Centriole Assembly Requires Both Centriolar and Pericentriolar Material Proteins

Sas-4 proteins are required during basal body duplication in Paramecium

This study investigated the role of Sas-4 in basal body duplication in Paramecium and found that Sas-4 proteins are required to assemble and stabilize the germinative disk and cartwheel, which suggests that Sas-4 plays an essential role in basal body duplication.

Centrioles and basal bodies are structurally related organelles composed of nine microtubule (MT) triplets. Studies performed in Caenorhabditis elegans embryos have shown that centriole duplication takes place in sequential way, in which different proteins are recruited in a specific order to assemble a procentriole. ZYG-1 initiates centriole duplication by triggering the recruitment of a complex of SAS-5 and SAS-6, which then recruits the final player, SAS-4, to allow the incorporation of MT singlets. It is thought that a similar mechanism (that also involves additional proteins) is present in other animal cells, but it remains to be investigated whether the same players and their ascribed functions are conserved during basal body duplication in cells that exclusively contain basal bodies. To investigate this question, we have used the multiciliated protist Paramecium tetraurelia. Here we show that in the absence of PtSas4, two types of defects in basal body duplication can be identified. In the majority of cases, the germinative disk and cartwheel, the first structures assembled during duplication, are not detected. In addition, if daughter basal bodies were formed, they invariably had defects in MT recruitment. Our results suggest that PtSas4 has a broader function than its animal orthologues.

Structures of SAS-6 suggest its organization in centrioles

Centrioles are cylindrical, ninefold symmetrical structures with peripheral triplet microtubules strictly required to template cilia and flagella. The highly conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. We determined the x-ray structure of the amino-terminal domain of SAS-6 from zebrafish, and we show that recombinant SAS-6 self-associates in vitro into assemblies that resemble cartwheel centers. Point mutations are consistent with the notion that centriole formation in vivo depends on the interactions that define the self-assemblies observed here. Thus, these interactions are probably essential to the structural organization of cartwheel centers.

Building the centriole

Centrioles are conserved microtubule-based organelles that lie at the core of the animal centrosome and play a crucial role in nucleating the formation of cilia and flagella in most eukaryotes. Centrioles have a complex ultrastructure with ninefold symmetry and a well-defined length. This structure is assembled from a host of proteins, including a variety of disease gene products. Over a century after the discovery of centrioles, the mechanisms underlying the assembly of these fascinating organelles, in particular the establishment of ninefold symmetry and the control of centriole length, are now starting to be uncovered.

DSas-6 and Ana2 coassemble into tubules to promote centriole duplication and engagement

Centrioles form cilia and centrosomes, organelles whose dysfunction is increasingly linked to human disease. Centriole duplication relies on a few conserved proteins (ZYG-1/Sak/Plk4, SAS-6, SAS-5/Ana2, and SAS-4), and is often initiated by the formation of an inner “cartwheel” structure. Here, we show that overexpressed Drosophila Sas-6 and Ana2 coassemble into extended tubules (SAStubules) that bear a striking structural resemblance to the inner cartwheel of the centriole. SAStubules specifically interact with centriole proximal ends, but extra DSas-6/Ana2 is only recruited onto centrioles when Sak/Plk4 kinase is also overexpressed. This extra centriolar DSas-6/Ana2 induces centriole overduplication and, surprisingly, increased centriole cohesion. Intriguingly, we observe tubules that are structurally similar to SAStubules linking the engaged centrioles in normal wild-type cells. We conclude that DSas-6 and Ana2 normally cooperate to drive the formation of the centriole inner cartwheel and that they promote both centriole duplication and centriole cohesion in a Sak/Plk4-dependent manner.

Cep152 interacts with Plk4 and is required for centriole duplication

Cep152, the orthologue of Drosophila Asterless, is a Plk4 target that functions with Plk4 in centriole assembly.

Centrioles are microtubule-based structures that organize the centrosome and nucleate cilia. Centrioles duplicate once per cell cycle, and duplication requires Plk4, a member of the Polo-like kinase family; however, the mechanism linking Plk4 activity and centriole formation is unknown. In this study, we show in human and frog cells that Plk4 interacts with the centrosome protein Cep152, the orthologue of Drosophila melanogaster Asterless. The interaction requires the N-terminal 217 residues of Cep152 and the crypto Polo-box of Plk4. Cep152 and Plk4 colocalize at the centriole throughout the cell cycle. Overexpression of Cep152 (1–217) mislocalizes Plk4, but both Cep152 and Plk4 are able to localize to the centriole independently of the other. Depletion of Cep152 prevents both normal centriole duplication and Plk4-induced centriole amplification and results in a failure to localize Sas6 to the centriole, an early step in duplication. Cep152 can be phosphorylated by Plk4 in vitro, suggesting that Cep152 acts with Plk4 to initiate centriole formation.

Ancestral centriole and flagella proteins identified by analysis of Naegleria differentiation

Naegleria gruberi is a single-celled eukaryote best known for its remarkable ability to form an entire microtubule cytoskeleton de novo during its metamorphosis from an amoeba into a flagellate, including basal bodies (equivalent to centrioles), flagella and a cytoplasmic microtubule array. Our publicly available full-genome transcriptional analysis, performed at 20-minute intervals throughout Naegleria differentiation, reveals vast transcriptional changes, including the differential expression of genes involved in metabolism, signaling and the stress response. Cluster analysis of the transcriptional profiles of predicted cytoskeletal genes reveals a set of 55 genes enriched in centriole components (induced early) and a set of 82 genes enriched in flagella proteins (induced late). The early set includes genes encoding nearly every known conserved centriole component, as well as eight previously uncharacterized, highly conserved genes. The human orthologs of at least five genes localize to the centrosomes of human cells, one of which (here named Friggin) localizes specifically to mother centrioles.

Basal body duplication in Paramecium: the key role of Bld10 in assembly and stability of the cartwheel

Basal bodies which nucleate cilia and flagella, and centrioles which organize centrosomes share the same architecture characterized by the ninefold symmetry of their microtubular shaft. Among the conserved proteins involved in the biogenesis of the canonical 9-triplet centriolar structures, Sas-6 and Bld10 proteins have been shown to play central roles in the early steps of assembly and in establishment/stabilization of the ninefold symmetry. Using fluorescent tagged proteins and RNAi to study the localization and function of these two proteins in Paramecium, we focused on the early effects of their depletion, the consequences of their overexpression and their functional interdependence. We find that both genes are essential and their depletion affects cartwheel assembly and hence basal body duplication. We also show that, contrary to Sas6p, Bld10p is not directly responsible for the establishment of the ninefold symmetry, but is required not only for new basal body assembly and stability but also for Sas6p maintenance at mature basal bodies. Finally, ultrastructural analysis of cells overexpressing either protein revealed two types of early assembly intermediates, hub-like structures and generative discs, suggesting a conserved scaffolding process.

Daughter centriole elongation is controlled by proteolysis

This report shows that inhibition of proteolysis can cause abnormal centriole elongation and identifies several centrosome proteins involved in this process by using a small interfering RNA screen.

