The spindle pole body of Saccharomyces cerevisiae: architecture and assembly of the core components

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

The role of centrosomal Nlp in the control of mitotic progression and tumourigenesis

The human centrosomal ninein-like protein (Nlp) is a new member of the γ-tubulin complexes binding proteins (GTBPs) that is essential for proper execution of various mitotic events. The primary function of Nlp is to promote microtubule nucleation that contributes to centrosome maturation, spindle formation and chromosome segregation. Its subcellular localisation and protein stability are regulated by several crucial mitotic kinases, such as Plk1, Nek2, Cdc2 and Aurora B. Several lines of evidence have linked Nlp to human cancer. Deregulation of Nlp in cell models results in aberrant spindle, chromosomal missegregation and multinulei, and induces chromosomal instability and renders cells tumourigenic. Overexpression of Nlp induces anchorage-independent growth and immortalised primary cell transformation. In addition, we first demonstrate that the expression of Nlp is elevated primarily due to NLP gene amplification in human breast cancer and lung carcinoma. Consistently, transgenic mice overexpressing Nlp display spontaneous tumours in breast, ovary and testicle, and show rapid onset of radiation-induced lymphoma, indicating that Nlp is involved in tumourigenesis. This review summarises our current knowledge of physiological roles of Nlp, with an emphasis on its potentials in tumourigenesis.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

An SMC-Domain Protein in Fission Yeast Links Telomeres to the Meiotic Centrosome

Abnormal centrosomal structures similar to those occurring in human cancers are induced in fission yeast by overexpression of the pericentrin homolog Pcp1p. Analysis of abnormal Pcp1p-containing structures with quantitative mass spectrometry and isotope-coded affinity tags identified a coiled-coil, structural maintenance of chromosomes (SMC) domain protein. This protein, termed Ccq1p (coiled-coil protein quantitatively enriched), localizes with Taz1p to telomeres in normal vegetative cells. Fluorescence resonance energy transfer (FRET) measurements indicate that Ccq1p also interacts with centrosomal Pcp1p in mating pheromone-stimulated cells containing centrosomally clustered telomeres. We provide evidence that the Ccq1p-Pcp1p interaction, while essential for meiosis, is deleterious when forced to occur during vegetative growth. Cells lacking one ccq1 allele exhibit a loss-of-function phenotype including abnormally long cell length, chromosome segregation failure, telomeric shortening, and defective telomeric clustering during meiotic prophase. Our data indicate a mechanism underlying meiotic chromosomal bouquet formation and suggest a recruitment model for supernumerary centrosome toxicity.

The spindle pole body assembly component mps3p/nep98p functions in sister chromatid cohesion

For successful chromosome segregation during mitosis, several processes must occur early in the cell cycle, including spindle pole duplication, DNA replication, and the establishment of cohesion between nascent sister chromatids. Spindle pole body duplication begins in G1 and continues during early S-phase as spindle pole bodies mature and start to separate. Key steps in spindle pole body duplication are the sequential recruitment of Cdc31p and Spc42p by the nuclear envelope transmembrane protein Msp3p/Nep98p (herein termed Mps3p). Concurrent with DNA replication, Ctf7p/Eco1p (herein termed Ctf7p) ensures that nascent sister chromatids are paired together, identifying the products of replication as sister chromatids. Here, we provide the first evidence that the nuclear envelope spindle pole body assembly component Mps3p performs a function critical to sister chromatid cohesion. Mps3p was identified as interacting with Ctf7p from a genome-wide two-hybrid screen, and the physical interaction was confirmed by both in vivo (co-immunoprecipitation) and in vitro (GST pull-down) assays. An in vivo cohesion assay on new mps3/nep98 alleles revealed that loss of Mps3p results in precocious sister chromatid separation and that Mps3p functions after G1, coincident with Ctf7p. Mps3p is not required for cohesion during mitosis, revealing that Mps3p functions in cohesion establishment and not maintenance. Mutated Mps3p that results in cohesion defects no longer binds to Ctf7p in vitro, demonstrating that the interaction between Mps3p and Ctf7p is physiologically relevant. In support of this model, mps3 ctf7 double mutant cells exhibit conditional synthetic lethality. These findings document a new role for Mps3p in sister chromatid cohesion and provide novel insights into the mechanism by which a spindle pole body component, when mutated, contributes to aneuploidy.

