Duplication and Nuclear Envelope Insertion of the Yeast Microtubule Organizing Centre, the Spindle Pole Body

The main microtubule organizing centre in the unicellular model organisms Saccharomyces cerevisiae and Schizosaccharomyces pompe is the spindle pole body (SPB). The SPB is a multilayer structure, which duplicates exactly once per cell cycle. Unlike higher eukaryotic cells, both yeast model organisms undergo mitosis without breakdown of the nuclear envelope (NE), a so-called closed mitosis. Therefore, in order to simultaneously nucleate nuclear and cytoplasmic MTs, it is vital to embed the SPB into the NE at least during mitosis, similarly to the nuclear pore complex (NPC). This review aims to embrace the current knowledge of the SPB duplication cycle with special emphasis on the critical step of the insertion of the new SPB into the NE.

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title: Lessons from yeast: the spindle pole body and the centrosome
abstract: The yeast spindle pole body (SPB) is the functional equivalent of the centrosome. Most SPB components have been identified and their functions partly established. This involved a large variety of techniques which are described here, and the potential use of some of these in the centrosome field is highlighted. In particular, very useful structural information on the SPB was obtained from a reconstituted complex, the γ-tubulin complex, and also from a sub-particle, SPB cores, prepared by extraction of an enriched SPB preparation. The labelling of SPB proteins with GFP at the N or C termini, using GFP tags inserted into the genome, gave informative electron microscopy localization and fluorescence resonance energy transfer data. Examples are given of more precise functional data obtained by removing domains from one SPB protein, Spc110p, without affecting its essential function. Finally, a structural model for SPB duplication is described and the differences between SPB and centrosome duplication discussed.

title: Once and only once: mechanisms of centriole duplication and their deregulation in disease
abstract: Centrioles are conserved microtubule-based organelles that form the core of the centrosome and act as templates for the formation of cilia and flagella. Centrioles have important roles in most microtubule-related processes, including motility, cell division and cell signalling. To coordinate these diverse cellular processes, centriole number must be tightly controlled. In cycling cells, one new centriole is formed next to each pre-existing centriole in every cell cycle. Advances in imaging, proteomics, structural biology and genome editing have revealed new insights into centriole biogenesis, how centriole numbers are controlled and how alterations in these processes contribute to diseases such as cancer and neurodevelopmental disorders. Moreover, recent work has uncovered the existence of surveillance pathways that limit the proliferation of cells with numerical centriole aberrations. Owing to this progress, we now have a better understanding of the molecular mechanisms governing centriole biogenesis, opening up new possibilities for targeting these pathways in the context of human disease.

title: Towards a molecular architecture of centriole assembly
abstract: The centriole is an evolutionarily conserved macromolecular structure that is crucial for the formation of flagella, cilia and centrosomes. The ultrastructure of the centriole was first characterized decades ago with the advent of electron microscopy, revealing a striking ninefold radial arrangement of microtubules. However, it is only recently that the molecular mechanisms governing centriole assembly have begun to emerge, including the elucidation of the crucial role of spindle assembly abnormal 6 (SAS-6) proteins in imparting the ninefold symmetry. These advances have brought the field to an exciting era in which architecture meets function.

title: The budding yeast spindle pole body: structure, duplication, and function.
abstract: Nucleation of microtubules by eukaryotic microtubule organizing centers (MTOCs) is required for a variety of functions, including chromosome segregation during mitosis and meiosis, cytokinesis, fertilization, cellular morphogenesis, cell motility, and intracellular trafficking. Analysis of MTOCs from different organisms shows that the structure of these organelles is widely varied even though they all share the function of microtubule nucleation. Despite their morphological diversity, many components and regulators of MTOCs, as well as principles in their assembly, seem to be conserved. This review focuses on one of the best-characterized MTOCs, the budding yeast spindle pole body (SPB). We review what is known about its structure, protein composition, duplication, regulation, and functions. In addition, we discuss how studies of the yeast SPB have aided investigation of other MTOCs, most notably the centrosome of animal cells.