The centrosome is the major microtubule-organizing center of most mammalian cells and consists of a pair of centrioles embedded in pericentriolar material. Before mitosis, the two centrioles duplicate and two new daughter centrioles form adjacent to each preexisting maternal centriole. After initiation of daughter centriole synthesis, the procentrioles elongate in a process that is poorly understood. Here, we show that inhibition of cellular proteolysis by Z-L₃VS or MG132 induces abnormal elongation of daughter centrioles to approximately 4 times their normal length. This activity of Z-L₃VS or MG132 was found to correlate with inhibition of intracellular protease-mediated substrate cleavage. Using a small interfering RNA screen, we identified a total of nine gene products that either attenuated (seven) or promoted (two) abnormal Z-L₃VS–induced daughter centriole elongation. Our hits included known regulators of centriole length, including CPAP and CP110, but, interestingly, several proteins involved in microtubule stability and anchoring as well as centrosome cohesion. This suggests that nonproteasomal functions, specifically inhibition of cellular proteases, may play an important and underappreciated role in the regulation of centriole elongation. They also highlight the complexity of daughter centriole length control and provide a framework for future studies to dissect the molecular details of this process.

SPICE--a previously uncharacterized protein required for centriole duplication and mitotic chromosome congression

Proper assembly and function of a bipolar mitotic spindle is crucial for faithful bidirectional chromosome segregation during cell division. In animal cells, the two poles of the mitotic spindle are organized by centrosomes, microtubule-organizing structures composed of a pair of centrioles surrounded by the so-called pericentriolar material. Proteomic studies have revealed a large number of centrosome proteins, but many remain uncharacterized. Here, we characterize SPICE, a protein that localizes to spindle microtubules in mitosis and to centrioles throughout the cell cycle. RNAi-mediated depletion of SPICE in human cells impairs centriole duplication and causes severe mitotic defects. SPICE depletion compromises spindle architecture, spindle pole integrity and chromosome congression, even in cells in which centriole duplication has occurred. Our data suggest that SPICE is an important dual-function regulator required for centriole duplication and for proper bipolar spindle formation and chromosome congression in mitosis.

Plk4/SAK/ZYG-1 in the regulation of centriole duplication

Centrioles organize both centrosomes and cilia. Centriole duplication is tightly regulated and coordinated with the cell cycle to limit duplication to only once per cell cycle. Defects in centriole number and structure are commonly found in cancer. Plk4/SAK and the functionally related Caenorhabditis elegans ZYG-1 kinases initiate centriole duplication. Several recent studies have elucidated the regulated activity of these kinases and potential downstream targets for centriole assembly.

The elegans of spindle assembly

The Caenorhabditis elegans one-cell embryo is a powerful system in which to study microtubule organization because this large cell assembles both meiotic and mitotic spindles within the same cytoplasm over the course of 1 h in a stereotypical manner. The fertilized oocyte assembles two consecutive acentrosomal meiotic spindles that function to reduce the replicated maternal diploid set of chromosomes to a single-copy haploid set. The resulting maternal DNA then unites with the paternal DNA to form a zygotic diploid complement, around which a centrosome-based mitotic spindle forms. The early C. elegans embryo is amenable to live-cell imaging and electron tomography, permitting a detailed structural comparison of the meiotic and mitotic modes of spindle assembly.

Centrioles: active players or passengers during mitosis?

Centrioles are cylinders made of nine microtubule (MT) triplets present in many eukaryotes. Early studies, where centrosomes were seen at the poles of the mitotic spindle led to their coining as "the organ for cell division". However, a variety of subsequent observational and functional studies showed that centrosomes might not always be essential for mitosis. Here we review the arguments in this debate. We describe the centriole structure and its distribution in the eukaryotic tree of life and clarify its role in the organization of the centrosome and cilia, with an historical perspective. An important aspect of the debate addressed in this review is how centrioles are inherited and the role of the spindle in this process. In particular, germline inheritance of centrosomes, such as their de novo formation in parthenogenetic species, poses many interesting questions. We finish by discussing the most likely functions of centrioles and laying out new research avenues.

PLK2 phosphorylation is critical for CPAP function in procentriole formation during the centrosome cycle

Control of centrosome duplication is tightly linked with the progression of the cell cycle. Recent studies suggest that the fundamental process of centriole duplication is evolutionally conserved. Here, we identified centrosomal P4.1-associated protein (CPAP), a human homologue of SAS-4, as a substrate of PLK2 whose activity oscillates during the cell cycle. PLK2 phosphorylates the S589 and S595 residues of CPAP in vitro and in vivo. This phosphorylation is critical for procentriole formation during the centrosome cycle. PLK4 also phosphorylates S595 of CPAP, but PLK4 phosphorylation is not a critical step in the PLK4 function in procentriole assembly. CPAP is phosphorylated in a cell cycle stage-specific manner, so that its phosphorylation increases at the G1/S transition phase and decreases during the exit of mitosis. Phosphorylated CPAP is preferentially located at the procentriole. Furthermore, overexpression of a phospho-resistant CPAP mutant resulted in the failure to form elongated centrioles. On the basis of these results, we propose that phosphorylated CPAP is involved in procentriole assembly, possibly for centriole elongation. This work demonstrates an example of how procentriole formation is linked to the progression of the cell cycle.

Reconstructing the evolutionary history of the centriole from protein components

Centrioles are highly conserved structures that fulfil important cellular functions, such as nucleation of cilia and flagella (basal-body function) and organisation of pericentriolar material to form the centrosome. The evolution of these functions can be inferred from the distribution of the molecular components of extant centrioles and centrosomes. Here, we undertake an evolutionary analysis of 53 proteins known either for centriolar association or for involvement in cilia-associated pathologies. By linking protein distribution in 45 diverse eukaryotes with organism biology, we provide molecular evidence to show that basal-body function is ancestral, whereas the presence of the centrosome is specific to the Holozoa. We define an ancestral centriolar inventory of 14 core proteins, Polo-like-kinase, and proteins associated with Bardet-Biedl syndrome (BBS) and Meckel-Gruber syndrome. We show that the BBSome is absent from organisms that produce cilia only for motility, predicting a dominant and ancient role for this complex in sensory function. We also show that the unusual centriole of Caenorhabditis elegans is highly divergent in both protein composition and sequence. Finally, we demonstrate a correlation between the presence of specific centriolar proteins and eye evolution. This correlation is used to predict proteins with functions in the development of ciliary, but not rhabdomeric, eyes.