MAPping the Eukaryotic Tree of Life: Structure, Function, and Evolution of the MAP215⧸Dis1 Family of Microtubule-Associated Proteins

The MAP215/Dis1 family of proteins is an evolutionarily ancient family of microtubule-associated proteins, with characterized members in all major kingdoms of eukaryotes, including fungi (Stu2 in S. cerevisiae, Dis1 and Alp14 in S. pombe), Dictyostelium (DdCP224), plants (Mor1 in A. thaliana and TMBP200 in N. tabaccum), and animals (Zyg9 in C. elegans, Msps in Drosophila, XMAP215 in Xenopus, and ch-TOG in humans). All MAP215/Dis1 proteins (with the exception of those in plants) localize to microtubule-organizing centers (MTOCs), including spindle pole bodies in yeast and centrosomes in animals, and all bind to microtubules in vitro and?or in vivo. Diverse roles in regulating microtubule assembly and organization have been proposed for individual family members, and a substantial body of evidence suggests that MAP215/Dis1-related proteins play critical roles in the assembly and function of the meiotic/mitotic spindles and/or cell division. An extensive search of public databases (including both EST and genome databases) identified partial sequences predicted to encode more than three dozen new members of the MAP215/Dis1 family, including putative MAP215/Dis1-related proteins in Giardia lamblia and four other protists, sixteen additional species of fungi, six plants, and twelve animals. The structure and function of MAP215/Dis1 proteins are discussed in relation to the evolution of this ancient family of microtubule-associated proteins.

Polo-like kinase 1 regulates Nlp, a centrosome protein involved in microtubule nucleation

In animal cells, most microtubules are nucleated at centrosomes. At the onset of mitosis, centrosomes undergo a structural reorganization, termed maturation, which leads to increased microtubule nucleation activity. Centrosome maturation is regulated by several kinases, including Polo-like kinase 1 (Plk1). Here, we identify a centrosomal Plk1 substrate, termed Nlp (ninein-like protein), whose properties suggest an important role in microtubule organization. Nlp interacts with two components of the gamma-tubulin ring complex and stimulates microtubule nucleation. Plk1 phosphorylates Nlp and disrupts both its centrosome association and its gamma-tubulin interaction. Overexpression of an Nlp mutant lacking Plk1 phosphorylation sites severely disturbs mitotic spindle formation. We propose that Nlp plays an important role in microtubule organization during interphase, and that the activation of Plk1 at the onset of mitosis triggers the displacement of Nlp from the centrosome, allowing the establishment of a mitotic scaffold with enhanced microtubule nucleation activity.

Genetic analysis of calmodulin and its targets in Saccharomyces cerevisiae

Calmodulin, a small, ubiquitous Ca2+-binding protein, regulates a wide variety of proteins and processes in all eukaryotes. CMD1, the single gene encoding calmodulin in S. cerevisiae, is essential, and this review discusses studies that identified many of calmodulin's physiological targets and their functions in yeast cells. Calmodulin performs essential roles in mitosis, through its regulation of Nuf1p/Spc110p, a component of the spindle pole body, and in bud growth, by binding Myo2p, an unconventional class V myosin required for polarized secretion. Surprisingly, mutant calmodulins that fail to bind Ca2+ can perform these essential functions. Calmodulin is also required for endocytosis in yeast and participates in Ca2+-dependent, stress-activated signaling pathways through its regulation of a protein phosphatase, calcineurin, and the protein kinases, Cmk1p and Cmk2p. Thus, calmodulin performs important physiological functions in yeast cells in both its Ca2+-bound and Ca2+-free form.