title: The centrosome and its mode of inheritance: the reduction of the centrosome during gametogenesis and its restoration during fertilization.
abstract: Neither the restoration of the centrosome during fertilization nor its reduction during gametogenesis is fully understood, but both are pivotal events in development. During each somatic cell cycle, the chromosomes, cytoplasm, and centrosomes duplicate in interphase, and all three split in two during each cell division. While it has long been recognized that both the sperm and the egg contribute equal haploid genomes during fertilization and that the vast majority of the cytoplasm is contributed by the egg, the relative contributions of the centrosome by each gamete are still in question. This article explores centrosome inheritance patterns and considers nine integral and secondarily derived activities of the centrosome. Boveri once hypothesized that "The ripe egg possesses all of the elements necessary for development save an active division-center. The sperm, on the other hand, possesses such a center but lacks the protoplasmic substratum in which to operate. In this respect the egg and sperm are complementary structures; their union in syngamy thus restores to each the missing element necessary to further development." This article reviews the evidence gathered from 11 experimental strategies used to test this theory. While the majority of these approaches supports the hypothesis that the sperm introduces the centrosome at fertilization, the pattern did not reveal itself as universal, since parthenogenesis occurs in nature and can be induced artificially, since centrosome and centriole form de novo in extracts from unfertilized eggs and since the centrosome is derived from maternal sources during fertilization in some systems--notably, in mice. Models of the centrosome are proposed, along with speculative mechanisms which might lead to the cloaking of the reproducing element of the maternal centrosome during oogenesis and the retention of this structure by the paternal centrosome during spermatogenesis. Proteins essential for microtubule nucleation, like gamma-tubulin, are retained in the cytoplasm during oogenesis, but are largely lost during spermatogenesis. It is further postulated that the restoration of the zygotic centrosome at fertilization requires the attraction of maternal centrosomal components (in particular, gamma-tubulin and the 25S "gamma-some" particle) to the paternal reproducing element; this, along with post-translational modifications (including phosphorylation, disulfide reduction, and calcium ion binding), creates a functional zygote centrosome by blending both maternal and paternal constituents.(ABSTRACT TRUNCATED AT 400

title: Loss and Rebirth of the Animal Microtubule Organizing Center: How Maternal Expression of Centrosomal Proteins Cooperates with the Sperm Centriole in Zygotic Centrosome Reformation
abstract: Centrosomes are the main microtubule organizing centers in animal cells. In particular during embryogenesis, they ensure faithful spindle formation and proper cell divisions. As metazoan centrosomes are eliminated during oogenesis, they have to be reassembled upon fertilization. Most metazoans use the sperm centrioles as templates for new centrosome biogenesis while the egg's cytoplasm re‐prepares all components for on‐going centrosome duplication in rapidly dividing embryonic cells. We discuss our knowledge and the experimental challenges to analyze zygotic centrosome reformation, which requires genetic experiments to enable scrutinizing respective male and female contributions. Male and female knockout animals and mRNA injection to mimic maternal expression of centrosomal proteins could point a way to the systematic molecular dissection of the process. The most recent data suggest that timely expression of centrosome components in oocytes is the key to zygotic centrosome reformation that uses male sperm as coordinators for de novo centrosome production.

title: Loss of p53 and centrosome hyperamplification
abstract: Loss or mutational inactivation of p53 has been shown to lead to abnormal amplification of centrosomes through deregulation of the centrosome duplication cycle and failure to undergo cytokinesis. In mouse cells, most cases of centrosome hyperamplification are attributed to deregulation of centrosome duplication. The presence of excess copies of centrosomes increases the frequency of mitotic defects, leading to unbalanced chromosome transmission to daughter cells. p53 controls centrosome duplication via transactivation-dependent and transactivation-independent mechanisms. In its transactivation-dependent control, p21Waf1/Cip1 acts as a major effector, likely guarding against untimely activation of CDK2/cyclin E kinase, hence ensuring the coordinated initiation of centrosome and DNA duplication. p53 appears to exert its transactivation-independent control through direct physical binding to the centrosomes.

title: Centrosome amplification: a suspect in breast cancer and racial disparities.
abstract: The multifaceted involvement of centrosome amplification (CA) in tumorigenesis is coming into focus following years of meticulous experimentation, which have elucidated the powerful abilities of CA to promote cellular invasion, disrupt stem cell division, drive chromosomal instability (CIN) and perturb tissue architecture, activities that can accelerate tumor progression. Integration of the extant in vitro, in vivo and clinical data suggests that in some tissues CA may be a tumor-initiating event, in others a consequential 'hit' in multistep tumorigenesis, and in some others, non-tumorigenic. However, in vivo data are limited and primarily focus on PLK4 (which has CA-independent mechanisms by which it promotes aggressive cellular phenotypes). In vitro breast cancer models suggest that CA can promote tumorigenesis in breast cancer cells in the setting of p53 loss or mutation, which can both trigger CA and promote cellular tolerance to its tendency to slow proliferation and induce aneuploidy. It is thus our perspective that CA is likely an early hit in multistep breast tumorigenesis that may sometimes be lost to preserve aggressive karyotypes acquired through centrosome clustering-mediated CIN, both numerical and structural. We also envision that the robust link between p53 and CA may underlie, to a considerable degree, racial health disparity in breast cancer outcomes. This question is clinically significant because, if it is true, then analysis of centrosomal profiles and administration of centrosome declustering drugs could prove highly efficacious in risk stratifying breast cancers and treating African American (AA) women with breast cancer.

