic spindle

The centrosome acts as the major microtubule-organizing center (MTOC) for cytoskeleton maintenance in interphase and mitotic spindle assembly in vertebrate cells. It duplicates only once per cell cycle in a highly spatiotemporally regulated manner. When the cell undergoes mitosis, the duplicated centrosomes separate to define spindle poles and monitor the assembly of the bipolar mitotic spindle for accurate chromosome separation and the maintenance of genomic stability. However, centrosome abnormalities occur frequently and often lead to monopolar or multipolar spindle formation, which results in chromosome instability and possibly tumorigenesis. A number of studies have begun to dissect the role of mitotic kinases, including NIMA-related kinases (Neks), cyclin-dependent kinases (CDKs), Polo-like kinases (Plks) and Aurora kinases, in regulating centrosome duplication, separation and maturation and subsequent mitotic spindle assembly during cell cycle progression. In this Commentary, we review the recent research progress on how these mitotic kinases are coordinated to couple the centrosome cycle with the cell cycle, thus ensuring bipolar mitotic spindle fidelity. Understanding this process will help to delineate the relationship between centrosomal abnormalities and spindle defects.

Cell Cycle Regulation of the Centrosome and Cilium

Centrosomes and cilia are conserved microtubule-based organelles whose structure and function depend on cell cycle stages. In dividing cells, centrosomes organize mitotic spindle poles, while in differentiating cells, centrosomes template ciliogenesis. Classically, this functional dichotomy has been attributed to regulation by cell cycle-dependent post-translational modifications, and recently PLK1, Nek2, Aurora A, and tubulin deacetylase were implicated in regulating the transition from cilia to centrosome. However, other recent studies suggest that tubulin dimers, the core structural components of centrosomes and cilia, also have a regulatory role. These regulatory mechanisms can be a target for chemotherapeutic intervention.

Spindle formation in the mouse embryo requires Plk4 in the absence of centrioles

During the first five rounds of cell division in the mouse embryo, spindles assemble in the absence of centrioles. Spindle formation initiates around chromosomes, but the microtubule nucleating process remains unclear. Here we demonstrate that Plk4, a protein kinase known as a master regulator of centriole formation, is also essential for spindle assembly in the absence of centrioles. Depletion of maternal Plk4 prevents nucleation and growth of microtubules and results in monopolar spindle formation. This leads to cytokinesis failure and, consequently, developmental arrest. We show that Plk4 function depends on its kinase activity and its partner protein, Cep152. Moreover, tethering Cep152 to cellular membranes sequesters Plk4 and is sufficient to trigger spindle assembly from ectopic membranous sites. Thus, the Plk4-Cep152 complex has an unexpected role in promoting microtubule nucleation in the vicinity of chromosomes to mediate bipolar spindle formation in the absence of centrioles.

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Plk4 is at acentriolar MTOCs and spindle poles in mouse embryos

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Plk4 is essential for acentriolar spindle assembly

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Depletion of maternal Plk4 prevents normal nucleation and growth of microtubules

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Plk4 MT-nucleating function depends on its kinase activity and its partner, Cep152

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Human Cep192 and Cep152 cooperate in Plk4 recruitment and centriole duplication

Polo-like kinase 4 (Plk4) is a key regulator of centriole duplication, but the mechanism underlying its recruitment to mammalian centrioles is not understood. In flies, Plk4 recruitment depends on Asterless, whereas nematodes rely on a distinct protein, Spd-2. Here, we have explored the roles of two homologous mammalian proteins, Cep152 and Cep192, in the centriole recruitment of human Plk4. We demonstrate that Cep192 plays a key role in centrosome recruitment of both Cep152 and Plk4. Double-depletion of Cep192 and Cep152 completely abolishes Plk4 binding to centrioles as well as centriole duplication, indicating that the two proteins cooperate. Most importantly, we show that Cep192 binds Plk4 through an N-terminal extension that is specific to the largest isoform. The Plk4 binding regions of Cep192 and Cep152 (residues 190-240 and 1-46, respectively) are rich in negatively charged amino acids, suggesting that Plk4 localization to centrioles depends on electrostatic interactions with the positively charged polo-box domain. We conclude that cooperation between Cep192 and Cep152 is crucial for centriole recruitment of Plk4 and centriole duplication during the cell cycle.