Characterization of Cep135, a novel coiled-coil centrosomal protein involved in microtubule organization in mammalian cells

By using monoclonal antibodies raised against isolated clam centrosomes, we have identified a novel 135-kD centrosomal protein (Cep135), present in a wide range of organisms. Cep135 is located at the centrosome throughout the cell cycle, and localization is independent of the microtubule network. It distributes throughout the centrosomal area in association with the electron-dense material surrounding centrioles. Sequence analysis of cDNA isolated from CHO cells predicted a protein of 1,145-amino acid residues with extensive alpha-helical domains. Expression of a series of deletion constructs revealed the presence of three independent centrosome-targeting domains. Overexpression of Cep135 resulted in the accumulation of unique whorl-like particles in both the centrosome and the cytoplasm. Although their size, shape, and number varied according to the level of protein expression, these whorls were composed of parallel dense lines arranged in a 6-nm space. Altered levels of Cep135 by protein overexpression and/or suppression of endogenous Cep135 by RNA interference caused disorganization of interphase and mitotic spindle microtubules. Thus, Cep135 may play an important role in the centrosomal function of organizing microtubules in mammalian cells.

Dictyostelium centrin-related protein (DdCrp), the most divergent member of the centrin family, possesses only two EF hands and dissociates from the centrosome during mitosis

We have identified a Dictyostelium discoideum cDNA sequence with homology to centrins. The derived protein, Dictyostelium discoideum centrinn-related protein (DdCrp), is the most divergent member of the centrin family. Most strikingly it lacks the first two EF-hand consensus motifs, whereas a number of other centrin-specific sequence features are conserved. Southern and Northern blot analysis and the data presently available from the Dictyostelium genome and cDNA projects suggest that DdCrp is the only centrin isoform present in Dictyostelium. Immunofluorescence analysis with anti-DdCrp antibodies revealed that the protein is localized to the centrosome, to a second, centrosome-associated structure close to the nucleus and to the nucleus itself. Confocal microscopy resolved that the centrosomal label is confined to the corona surrounding the centrosome core. Unlike for other centrins the localization of DdCrp is cell cycle-dependent. Both the centrosomal and the centrosome-associated label disappear during prometaphase, most likely in concert with the dissociation of the corona at this stage. The striking differences of DdCrp to all other centrins may be related to the distinct structure and duplication mode of the Dictyostelium centrosome.

Re-evaluating centrosome function

Over the past 100 years, the centrosome has risen in status from an enigmatic organelle, located at the focus of microtubules, to a key player in cell-cycle progression and cellular control. A growing body of evidence indicates that centrosomes might not be essential for spindle assembly, whereas recent data indicate that they might be important for initiating S phase and completing cytokinesis. Molecules that regulate centrosome duplication have been identified, and the expanding list of intriguing centrosome-anchored activities, the functions of which have yet to be determined, promises continued discovery.

Cnm67p is a spacer protein of the Saccharomyces cerevisiae spindle pole body outer plaque

In Saccharomyces cerevisiae, the spindle pole body (SPB) is the functional homolog of the mammalian centrosome, responsible for the organization of the tubulin cytoskeleton. Cytoplasmic (astral) microtubules essential for the proper segregation of the nucleus into the daughter cell are attached at the outer plaque on the SPB cytoplasmic face. Previously, it has been shown that Cnm67p is an integral component of this structure; cells deleted for CNM67 are lacking the SPB outer plaque and thus experience severe nuclear migration defects. With the use of partial deletion mutants of CNM67, we show that the N- and C-terminal domains of the protein are important for nuclear migration. The C terminus, not the N terminus, is essential for Cnm67p localization to the SPB. On the other hand, only the N terminus is subject to protein phosphorylation of a yet unknown function. Electron microscopy of SPB serial thin sections reveals that deletion of the N- or C-terminal domains disturbs outer plaque formation, whereas mutations in the central coiled-coil domain of Cnm67p change the distance between the SPB core and the outer plaque. We conclude that Cnm67p is the protein that connects the outer plaque to the central plaque embedded in the nuclear envelope, adjusting the space between them by the length of its coiled-coil.