title: Centrosome Amplification Is Sufficient to Promote Spontaneous Tumorigenesis in Mammals.
abstract: Centrosome amplification is a common feature of human tumors, but whether this is a cause or a consequence of cancer remains unclear. Here, we test the consequence of centrosome amplification by creating mice in which centrosome number can be chronically increased in the absence of additional genetic defects. We show that increasing centrosome number elevated tumor initiation in a mouse model of intestinal neoplasia. Most importantly, we demonstrate that supernumerary centrosomes are sufficient to drive aneuploidy and the development of spontaneous tumors in multiple tissues. Tumors arising from centrosome amplification exhibit frequent mitotic errors and possess complex karyotypes, recapitulating a common feature of human cancer. Together, our data support a direct causal relationship among centrosome amplification, genomic instability, and tumor development.

title: Maintaining centrosomes and cilia
abstract: Centrosomes and cilia are present in organisms from all branches of the eukaryotic tree of life. These structures are composed of microtubules and various other proteins, and are required for a plethora of cell processes such as structuring the cytoskeleton, sensing the environment, and motility. Deregulation of centrosome and cilium components leads to a wide range of diseases, some of which are incompatible with life. Centrosomes and cilia are thought to be very stable and can persist over long periods of time. However, these structures can disappear in certain developmental stages and diseases. Moreover, some centrosome and cilia components are quite dynamic. While a large body of knowledge has been produced regarding the biogenesis of these structures, little is known about how they are maintained. In this Review, we propose the existence of specific centrosome and cilia maintenance programs, which are regulated during development and homeostasis, and when deregulated can lead to disease.  Centrosomes and cilia are essential structures for many functions in development and disease. This Review discusses how their structure and functions are maintained.

title: Building the right centriole for each cell type
abstract: The centriole is a multifunctional structure that organizes centrosomes and cilia and is important for cell signaling, cell cycle progression, polarity, and motility. Defects in centriole number and structure are associated with human diseases including cancer and ciliopathies. Discovery of the centriole dates back to the 19th century. However, recent advances in genetic and biochemical tools, development of high-resolution microscopy, and identification of centriole components have accelerated our understanding of its assembly, function, evolution, and its role in human disease. The centriole is an evolutionarily conserved structure built from highly conserved proteins and is present in all branches of the eukaryotic tree of life. However, centriole number, size, and organization varies among different organisms and even cell types within a single organism, reflecting its cell type–specialized functions. In this review, we provide an overview of our current understanding of centriole biogenesis and how variations around the same theme generate alternatives for centriole formation and function.

title: Novel asymmetrically localizing components of human centrosomes identified by complementary proteomics methods
abstract: Centrosomes in animal cells are dynamic organelles with a proteinaceous matrix of pericentriolar material assembled around a pair of centrioles. They organize the microtubule cytoskeleton and the mitotic spindle apparatus. Mature centrioles are essential for biogenesis of primary cilia that mediate key signalling events. Despite recent advances, the molecular basis for the plethora of processes coordinated by centrosomes is not fully understood. We have combined protein identification and localization, using PCP‐SILAC mass spectrometry, BAC transgeneOmics, and antibodies to define the constituents of human centrosomes. From a background of non‐specific proteins, we distinguished 126 known and 40 candidate centrosomal proteins, of which 22 were confirmed as novel components. An antibody screen covering 4000 genes revealed an additional 113 candidates. We illustrate the power of our methods by identifying a novel set of five proteins preferentially associated with mother or daughter centrioles, comprising genes implicated in cell polarity. Pulsed labelling demonstrates a remarkable variation in the stability of centrosomal protein complexes. These spatiotemporal proteomics data provide leads to the further functional characterization of centrosomal proteins.