Stepwise evolution of the centriole-assembly pathway

The centriole and basal body (CBB) structure nucleates cilia and flagella, and is an essential component of the centrosome, underlying eukaryotic microtubule-based motility, cell division and polarity. In recent years, components of the CBB-assembly machinery have been identified, but little is known about their regulation and evolution. Given the diversity of cellular contexts encountered in eukaryotes, but the remarkable conservation of CBB morphology, we asked whether general mechanistic principles could explain CBB assembly. We analysed the distribution of each component of the human CBB-assembly machinery across eukaryotes as a strategy to generate testable hypotheses. We found an evolutionarily cohesive and ancestral module, which we term UNIMOD and is defined by three components (SAS6, SAS4/CPAP and BLD10/CEP135), that correlates with the occurrence of CBBs. Unexpectedly, other players (SAK/PLK4, SPD2/CEP192 and CP110) emerged in a taxon-specific manner. We report that gene duplication plays an important role in the evolution of CBB components and show that, in the case of BLD10/CEP135, this is a source of tissue specificity in CBB and flagella biogenesis. Moreover, we observe extreme protein divergence amongst CBB components and show experimentally that there is loss of cross-species complementation among SAK/PLK4 family members, suggesting species-specific adaptations in CBB assembly. We propose that the UNIMOD theory explains the conservation of CBB architecture and that taxon- and tissue-specific molecular innovations, gained through emergence, duplication and divergence, play important roles in coordinating CBB biogenesis and function in different cellular contexts.

Vertebrate maternal-effect genes: Insights into fertilization, early cleavage divisions, and germ cell determinant localization from studies in the zebrafish

In the earliest stages of animal development prior to the commencement of zygotic transcription, all critical cellular processes are carried out by maternally-provided molecular products accumulated in the egg during oogenesis. Disruption of these maternal products can lead to defective embryogenesis. In this review, we focus on maternal genes with roles in the fundamental processes of fertilization, cell division, centrosome regulation, and germ cell development with emphasis on findings from the zebrafish, as this is a unique and valuable model system for vertebrate reproduction.

Procentriole assembly revealed by cryo-electron tomography

Centrosomes are cellular organelles that have a major role in the spatial organisation of the microtubule network. The centrosome is comprised of two centrioles that duplicate only once during the cell cycle, generating a procentriole from each mature centriole. Despite the essential roles of centrosomes, the detailed structural mechanisms involved in centriole duplication remain largely unknown. Here, we describe human procentriole assembly using cryo-electron tomography. In centrosomes, isolated from human lymphoblasts, we observed that each one of the nine microtubule triplets grows independently around a periodic central structure. The proximal end of the A-microtubule is capped by a conical structure and the B- and C-microtubules elongate bidirectionally from its wall. These observations suggest that the gamma tubulin ring complex (gamma-TuRC) has a fundamental role in procentriole formation by nucleating the A-microtubule that acts as a template for B-microtubule elongation that, in turn, supports C-microtubule growth. This study provides new insights into the initial structural events involved in procentriole assembly and establishes the basis for determining the molecular mechanisms of centriole duplication on the nanometric scale.

Motility and protein phosphorylation in healthy and asthenozoospermic sperm

The majority of male infertility results from poor sperm motility. A direct link between altered protein phosphorylation and aberrant sperm motility has not been established. To address this issue, sperm samples obtained from 20 donors with healthy sperm and 20 donors with aberrantly motile sperm were subjected to computer assisted semen analysis (CASA), proteomic analysis, Western blot, and immunofluorescent staining. Proteomic analysis identified 12 protein spots as having differential phosphorylation, including gamma-tubulin complex associated protein 2 (GCP2). Western blot and immunofluorescence demonstrated differential expression of gamma-tubulin between healthy and aberrantly motile sperm. In conclusion, hypophosphorylated proteins and reduced expression of gamma-tubulin may be associated with low motility sperm.

The MST1 and hMOB1 tumor suppressors control human centrosome duplication by regulating NDR kinase phosphorylation

BACKGROUND

Human MST/hSAV/LATS/hMOB tumor suppressor cascades are regulators of cell death and proliferation; however, little is known about other functions of MST/hMOB signaling. Mob1p, one of two MOB proteins in yeast, appears to play a role in spindle pole body duplication (the equivalent of mammalian centrosome duplication). We therefore investigated the role of human MOB proteins in centrosome duplication. We also addressed the regulation of human centrosome duplication by mammalian serine/threonine Ste20-like (MST) kinases, considering that MOB proteins can function together with Ste20-like kinases in eukaryotes.

RESULTS

By studying the six human MOB proteins and five MST kinases, we found that MST1/hMOB1 signaling controls centrosome duplication. Overexpression of hMOB1 caused centrosome overduplication, whereas RNAi depletion of hMOB1 or MST1 impaired centriole duplication. Significantly, we delineated an hMOB1/MST1/NDR1 signaling pathway regulating centrosome duplication. More specifically, analysis of shRNA-resistant hMOB1 and NDR1 mutants revealed that a functional NDR/hMOB1 complex is critical for MST1 to phosphorylate NDR on the hydrophobic motif that in turn is required for human centrosome duplication. Furthermore, shRNA-resistant MST1 variants revealed that MST1 kinase activity is crucial for centrosome duplication whereas MST1 binding to the hSAV and RASSF1A tumor suppressor proteins is dispensable. Finally, by studying the PLK4/HsSAS-6/CP110 centriole assembly machinery, we also observed that normal daughter centriole formation depends on intact MST1/hMOB1/NDR signaling, although HsSAS-6 centriolar localization is not affected.

CONCLUSIONS

Our observations propose a novel pathway in control of human centriole duplication after recruitment of HsSAS-6 to centrioles.

Control of mitotic and meiotic centriole duplication by the Plk4-related kinase ZYG-1

Centriole duplication is of crucial importance during both mitotic and male meiotic divisions, but it is currently not known whether this process is regulated differently during the two modes of division. In Caenorhabditis elegans, the kinase ZYG-1 plays an essential role in both mitotic and meiotic centriole duplication. We have found that the C-terminus of ZYG-1 is necessary and sufficient for targeting to centrosomes and is important for differentiating mitotic and meiotic centriole duplication. Small truncations of the C-terminus dramatically lower the level of ZYG-1 at mitotic centrosomes but have little effect on the level of ZYG-1 at meiotic centrosomes. Interestingly, truncation of ZYG-1 blocks centrosome duplication in the mitotic cycle but leads to centrosome amplification in the meiotic cycle. Meiotic centriole amplification appears to result from the overduplication of centrioles during meiosis I and leads to the formation of multipolar meiosis II spindles. The extra centrioles also disrupt spermatogenesis by inducing the formation of supernumerary fertilization-competent spermatids that contain abnormal numbers of chromosomes and centrioles. Our data reveal differences in the regulation of mitotic and meiotic centrosome duplication, particularly with regard to ZYG-1 activity, and reveal an important role for centrosomes in spermatid formation.

Drosophila Ana2 is a conserved centriole duplication factor

In Caenorhabditis elegans, five proteins are required for centriole duplication: SPD-2, ZYG-1, SAS-5, SAS-6, and SAS-4. Functional orthologues of all but SAS-5 have been found in other species. In Drosophila melanogaster and humans, Sak/Plk4, DSas-6/hSas-6, and DSas-4/CPAP-orthologues of ZYG-1, SAS-6, and SAS-4, respectively-are required for centriole duplication. Strikingly, all three fly proteins can induce the de novo formation of centriole-like structures when overexpressed in unfertilized eggs. Here, we find that of eight candidate duplication factors identified in cultured fly cells, only two, Ana2 and Asterless (Asl), share this ability. Asl is now known to be essential for centriole duplication in flies, but no equivalent protein has been found in worms. We show that Ana2 is the likely functional orthologue of SAS-5 and that it is also related to the vertebrate STIL/SIL protein family that has been linked to microcephaly in humans. We propose that members of the SAS-5/Ana2/STIL family of proteins are key conserved components of the centriole duplication machinery.

