RESUMO
The microtubule (MT) cytoskeleton provides the architecture that governs intracellular organization and the regulated motion of macromolecules through the crowded cytoplasm. The key to establishing a functioning cytoskeletal architecture is regulating when and where new MTs are nucleated. Within the spindle, the vast majority of MTs are generated through a pathway known as branching MT nucleation, which exponentially amplifies MT number in a polar manner. Whereas other MT nucleation pathways generally require a complex organelle such as the centrosome or Golgi apparatus to localize nucleation factors, the branching site is based solely on a simple, preformed MT, making it an ideal system to study MT nucleation. In this review, we address recent developments in characterizing branching factors, the branching reaction, and its regulation, as well as branching MT nucleation in systems beyond the spindle and within human disease.
Assuntos
Centro Organizador dos Microtúbulos , Fuso Acromático , Humanos , Proteínas Associadas aos Microtúbulos/metabolismo , Centro Organizador dos Microtúbulos/metabolismo , Microtúbulos/metabolismo , Fuso Acromático/metabolismo , Tubulina (Proteína)/metabolismoRESUMO
The γ-tubulin ring complex (γ-TuRC) is an essential regulator of centrosomal and acentrosomal microtubule formation, yet its structure is not known. Here, we present a cryo-EM reconstruction of the native human γ-TuRC at â¼3.8 Å resolution, revealing an asymmetric, cone-shaped structure. Pseudo-atomic models indicate that GCP4, GCP5, and GCP6 form distinct Y-shaped assemblies that structurally mimic GCP2/GCP3 subcomplexes distal to the γ-TuRC "seam." We also identify an unanticipated structural bridge that includes an actin-like protein and spans the γ-TuRC lumen. Despite its asymmetric architecture, the γ-TuRC arranges γ-tubulins into a helical geometry poised to nucleate microtubules. Diversity in the γ-TuRC subunits introduces large (>100,000 Å2) surfaces in the complex that allow for interactions with different regulatory factors. The observed compositional complexity of the γ-TuRC could self-regulate its assembly into a cone-shaped structure to control microtubule formation across diverse contexts, e.g., within biological condensates or alongside existing filaments.
Assuntos
Centro Organizador dos Microtúbulos/metabolismo , Centro Organizador dos Microtúbulos/ultraestrutura , Tubulina (Proteína)/ultraestrutura , Actinas/metabolismo , Microscopia Crioeletrônica/métodos , Células HeLa , Humanos , Proteínas Associadas aos Microtúbulos/metabolismo , Proteínas Associadas aos Microtúbulos/ultraestrutura , Microtúbulos/metabolismo , Tubulina (Proteína)/metabolismoRESUMO
Life depends on cell proliferation and the accurate segregation of chromosomes, which are mediated by the microtubule (MT)-based mitotic spindle and â¼200 essential MT-associated proteins. Yet, a mechanistic understanding of how the mitotic spindle is assembled and achieves chromosome segregation is still missing. This is mostly due to the density of MTs in the spindle, which presumably precludes their direct observation. Recent insight has been gained into the molecular building plan of the metaphase spindle using bulk and single-molecule measurements combined with computational modeling. MT nucleation was uncovered as a key principle of spindle assembly, and mechanistic details about MT nucleation pathways and their coordination are starting to be revealed. Lastly, advances in studying spindle assembly can be applied to address the molecular mechanisms of how the spindle segregates chromosomes.
Assuntos
Centrossomo/metabolismo , Cinetocoros/metabolismo , Metáfase , Microtúbulos/metabolismo , Fuso Acromático/metabolismo , Animais , Centrossomo/ultraestrutura , Segregação de Cromossomos , Drosophila melanogaster/citologia , Drosophila melanogaster/metabolismo , Regulação da Expressão Gênica , Humanos , Cinesinas/genética , Cinesinas/metabolismo , Cinetocoros/ultraestrutura , Proteínas Associadas aos Microtúbulos/genética , Proteínas Associadas aos Microtúbulos/metabolismo , Microtúbulos/ultraestrutura , Transdução de Sinais , Fuso Acromático/ultraestrutura , Tubulina (Proteína)/genética , Tubulina (Proteína)/metabolismo , Proteínas de Xenopus/genética , Proteínas de Xenopus/metabolismo , Xenopus laevis/genética , Xenopus laevis/metabolismo , Zigoto/citologia , Zigoto/metabolismoRESUMO
The cell orchestrates the dance of chromosome segregation with remarkable speed and fidelity. The mitotic spindle is built from scratch after interphase through microtubule (MT) nucleation, which is dependent on the γ-tubulin ring complex (γ-TuRC), the universal MT template. Although several MT nucleation pathways build the spindle framework, the question of when and how γ-TuRC is targeted to these nucleation sites in the spindle and subsequently activated remains an active area of investigation. Recent advances facilitated the discovery of new MT nucleation effectors and their mechanisms of action. In this review, we illuminate each spindle assembly pathway and subsequently consider how the pathways are merged to build a spindle.