title: A FIBER APPARATUS IN THE NUCLEUS OF THE YEAST CELL
abstract: The structure and mode of division of the nucleus of budding yeast cells have been studied by phase-contrast microscopy during life and by ordinary microscopy after Helly fixation. The components of the nucleus were differentially stained by the Feulgen procedure, with Giemsa solution after hydrolysis, and with iron alum haematoxylin. New information was obtained in cells fixed in Helly's by directly staining them with 0.005% acid fuchsin in 1% acetic acid in water. Electron micrographs have been made of sections of cells that were first fixed with 3% glutaraldehyde, then divested of their walls with snail juice, and postfixed with osmium tetroxide. Light and electron microscopy have given concordant information about the organization of the yeast nucleus. A peripheral segment of the nucleus is occupied by relatively dense matter (the "peripheral cluster" of Mundkur) which is Feulgen negative. The greater part of the nucleus is filled with fine-grained Feulgen-positive matter of low density in which chromosomes could not be identified. Chromosomes become visible in this region under the light microscope at meiosis. In the chromatin lies a short fiber with strong affinity for acid fuchsin. The nucleus divides by elongation and constriction, and during this process the fiber becomes long and thin. Electron microscopy has resolved it into a bundle of dark-edged 150 to 180 A filaments which extends between "centriolar plaques" that are attached to the nuclear envelope.

title: The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds.
abstract: The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in some other yeasts. Like the centrosome of vertebrate cells, the SPB of S. pombe spends most of interphase in the cytoplasm, immediately next to the nuclear envelope. Some gamma-tubulin is localized on the SPB, suggesting that it plays a role in the organization of interphase microtubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. gamma-Tubulin is also found on osmiophilic material that lies near the inner surface of the nuclear envelope, immediately adjacent to the SPB, even though there are no MTs in the interphase nucleus. Apparently, the MT initiation activities of gamma-tubulin in S. pombe are regulated. The SPB duplicates in the cytoplasm during late G2 phase, and the two resulting structures are connected by a darkly staining bridge until the mitotic spindle forms. As the cell enters mitosis, the nuclear envelope invaginates beside the SPB, forming a pocket of cytoplasm that accumulates dark amorphous material. The nuclear envelope then opens to form a fenestra, and the duplicated SPB settles into it. Each part of the SPB initiates intranuclear MTs, and then the two structures separate to lie in distinct fenestrae as a bipolar spindle forms. Through metaphase, the SPBs remain in their fenestrae, bound to the polar ends of spindle MTs; at about this time, a small bundle of cytoplasmic MTs forms in association with each SPB. These MTs are situated with one end near to, but not on, the SPBs, and they project into the cytoplasm at an orientation that is oblique to the simple axis. As anaphase proceeds, the nuclear fenestrae close, and the SPBs are extruded back into the cytoplasm. These observations define new fields of enquiry about the control of SPB duplication and the dynamics of the nuclear envelope.

title: Big Lessons from Little Yeast: Budding and Fission Yeast Centrosome Structure, Duplication, and Function.
abstract: Centrosomes are a functionally conserved feature of eukaryotic cells that play an important role in cell division. The conserved γ-tubulin complex organizes spindle and astral microtubules, which, in turn, separate replicated chromosomes accurately into daughter cells. Like DNA, centrosomes are duplicated once each cell cycle. Although in some cell types it is possible for cell division to occur in the absence of centrosomes, these divisions typically result in defects in chromosome number and stability. In single-celled organisms such as fungi, centrosomes [known as spindle pole bodies (SPBs)] are essential for cell division. SPBs also must be inserted into the membrane because fungi undergo a closed mitosis in which the nuclear envelope (NE) remains intact. This poorly understood process involves events similar or identical to those needed for de novo nuclear pore complex assembly. Here, we review how analysis of fungal SPBs has advanced our understanding of centrosomes and NE events.

title: Genetic Control of the Cell Division Cycle in Yeast: V. Genetic Analysis of cdc Mutants.
abstract: One hundred and forty-eight temperature-sensitive cell division cycle (cdc) mutants of Saccharomyces cerevisiae have been isolated and characterized. Complementation studies ordered these recessive mutations into 32 groups and tetrad analysis revealed that each of these groups defines a single nuclear gene. Fourteen of these genes have been located on the yeast genetic map. Functionally related cistrons are not tightly clustered.Mutations in different cistrons frequently produce different cellular and nuclear morphologies in the mutant cells following incubation at the restrictive temperature, but all the mutations in the same cistron produce essentially the same morphology. The products of these genes appear, therefore, each to function individually in a discrete step of the cell cycle and they define collectively a large number of different steps.The mutants were examined by time-lapse photomicroscopy to determine the number of cell cycles completed at the restrictive temperature before arrest. For most mutants, cells early in the cell cycle at the time of the temperature shift (before the execution point) arrest in the first cell cycle while those later in the cycle (after the execution point) arrest in the second cell cycle. Execution points for allelic mutations that exhibit first or second cycle arrest are rather similar and appear to be cistron-specific. Other mutants traverse several cycles before arrest, and its suggested that the latter type of response may reveal gene products that are temperature-sensitive for synthesis, whereas the former may be temperature-sensitive for function.The gene products that are defined by the cdc cistrons are essential for the completion of the cell cycle in haploids of a and alpha mating type and in a/alpha diploid cells. The same genes, therefore, control the cell cycle in each of these stages of the life cycle.