Phosphorylation of SAS-6 by ZYG-1 is critical for centriole formation in C. elegans embryos

Despite being essential for proper cell division, the mechanisms governing centrosome duplication are incompletely understood and represent an important open question in cell biology. Formation of a new centriole next to each existing one is critical for centrosome duplication. In Caenorhabditis elegans embryos, the proteins SPD-2, ZYG-1, SAS-6, SAS-5, and SAS-4 are essential for centriole formation, but the mechanisms underlying their requirement remain unclear. Here, we demonstrate that the kinase ZYG-1 phosphorylates the coiled-coil protein SAS-6 at serine 123 in vitro. Importantly, we show that this phosphorylation event is crucial for centriole formation in vivo. Furthermore, we establish that such phosphorylation ensures the maintenance of SAS-6 at the emerging centriole. Overall, our findings establish that phosphorylation of the evolutionarily conserved protein SAS-6 is critical for centriole formation and thus for faithful cell division.

Self-assembling SAS-6 multimer is a core centriole building block

Centrioles are conserved microtubule-based organelles with 9-fold symmetry that are essential for cilia and mitotic spindle formation. A conserved structure at the onset of centriole assembly is a "cartwheel" with 9-fold radial symmetry and a central tubule in its core. It remains unclear how the cartwheel is formed. The conserved centriole protein, SAS-6, is a cartwheel component that functions early in centriole formation. Here, combining biochemistry and electron microscopy, we characterize SAS-6 and show that it self-assembles into stable tetramers, which serve as building blocks for the central tubule. These results suggest that SAS-6 self-assembly may be an initial step in the formation of the cartwheel that provides the 9-fold symmetry. Electron microscopy of centrosomes identified 25-nm central tubules with repeating subunits and show that SAS-6 concentrates at the core of the cartwheel. Recombinant and native SAS-6 self-oligomerizes into tetramers with approximately 6-nm subunits, and these tetramers are components of the centrosome, suggesting that tetramers are the building blocks of the central tubule. This is further supported by the observation that elevated levels of SAS-6 in Drosophila cells resulted in higher order structures resembling central tubule morphology. Finally, in the presence of embryonic extract, SAS-6 tetramers assembled into high density complexes, providing a starting point for the eventual in vitro reconstruction of centrioles.

Basal body stability and ciliogenesis requires the conserved component Poc1

Poc1 shores up basal bodies to support cilia formation in Tetrahymena thermophila, zebrafish, and humans; Poc1 depletion causes phenotypes commonly seen in ciliopathies.

Centrioles are the foundation for centrosome and cilia formation. The biogenesis of centrioles is initiated by an assembly mechanism that first synthesizes the ninefold symmetrical cartwheel and subsequently leads to a stable cylindrical microtubule scaffold that is capable of withstanding microtubule-based forces generated by centrosomes and cilia. We report that the conserved WD40 repeat domain–containing cartwheel protein Poc1 is required for the structural maintenance of centrioles in Tetrahymena thermophila. Furthermore, human Poc1B is required for primary ciliogenesis, and in zebrafish, DrPoc1B knockdown causes ciliary defects and morphological phenotypes consistent with human ciliopathies. T.thermophila Poc1 exhibits a protein incorporation profile commonly associated with structural centriole components in which the majority of Poc1 is stably incorporated during new centriole assembly. A second dynamic population assembles throughout the cell cycle. Our experiments identify novel roles for Poc1 in centriole stability and ciliogenesis.

Electron microscopy and high-pressure freezing of Arabidopsis

In this chapter, we will discuss methods and protocols for high-pressure freezing (HPF) and freeze substitution (FS) to examine Arabidopsis tissues by transmission electron microscopy (TEM). By use of HPF in combination with FS, it is possible to obtain Arabidopsis samples that are far better preserved for both ultrastructural analysis and immunogold labeling than by conventional chemical fixation. Like other cryofixation methods, ice crystal growth is still a problem in HPF if samples are too thick (> 200 μm) or if their water content is too high. Furthermore, damage done to cells/tissues prior to freezing cannot be "reverted" by HPF. In general, FS of plant tissues is more difficult than that of nonplant tissues because plant cell walls impede removal of water from the enclosed cells as well as from the walls themselves. To overcome these challenges, we describe the details of a HPF, FS, and resin-embedding protocol for Arabidopsis tissues here. In addition, the generation of ribbons of serial sections from Arabidopsis TEM blocks, three-dimensional (3D) analysis of organelle shapes and distribution within the tissue, and immunogold labeling are also explained. The Arabidopsis research community has developed many research tools to investigate gene functions such as knockout mutant lines, antibodies, and transgenic lines expressing epitope-tagged proteins. The TEM techniques explained here have been combined with these tools to elucidate how a particular gene of interest functions in the Arabidopsis cell.

Centrioles, centrosomes, and cilia in health and disease

Centrioles are barrel-shaped structures that are essential for the formation of centrosomes, cilia, and flagella. Here we review recent advances in our understanding of the function and biogenesis of these organelles, and we emphasize their connection to human disease. Deregulation of centrosome numbers has long been proposed to contribute to genome instability and tumor formation, whereas mutations in centrosomal proteins have recently been genetically linked to microcephaly and dwarfism. Finally, structural or functional centriole aberrations contribute to ciliopathies, a variety of complex diseases that stem from the absence or dysfunction of cilia.

Ana3 is a conserved protein required for the structural integrity of centrioles and basal bodies

Recent studies have identified a conserved "core" of proteins that are required for centriole duplication. A small number of additional proteins have recently been identified as potential duplication factors, but it is unclear whether any of these proteins are components of the core duplication machinery. In this study, we investigate the function of one of these proteins, Drosophila melanogaster Ana3. We show that Ana3 is present in centrioles and basal bodies, but its behavior is distinct from that of the core duplication proteins. Most importantly, we find that Ana3 is required for the structural integrity of both centrioles and basal bodies and for centriole cohesion, but it is not essential for centriole duplication. We show that Ana3 has a mammalian homologue, Rotatin, that also localizes to centrioles and basal bodies and appears to be essential for cilia function. Thus, Ana3 defines a conserved family of centriolar proteins and plays an important part in ensuring the structural integrity of centrioles and basal bodies.