Assuntos
Proteínas Associadas aos Microtúbulos , Tubulina (Proteína) , Proteínas Associadas aos Microtúbulos/genética , Proteínas Associadas aos Microtúbulos/metabolismo , Tubulina (Proteína)/genética , Tubulina (Proteína)/metabolismo , Microtúbulos/genética , Microtúbulos/metabolismo , Fuso Acromático/genética , Fuso Acromático/metabolismo , Centro Organizador dos Microtúbulos/metabolismoRESUMO
The centrosome, a non-membranous organelle, constrains various soluble molecules locally to execute its functions. As the centrosome is surrounded by various dense components, we hypothesized that it may be bordered by a putative diffusion barrier. After quantitatively measuring the trapping kinetics of soluble proteins of varying size at centrosomes by a chemically inducible diffusion trapping assay, we find that centrosomes are highly accessible to soluble molecules with a Stokes radius of less than 5.8 nm, whereas larger molecules rarely reach centrosomes, indicating the existence of a size-dependent diffusion barrier at centrosomes. The permeability of this barrier is tightly regulated by branched actin filaments outside of centrosomes and it decreases during anaphase when branched actin temporally increases. The actin-based diffusion barrier gates microtubule nucleation by interfering with γ-tubulin ring complex recruitment. We propose that actin filaments spatiotemporally constrain protein complexes at centrosomes in a size-dependent manner.
Assuntos
Microtúbulos , Tubulina (Proteína) , Tubulina (Proteína)/metabolismo , Microtúbulos/metabolismo , Actinas/metabolismo , Centrossomo/metabolismo , Citoesqueleto de Actina/metabolismoRESUMO
Cytoskeletal microtubules (MTs) are nucleated from γ-tubulin ring complexes (γTuRCs) located at MT organizing centers (MTOCs), such as the centrosome. However, the exact regulatory mechanism of γTuRC assembly is not fully understood. Here, we showed that the nonreceptor tyrosine kinase c-Abl was associated with and phosphorylated γ-tubulin, the essential component of the γTuRC, mainly on the Y443 residue by in vivo (immunofluorescence and immunoprecipitation) or in vitro (surface plasmon resonance) detection. We further demonstrated that phosphorylation deficiency significantly impaired γTuRC assembly, centrosome construction, and MT nucleation. c-Abl/Arg deletion and γ-tubulin Y443F mutation resulted in an abnormal morphology and compromised spindle function during mitosis, eventually causing uneven chromosome segregation. Our findings reveal that γTuRC assembly and nucleation function are regulated by Abl kinase-mediated γ-tubulin phosphorylation, revealing a fundamental mechanism that contributes to the maintenance of MT function.
Assuntos
Centro Organizador dos Microtúbulos , Microtúbulos , Proteínas Proto-Oncogênicas c-abl , Tubulina (Proteína) , Centrossomo/metabolismo , Centro Organizador dos Microtúbulos/metabolismo , Microtúbulos/metabolismo , Fosforilação , Proteínas Proto-Oncogênicas c-abl/genética , Proteínas Proto-Oncogênicas c-abl/metabolismo , Tubulina (Proteína)/genética , Tubulina (Proteína)/metabolismoRESUMO
γ-Tubulin is the main protein involved in the nucleation of microtubules in all eukaryotes. It forms two different complexes with proteins of the GCP family (γ-tubulin complex proteins): γ-tubulin small complexes (γTuSCs) that contain γ-tubulin, and GCPs 2 and 3; and γ-tubulin ring complexes (γTuRCs) that contain multiple γTuSCs in addition to GCPs 4, 5 and 6. Whereas the structure and assembly properties of γTuSCs have been intensively studied, little is known about the assembly of γTuRCs and the specific roles of GCPs 4, 5 and 6. Here, we demonstrate that two copies of GCP4 and one copy each of GCP5 and GCP6 form a salt (KCl)-resistant sub-complex within the γTuRC that assembles independently of the presence of γTuSCs. Incubation of this sub-complex with cytoplasmic extracts containing γTuSCs leads to the reconstitution of γTuRCs that are competent to nucleate microtubules. In addition, we investigate sequence extensions and insertions that are specifically found at the N-terminus of GCP6, and between the GCP6 grip1 and grip2 motifs. We also demonstrate that these are involved in the assembly or stabilization of the γTuRC.