Malaria parasite cGMP-dependent protein kinase regulates blood stage merozoite secretory organelle discharge and egress

The malaria parasite replicates within an intraerythrocytic parasitophorous vacuole (PV). Eventually, in a tightly regulated process called egress, proteins of the PV and intracellular merozoite surface are modified by an essential parasite serine protease called PfSUB1, whilst the enclosing PV and erythrocyte membranes rupture, releasing merozoites to invade fresh erythrocytes. Inhibition of the Plasmodium falciparum cGMP-dependent protein kinase (PfPKG) prevents egress, but the underlying mechanism is unknown. Here we show that PfPKG activity is required for PfSUB1 discharge into the PV, as well as for release of distinct merozoite organelles called micronemes. Stimulation of PfPKG by inhibiting parasite phosphodiesterase activity induces premature PfSUB1 discharge and egress of developmentally immature, non-invasive parasites. Our findings identify the signalling pathway that regulates PfSUB1 function and egress, and raise the possibility of targeting PfPKG or parasite phosphodiesterases in therapeutic approaches to dysregulate critical protease-mediated steps in the parasite life cycle.

Building a centriole

Centrioles are the key foundation of centrosomes and cilia, yet a molecular understanding of how they form has only recently begun to emerge. Building a fully functional centriole that can form a centrosome and cilium requires two cell cycles. Centriole building starts with procentriole nucleation, a process that is coordinated by the conserved proteins Plk4/Zyg-1, and Asterless/Cep152. Subsequently, Sas-6, a conserved procentriole protein, self-assembles to provide nine-fold symmetry to the centriole scaffold. The procentriole then continues to elongate into a centriole, a process controlled by Sas-4/CPAP and CP110. Then, centrioles recruit Sas-4-mediated pre-assembled centrosomal complexes from the cytoplasm to form the pericentriolar material (PCM). Finally, CP110 and its interacting proteins are involved in controlling the timing of centriole templating of the cilium.

3D-structured illumination microscopy provides novel insight into architecture of human centrosomes

Centrioles are essential for the formation of cilia and flagella. They also form the core of the centrosome, which organizes microtubule arrays important for cell shape, polarity, motility and division. Here, we have used super-resolution 3D-structured illumination microscopy to analyse the spatial relationship of 18 centriole and pericentriolar matrix (PCM) components of human centrosomes at different cell cycle stages. During mitosis, PCM proteins formed extended networks with interspersed γ-Tubulin. During interphase, most proteins were arranged at specific distances from the walls of centrioles, resulting in ring staining, often with discernible density masses. Through use of site-specific antibodies, we found the C-terminus of Cep152 to be closer to centrioles than the N-terminus, illustrating the power of 3D-SIM to study protein disposition. Appendage proteins showed rings with multiple density masses, and the number of these masses was strongly reduced during mitosis. At the proximal end of centrioles, Sas-6 formed a dot at the site of daughter centriole assembly, consistent with its role in cartwheel formation. Plk4 and STIL co-localized with Sas-6, but Cep135 was associated mostly with mother centrioles. Remarkably, Plk4 formed a dot on the surface of the mother centriole before Sas-6 staining became detectable, indicating that Plk4 constitutes an early marker for the site of nascent centriole formation. Our study provides novel insights into the architecture of human centrosomes and illustrates the power of super-resolution microscopy in revealing the relative localization of centriole and PCM proteins in unprecedented detail.

Mouse Rif1 is a key regulator of the replication-timing programme in mammalian cells

The eukaryotic genome is replicated according to a specific spatio-temporal programme. However, little is known about both its molecular control and biological significance. Here, we identify mouse Rif1 as a key player in the regulation of DNA replication timing. We show that Rif1 deficiency in primary cells results in an unprecedented global alteration of the temporal order of replication. This effect takes place already in the first S-phase after Rif1 deletion and is neither accompanied by alterations in the transcriptional landscape nor by major changes in the biochemical identity of constitutive heterochromatin. In addition, Rif1 deficiency leads to both defective G1/S transition and chromatin re-organization after DNA replication. Together, these data offer a novel insight into the global regulation and biological significance of the replication-timing programme in mammalian cells.

The where, when and how of microtubule nucleation – one ring to rule them all

The function of microtubules depends on their arrangement into highly ordered arrays. Spatio-temporal control over the formation of new microtubules and regulation of their properties are central to the organization of these arrays. The nucleation of new microtubules requires γ-tubulin, an essential protein that assembles into multi-subunit complexes and is found in all eukaryotic organisms. However, the way in which γ-tubulin complexes are regulated and how this affects nucleation and, potentially, microtubule behavior, is poorly understood. γ-tubulin has been found in complexes of various sizes but several lines of evidence suggest that only large, ring-shaped complexes function as efficient microtubule nucleators. Human γ-tubulin ring complexes (γTuRCs) are composed of γ-tubulin and the γ-tubulin complex components (GCPs) 2, 3, 4, 5 and 6, which are members of a conserved protein family. Recent work has identified additional unrelated γTuRC subunits, as well as a large number of more transient γTuRC interactors. In this Commentary, we discuss the regulation of γTuRC-dependent microtubule nucleation as a key mechanism of microtubule organization. Specifically, we focus on the regulatory roles of the γTuRC subunits and interactors and present an overview of other mechanisms that regulate γTuRC-dependent microtubule nucleation and organization.