A ZYG-12-dynein interaction at the nuclear envelope defines cytoskeletal architecture in the C. elegans gonad

Changes in cellular microtubule organization often accompany developmental progression. In the Caenorhabditis elegans embryo, the centrosome, which is attached to the nucleus via ZYG-12, organizes the microtubule network. In this study, we investigate ZYG-12 function and microtubule organization before embryo formation in the gonad. Surprisingly, ZYG-12 is dispensable for centrosome attachment in the germline. However, ZYG-12-mediated recruitment of dynein to the nuclear envelope is required to maintain microtubule organization, membrane architecture, and nuclear positioning within the syncytial gonad. We examined gamma-tubulin localization and microtubule regrowth after depolymerization to identify sites of nucleation in germ cells. gamma-Tubulin localizes to the plasma membrane in addition to the centrosome, and regrowth initiates at both sites. Because we do not observe organized microtubules around zyg-12(ct350) mutant nuclei with attached centrosomes, we propose that gonad architecture, including membrane and nuclear positioning, is determined by microtubule nucleation at the plasma membrane combined with tension on the microtubules by dynein anchored at the nucleus by ZYG-12.

Procentriole elongation and recruitment of pericentriolar material are downregulated in cyst cells as they enter quiescence

The apical region of the Drosophila testis contains a niche with two stem cell populations: germline stem cells (GSCs) and cyst progenitor cells (CPCs). Asymmetrical division of these stem cells leads to gonioblast daughters (which undergo further mitoses) and cyst cell daughters (which withdraw from the cell cycle and become quiescent). Although a considerable body of evidence indicates important roles for centrosomes in spindle orientation and asymmetrical division of GSCs, the behaviour and function of the centrioles in CPCs and their daughters remain unknown. Here, we show that quiescent cyst cells lose centrosome components after two divisions of the spermatogonia they envelop, but keep the centriolar component SAS-6. Cyst cells do have centriole pairs, but they are formed by a mother and a very short daughter that does not elongate or mature. The presence of procentrioles in quiescent cyst cells suggests that the centriole duplication cycle is uncoupled from the G1-S transition and that it might begin even earlier, in mitosis. Failure to enter the cell cycle might result in the improper recruitment of centriolar components at the mother centriole, thus hampering the full elongation of its daughter. Procentriole maturation defects could thus lead to the inability to maintain centrosomal components during development.

CP250, a novel acidic coiled-coil protein of the Dictyostelium centrosome, affects growth, chemotaxis, and the nuclear envelope

The Dictyostelium centrosome is a nucleus associated body consisting of a box-shaped core surrounded by the corona, an amorphous matrix functionally equivalent to the pericentriolar material of animal centrosomes which is responsible for the nucleation and anchoring of microtubules. Here we describe CP250 a component of the corona, an acidic coiled coil protein that is present at the centrosome throughout interphase while disappearing during prophase and reappearing at the end of late telophase. Amino acids 756-1148 of the 2110 amino acids are sufficient for centrosomal targeting and cell cycle-dependent centrosome association. Mutant cells lacking CP250 are smaller in size, growth on bacteria is delayed, chemotaxis is altered, and development is affected, which, in general, are defects observed in cytoskeletal mutants. Furthermore, loss of CP250 affected the nuclear envelope and led to reduced amounts and altered distribution of Sun-1, a conserved nuclear envelope protein that connects the centrosome to chromatin.

Centrosomes and cancer: how cancer cells divide with too many centrosomes

Precise control of centrosome number is crucial for bipolar spindle assembly and accurate transmission of genetic material to daughter cells. Failure to properly control centrosome number results in supernumerary centrosomes, which are frequently found in cancer cells. This presents a paradox: during mitosis, cells with more than two centrosomes are prone to multipolar mitoses and cell death, however, cancer cells possessing extra centrosomes usually divide successfully. One mechanism frequently utilized by cancer cells to escape death caused by multipolar mitoses is the clustering of supernumerary centrosomes into bipolar arrays. An understanding of the molecular mechanisms by which cancer cells can suppress multipolar mitoses is beginning to emerge. Here, we review what's currently known about centrosome clustering mechanisms and discuss potential strategies to target these mechanisms for the selective killing of cancer cells.

The hydrolethalus syndrome protein HYLS-1 links core centriole structure to cilia formation

Centrioles are subcellular organelles composed of a ninefold symmetric microtubule array that perform two important functions: (1) They build centrosomes that organize the microtubule cytoskeleton, and (2) they template cilia, microtubule-based projections with sensory and motile functions. We identified HYLS-1, a widely conserved protein, based on its direct interaction with the core centriolar protein SAS-4. HYLS-1 localization to centrioles requires SAS-4 and, like SAS-4, HYLS-1 is stably incorporated into the outer centriole wall. Unlike SAS-4, HYLS-1 is dispensable for centriole assembly and centrosome function in cell division. Instead, HYLS-1 plays an essential role in cilia formation that is conserved between Caenorhabditis elegans and vertebrates. A single amino acid change in human HYLS1 leads to a perinatal lethal disorder termed hydrolethalus syndrome, and we show that this mutation impairs HYLS-1 function in ciliogenesis. HYLS-1 is required for the apical targeting/anchoring of centrioles at the plasma membrane but not for the intraflagellar transport-dependent extension of the ciliary axoneme. These findings classify hydrolethalus syndrome as a severe human ciliopathy and shed light on the dual functionality of centrioles, defining the first stably incorporated centriolar protein that is not required for centriole assembly but instead confers on centrioles the capacity to initiate ciliogenesis.

Semi-automated curation of protein subcellular localization: a text mining-based approach to Gene Ontology (GO) Cellular Component curation

Background

Manual curation of experimental data from the biomedical literature is an expensive and time-consuming endeavor. Nevertheless, most biological knowledge bases still rely heavily on manual curation for data extraction and entry. Text mining software that can semi- or fully automate information retrieval from the literature would thus provide a significant boost to manual curation efforts.

Results

We employ the Textpresso category-based information retrieval and extraction system , developed by WormBase to explore how Textpresso might improve the efficiency with which we manually curate C. elegans proteins to the Gene Ontology's Cellular Component Ontology. Using a training set of sentences that describe results of localization experiments in the published literature, we generated three new curation task-specific categories (Cellular Components, Assay Terms, and Verbs) containing words and phrases associated with reports of experimentally determined subcellular localization. We compared the results of manual curation to that of Textpresso queries that searched the full text of articles for sentences containing terms from each of the three new categories plus the name of a previously uncurated C. elegans protein, and found that Textpresso searches identified curatable papers with recall and precision rates of 79.1% and 61.8%, respectively (F-score of 69.5%), when compared to manual curation. Within those documents, Textpresso identified relevant sentences with recall and precision rates of 30.3% and 80.1% (F-score of 44.0%). From returned sentences, curators were able to make 66.2% of all possible experimentally supported GO Cellular Component annotations with 97.3% precision (F-score of 78.8%). Measuring the relative efficiencies of Textpresso-based versus manual curation we find that Textpresso has the potential to increase curation efficiency by at least 8-fold, and perhaps as much as 15-fold, given differences in individual curatorial speed.

Conclusion

Textpresso is an effective tool for improving the efficiency of manual, experimentally based curation. Incorporating a Textpresso-based Cellular Component curation pipeline at WormBase has allowed us to transition from strictly manual curation of this data type to a more efficient pipeline of computer-assisted validation. Continued development of curation task-specific Textpresso categories will provide an invaluable resource for genomics databases that rely heavily on manual curation.