Assuntos
Proteínas Associadas aos Microtúbulos , Tubulina (Proteína) , Centrossomo , Proteínas Associadas aos Microtúbulos/genética , Centro Organizador dos Microtúbulos , Microtúbulos , Tubulina (Proteína)/genéticaRESUMO
Overexpression of γ-tubulin leads to the formation of filaments, but nothing is known about such filaments with regard to possible presence in cells, structure and probable dynamics. Here, we used mammalian cell lines to investigate the ability of γ-tubulin to form filaments. We found that γ-tubulin produces fibers called γ-tubules in a GTP-dependent manner and that γ-tubules are made up of pericentrin and the γ-tubulin complex proteins 2, 3, 5 and 6. Furthermore, we noted that the number of cells with cytosolic γ-tubules is increased in non-dividing cells. Our experiments showed that γ-tubules are polar structures that have a low regrowth rate compared to microtubules. Also, we observed that γ-tubules were disassembled by treatment with cold, colcemid, citral dimethyl acetal, dimethyl fumarate or mutation of γ-tubulin GTPase domain, but were increased in number by treatment with taxol or by stable expression of the γ-tubulin1-333 GTPase domain. Our results demonstrate that γ-tubulin forms filaments, and such assembly is facilitated by the GTPase domain of γ-tubulin.
Assuntos
Citoesqueleto/metabolismo , Células Eucarióticas/metabolismo , Tubulina (Proteína)/química , Tubulina (Proteína)/metabolismo , Animais , Células COS , Células Cultivadas , Chlorocebus aethiops , Citoesqueleto/ultraestrutura , Células Eucarióticas/ultraestrutura , Humanos , Mamíferos , Camundongos , Microtúbulos/metabolismo , Microtúbulos/ultraestrutura , Células NIH 3T3 , Multimerização ProteicaRESUMO
Microtubules are nucleated from multiprotein complexes containing γ-tubulin and associated γ-tubulin complex proteins (GCPs). Small complexes (γTuSCs) comprise two molecules of γ-tubulin bound to the C-terminal domains of GCP2 and GCP3. γTuSCs associate laterally into helical structures, providing a structural template for microtubule nucleation. In most eukaryotes γTuSCs associate with additional GCPs (4, 5, and 6) to form the core of the so-called γ-tubulin ring complex (γTuRC). GCPs 2-6 constitute a family of homologous proteins. Previous structural analysis and modeling of GCPs suggest that all family members can potentially integrate into the helical structure. Here we provide experimental evidence for this model. Using chimeric proteins in which the N- and C-terminal domains of different GCPs are swapped, we show that the N-terminal domains define the functional identity of GCPs, whereas the C-terminal domains are exchangeable. FLIM-FRET experiments indicate that GCP4 and GCP5 associate laterally within the complex, and their interaction is mediated by their N-terminal domains as previously shown for γTuSCs. Our results suggest that all GCPs are incorporated into the helix via lateral interactions between their N-terminal domains, whereas the C-terminal domains mediate longitudinal interactions with γ-tubulin. Moreover, we show that binding to γ-tubulin is not essential for integrating into the helical complex.
Assuntos
Proteínas Associadas aos Microtúbulos/química , Proteínas Associadas aos Microtúbulos/metabolismo , Tubulina (Proteína)/química , Tubulina (Proteína)/metabolismo , Cristalografia por Raios X , Humanos , Proteínas Associadas aos Microtúbulos/genética , Complexos Multiproteicos/química , Complexos Multiproteicos/genética , Complexos Multiproteicos/metabolismo , Ligação Proteica , Domínios Proteicos , Tubulina (Proteína)/genéticaRESUMO
We investigated the cause of situs inversus totalis (SIT) in two siblings from a consanguineous family. Genotyping and whole-exome analysis revealed a homozygous change in NME7, resulting in deletion of an exon causing an in-frame deletion of 34 amino acids located in the second NDK domain of the protein and segregated with the defective lateralization in the family. NME7 is an important developmental gene, and NME7 protein is a component of the γ-tubulin ring complex. This mutation is predicted to affect the interaction of NME7 protein with this complex as it deletes the amino acids crucial for the binding. SIT associated with homozygous deletion in our patients is in line with Nme7(-/-) mutant mice phenotypes consisting of congenital hydrocephalus and SIT, indicating a novel human laterality patterning role for NME7. Further cases are required to elaborate the full human phenotype associated with NME7 mutations.
Assuntos
Núcleosídeo-Difosfato Quinase/genética , Deleção de Sequência , Situs Inversus/genética , Sequência de Aminoácidos , Feminino , Humanos , Masculino , Proteínas Associadas aos Microtúbulos/metabolismo , Modelos Moleculares , Núcleosídeo-Difosfato Quinase/química , Núcleosídeo-Difosfato Quinase/metabolismo , Linhagem , Domínios ProteicosRESUMO
Golgi-derived microtubules constitute an asymmetrical microtubule network that drives polarized transport of vesicles to support cell polarization and directional migration. Golgi-based microtubule nucleation requires the γ-tubulin ring complex (γTuRC), the principal microtubule nucleator in animal cells. In this chapter, we present methods for detecting γTuRC components and associated proteins on the Golgi, examining Golgi-based microtubule nucleation, and measuring the microtubule-nucleating activity of isolated γTuRCs. These approaches have been demonstrated to be effective for assessing the microtubule-organizing function of the Golgi complex.
Assuntos
Microtúbulos , Tubulina (Proteína) , Animais , Tubulina (Proteína)/metabolismo , Microtúbulos/metabolismo , Centro Organizador dos Microtúbulos/metabolismo , Complexo de Golgi/metabolismo , Proteínas Associadas aos Microtúbulos/metabolismo , Centrossomo/metabolismoRESUMO
The primary cilium, a microtubule-based sensory organelle, undergoes cycles of assembly and disassembly that govern the cell cycle progression critical to cell proliferation and differentiation. Although cilia assembly has been studied extensively, the molecular mechanisms underlying cilia disassembly are less well understood. Here, we uncover a γ-tubulin ring complex (γ-TuRC)-dependent pathway that promotes cilia disassembly and thereby prevents cilia formation. We further demonstrate that Kif2A, a kinesin motor that bears microtubule-depolymerizing activity, is recruited to the cilium basal body in a γ-TuRC-dependent manner. Our mechanistic analyses show that γ-TuRC specifically recruits Kif2A via the GCP2 subunit and its binding partner Mzt2. Hence, despite the long-standing view that γ-TuRC acts mainly as a microtubule template, we illustrate that its functional heterogeneity at the basal body facilitates both microtubule nucleation and Kif2A recruitment-mediated regulation of ciliogenesis, ensuring cell cycle progression.
Assuntos
Proteínas Associadas aos Microtúbulos , Tubulina (Proteína) , Tubulina (Proteína)/metabolismo , Proteínas Associadas aos Microtúbulos/metabolismo , Cílios/metabolismo , Centro Organizador dos Microtúbulos/metabolismo , Microtúbulos/metabolismoRESUMO
The centrosome lacks microtubule (MT)-nucleation activity in differentiated neurons. We have previously demonstrated that MTs were nucleated at the cytoplasm of mouse neurons. They are supposed to serve seeds for MTs required for dendrite growth. However, the factors that activate the cytoplasmic γ-tubulin ring complex (γTuRC) are unknown. Here we report an alternative splicing isoform of cyclin-dependent kinase 5 regulatory subunit-associated protein 2 (CKD5RAP2) as a candidate for the cytoplasmic γTuRC activator. This isoform lacked exon 17 and was expressed predominantly in the brain and testis. The expression was transient during the development of cortical neurons, which period coincided with the period we reported cytoplasmic MT nucleation. This isoform resulted in a frameshift and generated truncated protein without a centrosomal localization signal. When this isoform was expressed in cells, it localized diffusely in the cytoplasm. It was co-immunoprecipitated with γ-tubulin and MOZART2, suggesting that it can activate cytosolic γTuRCs. After cold-nocodazole depolymerization of MTs and subsequent washout, we observed numerous short MTs in the cytoplasm of cells transfected with the cDNA of this isoform. The isoform-overexpressing cells exhibited an increased amount of MTs and a decreased ratio of acetylated tubulin, suggesting that MT generation and turnover were enhanced by the isoform. Our data suggest the possibility that alternative splicing of CDK5RAP2 induces cytoplasmic nucleation of MTs in developing neurons.