Gamma-tubulin-containing abnormal centrioles are induced by insufficient Plk4 in human HCT116 colorectal cancer cells

Cancer cells frequently induce aberrant centrosomes, which have been implicated in cancer initiation and progression. Human colorectal cancer cells, HCT116, contain aberrant centrioles composed of disorganized cylindrical microtubules and displaced appendages. These cells also express unique centrosome-related structures associated with a subset of centrosomal components, including gamma-tubulin, centrin and PCM1. During hydroxyurea treatment, these abnormal structures become more abundant and undergo a change in shape from small dots to elongated fibers. Although gamma-tubulin seems to exist as a ring complex, the abnormal structures do not support microtubule nucleation. Several lines of evidence suggest that the fibers correspond to a disorganized form of centriolar microtubules. Plk4, a mammalian homolog of ZYG-1 essential for initiation of centriole biogenesis, is not associated with the gamma-tubulin-specific abnormal centrosomes. The amount of Plk4 at each centrosome was less in cells with abnormal centrosomes than cells without gamma-tubulin-specific abnormal centrosomes. In addition, the formation of abnormal structures was abolished by expression of exogenous Plk4, but not SAS6 and Cep135/Bld10p, which are downstream regulators required for the organization of nine-triplet microtubules. These results suggest that HCT116 cells fail to organize the ninefold symmetry of centrioles due to insufficient Plk4.

Molecular cold-adaptation of protein function and gene regulation: The case for comparative genomic analyses in marine ciliated protozoa

Euplotes focardii is a marine ciliated protozoan discovered in the Ross Sea near Terra Nova Bay, Antarctica. This organism is strictly psychrophilic, survives and reproduces optimally at 4-5 °C, and has a genome rich in A/T base pairs. Like other ciliated protozoans, Euplotes spp. are characterized by nuclear dimorphism: 1) the germline micronucleus contains the entire genome as large chromosomes; and 2) the somatic macronucleus (∼50 megabases, or 5% of the micronuclear genome) contains small linear DNA nanochromosomes [1-12 kilobases], each of which constitutes a single genetic unit. These characteristics make E. focardii an ideal model for genome-level analysis to understand the evolutionary mechanisms that determine the adaptation of organisms to cold environments. Here we describe two examples that are controlled by phylogenetically appropriate comparison with mesophilic and psychrotolerant Euplotes species: 1) the genes and encoded proteins of the E. focardii tubulin superfamily, including α-, β-, and γ-tubulins; and 2) the genes of the heat-shock protein (Hsp) 70 family. The tubulins provide particular insight into protein-level structural changes that are likely to facilitate microtubule nucleation and polymerization in an energy poor environment. By contrast, the hsp70 genes of E. focardii and of its psychrotolerant relative E. nobilii reveal adaptive alterations in the regulation of gene expression in the cold. The unique characteristics of the E. focardii genome and the results that we present here argue strongly for a concerted effort to characterize the relatively low complexity macronuclear genome of this psychrophilic organism.

Regulation of primary cilia formation by ceramide

The primary cilium is an important sensory organelle, the regulation of which is not fully understood. We found that in polarized Madin-Darby Canine Kidney cells, the sphingolipid ceramide is specifically distributed to a cis-Golgi compartment at the base of the primary cilium. This compartment immunostained for the centrosome marker gamma-tubulin, the Rho type GTPase cell division cycle 42 (Cdc42), and atypical protein kinase Czeta/lambda (aPKC), a kinase activated by ceramide and associated with a polarity protein complex consisting of partitioning defective (Par)6 and Cdc42. Inhibition of ceramide biosynthesis with Fumonisin B1 prevented codistribution of aPKC and Cdc42 in the centrosomal/pericentriolar compartment and severely impaired ciliogenesis. Cilium formation and codistribution of aPKC and Cdc42 were restored by incubation with N-acetyl or N-palmitoyl sphingosine (C2 or C16 ceramide), or the ceramide analog N-oleoyl serinol (S18). Cilium formation was also restored by the glycogen synthase kinase-3beta (GSK-3beta) inhibitor indirubin-3-monoxime, suggesting that regulation of ciliogenesis depends on the inhibition of GSK-3beta by ceramide-activated aPKC. Consistently, inhibition of aPKC with a pseudosubstrate inhibitor prevented restoration of ciliogenesis by C2 ceramide or S18. Our data show for the first time that ceramide is required for primary cilium formation.

Stability of the small gamma-tubulin complex requires HCA66, a protein of the centrosome and the nucleolus

To investigate changes at the centrosome during the cell cycle, we analyzed the composition of the pericentriolar material from unsynchronized and S-phase-arrested cells by gel electrophoresis and mass spectrometry. We identified HCA66, a protein that localizes to the centrosome from S-phase to mitosis and to the nucleolus throughout interphase. Silencing of HCA66 expression resulted in failure of centrosome duplication and in the formation of monopolar spindles, reminiscent of the phenotype observed after gamma-tubulin silencing. Immunofluorescence microscopy showed that proteins of the gamma-tubulin ring complex were absent from the centrosome in these monopolar spindles. Immunoblotting revealed reduced protein levels of all components of the gamma-tubulin small complex (gamma-tubulin, GCP2, and GCP3) in HCA66-depleted cells. By contrast, the levels of gamma-tubulin ring complex proteins such as GCP4 and GCP-WD/NEDD1 were unaffected. We propose that HCA66 is a novel regulator of gamma-tubulin function that plays a role in stabilizing components of the gamma-tubulin small complex, which is in turn essential for assembling the larger gamma-tubulin ring complex.

Ab ovo or de novo? Mechanisms of centriole duplication

The centrosome, an organelle comprising centrioles and associated pericentriolar material, is the major microtubule organizing center in animal cells. For the cell to form a bipolar mitotic spindle and ensure proper chromosome segregation at the end of each cell cycle, it is paramount that the cell contains two and only two centrosomes. Because the number of centrosomes in the cell is determined by the number of centrioles, cells have evolved elaborate mechanisms to control centriole biogenesis and to tightly coordinate this process with DNA replication. Here we review key proteins involved in centriole assembly, compare two major modes of centriole biogenesis, and discuss the mechanisms that ensure stringency of centriole number.

The SCF Slimb ubiquitin ligase regulates Plk4/Sak levels to block centriole reduplication

Restricting centriole duplication to once per cell cycle is critical for chromosome segregation and genomic stability, but the mechanisms underlying this block to reduplication are unclear. Genetic analyses have suggested an involvement for Skp/Cullin/F box (SCF)-class ubiquitin ligases in this process. In this study, we describe a mechanism to prevent centriole reduplication in Drosophila melanogaster whereby the SCF E3 ubiquitin ligase in complex with the F-box protein Slimb mediates proteolytic degradation of the centrosomal regulatory kinase Plk4. We identified SCF(Slimb) as a regulator of centriole duplication via an RNA interference (RNAi) screen of Cullin-based ubiquitin ligases. We found that Plk4 binds to Slimb and is an SCF(Slimb) target. Both Slimb and Plk4 localize to centrioles, with Plk4 levels highest at mitosis and absent during S phase. Using a Plk4 Slimb-binding mutant and Slimb RNAi, we show that Slimb regulates Plk4 localization to centrioles during interphase, thus regulating centriole number and ensuring the block to centriole reduplication.