RESUMO
Microtubule nucleation is mainly mediated by the γ-tubulin ring complex (γTuRC), whose core components are γ-tubulin and γ-tubulin complex proteins GCP2-6. A substantial fraction of γ-tubulin also exists with GCP2 and GCP3 in a tetramer called the γ-tubulin small complex (γTuSC). To date, the mechanisms underlying the turnover of γ-tubulin and GCPs have remained unclear. Here, we show that γ-tubulin, GCP2, and GCP3 are proteolyzed by the ubiquitin-proteasome system, and we identify cullin 1, cullin 4A, and cullin 4B as the E3 ligases that mediate the ubiquitination and, consequently, the degradation of γ-tubulin. Notably, we found that γTuSC disassembly promotes the degradation of γ-tubulin, GCP2, and GCP3, which indicates a role for γTuSCs in the stabilization of its components.
Assuntos
Complexo de Endopeptidases do Proteassoma/metabolismo , Tubulina (Proteína)/metabolismo , Ubiquitina/metabolismo , Linhagem Celular Tumoral , Humanos , Proteínas Associadas aos Microtúbulos/metabolismo , Estabilidade Proteica , Proteólise , UbiquitinaçãoRESUMO
Microtubules are essential cytoskeletal elements assembled from αß-tubulin dimers. In high eukaryotes, microtubule nucleation, the de novo assembly of a microtubule from its minus end, is initiated by the γ-tubulin ring complex (γ-TuRC). Despite many years of research, the structural and mechanistic principles of the microtubule nucleation machinery remained poorly understood. Only recently, cryoelectron microscopy studies uncovered the molecular organization and potential activation mechanisms of γ-TuRC. In vitro assays further deciphered the spatial and temporal cooperation between γ-TuRC and additional factors, for example, the augmin complex, the phase separation protein TPX2, and the microtubule polymerase XMAP215. These breakthroughs deepen our understanding of microtubule nucleation mechanisms and will link the assembly of individual microtubules to the organization of cellular microtubule networks.
Assuntos
Centro Organizador dos Microtúbulos/química , Microtúbulos/química , Tubulina (Proteína)/química , Animais , Microscopia Crioeletrônica , Humanos , Proteínas Associadas aos Microtúbulos/química , Proteínas Associadas aos Microtúbulos/metabolismo , Proteínas Associadas aos Microtúbulos/ultraestrutura , Centro Organizador dos Microtúbulos/metabolismo , Centro Organizador dos Microtúbulos/ultraestrutura , Microtúbulos/metabolismo , Microtúbulos/ultraestrutura , Polimerização , Tubulina (Proteína)/metabolismo , Tubulina (Proteína)/ultraestruturaRESUMO
Neurons are highly polarized cells with morphologically and functionally distinct dendritic and axonal processes. The molecular mechanisms that establish axon-dendrite polarity in vivo are poorly understood. Here, we describe the initial polarization of posterior deirid (PDE), a ciliated mechanosensory neuron, during development in vivo through 4D live imaging with endogenously tagged proteins. PDE inherits and maintains apicobasal polarity from its epithelial precursor. Its apical domain is directly transformed into the ciliated dendritic tip through apical constriction, which is followed by axonal outgrowth from the opposite basal side of the cell. The apical Par complex and junctional proteins persistently localize at the developing dendritic domain throughout this transition. Consistent with their instructive role in axon-dendrite polarization, conditional depletion of the Par complex and junctional proteins results in robust defects in dendrite and axon formation. During apical constriction, a microtubule-organizing center (MTOC) containing the microtubule nucleator γ-tubulin ring complex (γ-TuRC) forms along the apical junction between PDE and its sister cell in a manner dependent on the Par complex and junctional proteins. This junctional MTOC patterns neuronal microtubule polarity and facilitate the dynein-dependent recruitment of the basal body for ciliogenesis. When non-ciliated neurons are genetically manipulated to obtain ciliated neuronal fate, inherited apicobasal polarity is required for generating ciliated dendritic tips. We propose that inherited apicobasal polarity, together with apical cell-cell interactions drive the morphological and cytoskeletal polarity in early neuronal differentiation.
Assuntos
Axônios , Centro Organizador dos Microtúbulos , Polaridade Celular/fisiologia , Dendritos/metabolismo , Centro Organizador dos Microtúbulos/metabolismo , Microtúbulos/metabolismo , Células Receptoras SensoriaisRESUMO
The γ-tubulin ring complex (γTuRC) is the major microtubule nucleator in cells. The mechanism of its regulation is not understood. We purified human γTuRC and measured its nucleation properties in a total internal reflection fluorescence (TIRF) microscopy-based real-time nucleation assay. We find that γTuRC stably caps the minus ends of microtubules that it nucleates stochastically. Nucleation is inefficient compared with microtubule elongation. The 4 Šresolution cryoelectron microscopy (cryo-EM) structure of γTuRC, combined with crosslinking mass spectrometry analysis, reveals an asymmetric conformation with only part of the complex in a "closed" conformation matching the microtubule geometry. Actin in the core of the complex, and MZT2 at the outer perimeter of the closed part of γTuRC appear to stabilize the closed conformation. The opposite side of γTuRC is in an "open," nucleation-incompetent conformation, leading to a structural asymmetry explaining the low nucleation efficiency of purified human γTuRC. Our data suggest possible regulatory mechanisms for microtubule nucleation by γTuRC closure.
Assuntos
Proteínas Associadas aos Microtúbulos/metabolismo , Microtúbulos/ultraestrutura , Tubulina (Proteína)/química , Actinas/química , Actinas/metabolismo , Microscopia Crioeletrônica , Células HeLa , Humanos , Proteínas Associadas aos Microtúbulos/química , Microtúbulos/química , Microtúbulos/metabolismo , Simulação de Dinâmica Molecular , Conformação Proteica , Imagem Individual de Molécula , Tubulina (Proteína)/metabolismoRESUMO
Microtubule organization depends on the γ-tubulin ring complex (γ-TuRC), a â¼2.3-MDa nucleation factor comprising an asymmetric assembly of γ-tubulin and GCP2-GCP6. However, it is currently unclear how the γ-TuRC-associated microproteins MZT1 and MZT2 contribute to the structure and regulation of the holocomplex. Here, we report cryo-EM structures of MZT1 and MZT2 in the context of the native human γ-TuRC. MZT1 forms two subcomplexes with the N-terminal α-helical domains of GCP3 or GCP6 (GCP-NHDs) within the γ-TuRC "lumenal bridge." We determine the X-ray structure of recombinant MZT1/GCP6-NHD and find it is similar to that within the native γ-TuRC. We identify two additional MZT/GCP-NHD-like subcomplexes, one of which is located on the outer face of the γ-TuRC and comprises MZT2 and GCP2-NHD in complex with a centrosomin motif 1 (CM1)-containing peptide. Our data reveal how MZT1 and MZT2 establish multi-faceted, structurally mimetic "modules" that can expand structural and regulatory interfaces in the γ-TuRC.
Assuntos
Proteínas Associadas aos Microtúbulos/metabolismo , Complexos Multiproteicos/metabolismo , Tubulina (Proteína)/metabolismo , Células HeLa , Humanos , Proteínas Associadas aos Microtúbulos/química , Modelos Moleculares , Complexos Multiproteicos/ultraestrutura , Peptídeos/metabolismo , Ligação Proteica , Multimerização Proteica , Estrutura Secundária de Proteína , Tubulina (Proteína)/química , Tubulina (Proteína)/ultraestruturaRESUMO
The cytoplasmic intermediate filament cytoskeleton provides a tissue-specific three-dimensional scaffolding with unique context-dependent organizational features. This is particularly apparent in the intestinal epithelium, in which the intermediate filament network is localized below the apical terminal web region and is anchored to the apical junction complex. This arrangement is conserved from the nematode Caenorhabditis elegans to humans. The review summarizes compositional, morphological and functional features of the polarized intermediate filament cytoskeleton in intestinal cells of nematodes and mammals. We emphasize the cross talk of intermediate filaments with the actin- and tubulin-based cytoskeleton. Possible links of the intermediate filament system to the distribution of apical membrane proteins and the cell polarity complex are highlighted. Finally, we discuss how these properties relate to the establishment and maintenance of polarity in the intestine.
RESUMO
Microtubule organization by microtubule-organizing centers such as the centrosome requires γ-tubulin, which exists in the γ-tubulin ring complex (γTuRC) that nucleates microtubules. The γTuRC is a ring-shaped, macromolecular complex whose core components are γ-tubulin and the γ-tubulin complex proteins. Despite the recent identification of additional γTuRC components, the molecular composition and regulatory properties of the complex remain poorly understood. The ability to purify the γTuRC at a large scale for characterization may hold a key to understanding the mechanism by which the γTuRC nucleates microtubules. In this chapter, we describe methods to isolate the γTuRC from human cell cultures and to perform assays on the purified γTuRC.