Molecular architecture of the centriole proteome: the conserved WD40 domain protein POC1 is required for centriole duplication and length control

Centrioles are intriguing cylindrical organelles composed of triplet microtubules. Proteomic data suggest that a large number of proteins besides tubulin are necessary for the formation and maintenance of a centriole's complex structure. Expansion of the preexisting centriole proteome from the green alga Chlamydomonas reinhardtii revealed additional human disease genes, emphasizing the significance of centrioles in normal human tissue homeostasis. We found that two classes of ciliary disease genes were highly represented among the basal body proteome: cystic kidney disease (especially nephronophthisis) syndromes, including Meckel/Joubert-like and oral-facial-digital syndrome, caused by mutations in CEP290, MKS1, OFD1, and AHI1/Jouberin proteins and cone-rod dystrophy syndrome genes, including UNC-119/HRG4, NPHP4, and RPGR1. We further characterized proteome of the centriole (POC) 1, a highly abundant WD40 domain-containing centriole protein. We found that POC1 is recruited to nascent procentrioles and localizes in a highly asymmetrical pattern in mature centrioles corresponding to sites of basal-body fiber attachment. Knockdown of POC1 in human cells caused a reduction in centriole duplication, whereas overexpression caused the appearance of elongated centriole-like structures. Together, these data suggest that POC1 is involved in early steps of centriole duplication as well as in the later steps of centriole length control.

Electron tomography study of isolated human centrioles

Centrioles are components of the centrosome, which is present in most eukaryotic cells (from protozoa to mammals). They organize the microtubule skeleton during interphase and the mitotic spindle during cell division. In ciliate cells, centrioles form basal bodies that are involved in cellular motility. Despite their important roles in biology, the detailed structure of centrioles remains obscure. This work contributes to a more complete model of centriole structure. The authors used electron tomography of isolated centrosomes from the human lymphoblast KE37 to explore the details of subdistal appendages and centriole lumen organization in mother centrioles. Their results reveal that each of the nine subdistal appendages is composed of two halves (20 nm diameter each) fused in a 40 nm tip that extends 100 nm from where it anchors to microtubules. The centriole lumen is filled at the distal domain by a 45 nm periodic stack of rings. Each ring has a 30 nm diameter, is 15 nm thick, and appears to be tilted at 53 degrees perpendicular to the centriole axis. The rings are anchored to microtubules by arms. Based on their results, the authors propose a model of the mother centriole distal structure.

Centrosome function: sometimes less is more

Tight regulation of centrosome duplication is critical to ensure that centrosome number doubles once and only once per cell cycle. Superimposed onto this centrosome duplication cycle is a functional centrosome cycle in which they alternate between phases of quiescence and robust microtubule (MT) nucleation and MT-anchoring activities. In vertebrate cycling cells, interphase centrioles accumulate less pericentriolar material (PCM), reducing their MT nucleation capacity. In mitosis, centrosomes mature, accumulating more PCM to increase their nucleation and anchoring capacities to form robust MT asters. Interestingly, functional cycles of centrosomes can be altered to suit the cell's needs. Some interphase centrosomes function as a microtubule-organizing center by increasing their ability to anchor MTs to form centrosomal radial arrays. Other interphase centrosomes maintain their MT nucleation capacity but reduce/eliminate their MT-anchoring capacity. Recent work demonstrates that Drosophila cells take this to the extreme, whereby centrioles lose all detectable PCM during interphase, offering an explanation as to how centrosome-deficient flies develop to adulthood. Drosophila stem cells further modify the functional cycle by differentially regulating their two centrioles - a situation that seems important for stem cell asymmetric divisions, as misregulation of centrosome duplication in stem/progenitor cells can promote tumor formation. Here, we review recent findings that describe variations in the functional cycle of centrosomes.

The two SAS-6 homologs in Tetrahymena thermophila have distinct functions in basal body assembly

Cilia and flagella are structurally and functionally conserved organelles present in basal as well as higher eukaryotes. The assembly of cilia requires a microtubule based scaffold called a basal body. The ninefold symmetry characteristic of basal bodies and the structurally similar centriole is organized around a hub and spoke structure termed the cartwheel. To date, SAS-6 is one of the two clearly conserved components of the cartwheel. In some organisms, overexpression of SAS-6 causes the formation of supernumerary centrioles. We questioned whether the centriole assembly initiation capacity of SAS-6 is separate from or directly related to its structural role at the cartwheel. To address this question we used Tetrahymena thermophila, which expresses two SAS-6 homologues, TtSAS6a and TtSAS6b. Cells lacking either TtSAS6a or TtSAS6b are defective in new basal body assembly. TtSas6a localizes to all basal bodies equally, whereas TtSas6b is enriched at unciliated and assembling basal bodies. Interestingly, overexpression of TtSAS6b but not TtSAS6a, led to the assembly of clusters of new basal bodies in abnormal locations. Our data suggest a model where TtSAS6a and TtSAS6b have diverged such that TtSAS6a acts as a structural component of basal bodies, whereas TtSAS6b influences the location of new basal body assembly.

The conserved protein SZY-20 opposes the Plk4-related kinase ZYG-1 to limit centrosome size

Microtubules are organized by the centrosome, a dynamic organelle that exhibits changes in both size and number during the cell cycle. Here we show that SZY-20, a putative RNA-binding protein, plays a critical role in limiting centrosome size in C. elegans. SZY-20 localizes in part to centrosomes and in its absence centrosomes possess increased levels of centriolar and pericentriolar components including gamma-tubulin and the centriole duplication factors ZYG-1 and SPD-2. These enlarged centrosomes possess normal centrioles, nucleate more microtubules, and fail to properly direct a number of microtubule-dependent processes. Depletion of ZYG-1 restores normal centrosome size and function to szy-20 mutants, whereas loss of szy-20 suppresses the centrosome duplication defects in both zyg-1 and spd-2 mutants. Our results describe a pathway that determines centrosome size and implicate centriole duplication factors in this process.

Different roles of two gamma-tubulin isotypes in the cytoskeleton of the Antarctic ciliate Euplotes focardii: remodelling of interaction surfaces may enhance microtubule nucleation at low temperature

Gamma-tubulin belongs to the tubulin superfamily and plays an essential role in the nucleation of cellular microtubules. In the present study, we report the characterization of gamma-tubulin from the psychrophilic Antarctic ciliate Euplotes focardii. In this organism, gamma-tubulin is encoded by two genes, gamma-T1 and gamma-T2, that produce distinct isotypes. Comparison of the gamma-T1 and gamma-T2 primary sequences to a Euplotesgamma-tubulin consensus, derived from mesophilic (i.e. temperate) congeneric species, revealed the presence of numerous unique amino acid substitutions, particularly in gamma-T2. Structural models of gamma-T1 and gamma-T2, obtained using the 3D structure of human gamma-tubulin as a template, suggest that these substitutions are responsible for conformational and/or polarity differences located: (a) in the regions involved in longitudinal 'plus end' contacts; (b) in the T3 loop that participates in binding GTP; and (c) in the M loop that forms lateral interactions. Relative to gamma-T1, the gamma-T2 gene is amplified by approximately 18-fold in the macronuclear genome and is very strongly transcribed. Using confocal immunofluorescence microscopy, we found that the gamma-tubulins of E. focardii associate throughout the cell cycle with basal bodies of the non-motile dorsal cilia and of all of the cirri of the ventral surface (i.e. adoral membranelles, paraoral membrane, and frontoventral transverse, caudal and marginal cirri). By contrast, only gamma-T2 interacts with the centrosomes of the spindle during micronuclear mitosis. We also established that the gamma-T1 isotype associates only with basal bodies. Our results suggest that gamma-T1 and gamma-T2 perform different functions in the organization of the microtubule cytoskeleton of this protist and are consistent with the hypothesis that gamma-T1 and gamma-T2 have evolved sequence-based structural alterations that facilitate template nucleation of microtubules by the gamma-tubulin ring complex at cold temperatures.

Basal body components exhibit differential protein dynamics during nascent basal body assembly

Basal bodies organize cilia that are responsible for both mechanical beating and sensation. Nascent basal body assembly follows a series of well characterized morphological events; however, the proteins and their assembly dynamics for new basal body formation and function are not well understood. High-resolution light and electron microscopy studies were performed in Tetrahymena thermophila to determine how proteins assemble into the structure. We identify unique dynamics at basal bodies for each of the four proteins analyzed (alpha-tubulin, Spag6, centrin, and Sas6a). alpha-Tubulin incorporates only during new basal body assembly, Spag6 continuously exchanges at basal bodies, and centrin and Sas6a exhibit both of these patterns. Centrin loads and exchanges at the basal body distal end and stably incorporates during new basal body assembly at the nascent site of assembly and the microtubule cylinder. Conversely, both dynamic and stable populations of Sas6a are found only at a single site, the cartwheel. The bimodal dynamics found for centrin and Sas6a reveal unique protein assembly mechanisms at basal bodies that may reflect novel functions for these important basal body and centriolar proteins.

The UNI1 and UNI2 genes function in the transition of triplet to doublet microtubules between the centriole and cilium in Chlamydomonas

One fundamental role of the centriole in eukaryotic cells is to nucleate the growth of cilia. The unicellular alga Chlamydomonas reinhardtii provides a simple genetic system to study the role of the centriole in ciliogenesis. Wild-type cells are biflagellate, but "uni" mutations result in failure of some centrioles (basal bodies) to assemble cilia (flagella). Serial transverse sections through basal bodies in uni1 and uni2 single and double mutant cells revealed a previously undescribed defect in the transition of triplet microtubules to doublet microtubules, a defect correlated with failure to assemble flagella. Phosphorylation of the Uni2 protein is reduced in uni1 mutant cells. Immunogold electron microscopy showed that the Uni2 protein localizes at the distal end of the basal body where microtubule transition occurs. These results provide the first mechanistic insights into the function of UNI1 and UNI2 genes in the pathway mediating assembly of doublet microtubules in the axoneme from triplet microtubules in the basal body template.

A genome-wide RNAi screen to dissect centriole duplication and centrosome maturation in Drosophila

Centrosomes comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Here, we have performed a microscopy-based genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins required for centriole duplication and mitotic PCM recruitment. We analysed 92% of the Drosophila genome (13,059 genes) and identified 32 genes involved in centrosome function. An extensive series of secondary screens classified these genes into four categories: (1) nine are required for centriole duplication, (2) 11 are required for centrosome maturation, (3) nine are required for both functions, and (4) three genes regulate centrosome separation. These 32 hits include several new centrosomal components, some of which have human homologs. In addition, we find that the individual depletion of only two proteins, Polo and Centrosomin (Cnn) can completely block centrosome maturation. Cnn is phosphorylated during mitosis in a Polo-dependent manner, suggesting that the Polo-dependent phosphorylation of Cnn initiates centrosome maturation in flies.

A major goal of the cell cycle is to accurately separate the duplicated chromosomes between two daughter cells. To achieve this, a pair of centrosomes organise a bipolar spindle made of microtubules; the chromosomes line up on the spindle and are then separated to the two spindle poles. Centrosomes are also required for the formation of cilia and flagella, which are present in many eukaryotic cells; centrosome dysfunction is a common feature of many human cancers and several neurological disorders, whereas mutations in genes that affect cilia function give rise to several human diseases. Here, we perform a genome-wide screen using RNA interference to try to identify all of the proteins required for centrosome function in the model organism Drosophila melanogaster (a fruitfly). We identified all 16 of the centrosomal proteins that were already known to be required for centrosome function in Drosophila, as well as 16 new centrosomal components or regulators. We confirmed the centrosomal location of several of the components and performed some analysis of their functions. We believe we are approaching a complete inventory of the proteins required for centrosome function in flies.

An RNAi screen identifies 16 new centrosomal components or regulators inDrosophila, and molecular dissection of their function addresses the role of Polo kinase in the maturation of pericentriolar material.

Centrioles: some self-assembly required

Centrioles play an important role in organizing microtubules and are precisely duplicated once per cell cycle. New (daughter) centrioles typically arise in association with existing (mother) centrioles (canonical assembly), suggesting that mother centrioles direct the formation of daughter centrioles. However, under certain circumstances, centrioles can also selfassemble free of an existing centriole (de novo assembly). Recent work indicates that the canonical and de novo pathways utilize a common mechanism and that a mother centriole spatially constrains the self-assembly process to occur within its immediate vicinity. Other recently identified mechanisms further regulate canonical assembly so that during each cell cycle, one and only one daughter centriole is assembled per mother centriole.

Drosophila asterless and vertebrate Cep152 Are orthologs essential for centriole duplication

The centriole is the core structure of centrosome and cilium. Failure to restrict centriole duplication to once per cell cycle has serious consequences and is commonly observed in cancer. Despite its medical importance, the mechanism of centriole formation is poorly understood. Asl was previously reported to be a centrosomal protein essential for centrosome function. Here we identify mecD, a severe loss-of-function allele of the asl gene, and demonstrate that it is required for centriole and cilia formation. Similarly, Cep152, the Asl ortholog in vertebrates, is essential for cilia formation and its function can be partially rescued by the Drosophila Asl. The study of Asl localization suggests that it is closely associated with the centriole wall, but is not part of the centriole structure. By analyzing the biogenesis of centrosomes in cells depleted of Asl, we found that, while pericentriolar material (PCM) function is mildly affected, Asl is essential for daughter centriole formation. The clear absence of several centriolar markers in mecD mutants suggests that Asl is critical early in centriole duplication.