title: An endomitotic effect of a cell cycle mutation of Saccharomyces cerevisiae.
abstract: A recessive temperature-sensitive mutation of Saccharomyces cerevisiae has been isolated and shown to cause an increase in ploidy in both haploids and diploids. Genetic analysis revealed that the strain carrying the mutation was an aa diploid, although MNNG mutagenesis had been done on an a haploid strain. When the mutant strain was crossed with an alpha alpha diploid and the resultant tetraploid sporulated, some of the meiotic progeny of this tetraploid were themselves tetraploid, as shown by both genetic analysis and DNA measurements, instead of diploid as expected of tetraploid meiosis. The ability of these tetraploids to continue to produce tetraploid meiotic progeny was followed for four generations. Homothallism was excluded as a cause of the increase in ploidy; visual pedigree analysis of spore clones to about the 32-cell stage failed to reveal any zygotes, and haploids that diploidized retained their mating type. An extra round of meiotic DNA synthesis was also considered and excluded. It was found that tetraploidization was independent of sporulation temperature, but was dependent on the temperature of germination and the growth of the spores. Increase in ploidy occurred when the spores were germinated and grown at 30 degrees, but did not occur at 23 degrees. Two cycles of sporulation and growth at 23 degrees resulted in haploids, which were shown to diploidize within 24 hr when grown at 30 degrees. Visual observation of the haploid cells incubated at 36 degrees revealed a cell-division-cycle phenotype characteristic of mutations that affect nuclear division; complementation analysis demonstrated that the mutation, cdc31-2, is allelic to cdc31-1, a mutation isolated by Hartwell et al. (1973) and characterized as causing a temperature-sensitive arrest during late nuclear division. The segregation of cdc31-2 in heterozygous diploids was 2:2 and characteristic of a noncentromere-linked gene.

title: The calcium-binding protein cell division cycle 31 of Saccharomyces cerevisiae is a component of the half bridge of the spindle pole body
abstract: Cdc31 mutants of Saccharomyces cerevisiae arrest at the nonpermissive temperature with large buds, G2 DNA content and, a single, abnormally large spindle pole body (SPB) (Byers, B. 1981. Molecular Genetics in Yeast. Alfred Benzon Symposium. . In this report, we show that the CDC31 gene product is essential for cell viability. We demonstrate that purified CDC31 protein binds Ca2+ and that this binding is highly specific. Taken together, three lines of evidence indicate that CDC31 is a component of the SPB. First, CDC31 cofractionates with enriched preparations of SPBs. Second, immunofluorescence staining indicates that CDC31 colocalizes with a known SPB component. Third, immunoelectron microscopy with whole cells and with isolated SPBs reveals that CDC31 is localized to the half bridge of the SPB, which lies immediately adjacent to the SPB plaques. CDC31 was detected mainly at the cytoplasmic side of the half bridge and, therefore, defines a further substructure of the SPB. We suggest that CDC31 is a member of a family of calcium-binding, centrosome- associated proteins from a phylogenetically diverse group of organisms.

title: The Saccharomyces cerevisiae spindle pole body duplication gene MPS1 is part of a mitotic checkpoint
abstract: M-phase checkpoints inhibit cell division when mitotic spindle function is perturbed. Here we show that the Saccharomyces cerevisiae MPS1 gene product, an essential protein kinase required for spindle pole body (SPB) duplication (Winey et al., 1991; Lauze et al., 1995), is also required for M-phase check-point function. In cdc31-2 and mps2-1 mutants, conditional failure of SPB duplication results in cell cycle arrest with high p34CDC28 kinase activity that depends on the presence of the wild-type MAD1 checkpoint gene, consistent with checkpoint arrest of mitosis. In contrast, mps1 mutant cells fail to duplicate their SPBs and do not arrest division at 37 degrees C, exhibiting a normal cycle of p34CDC28 kinase activity despite the presence of a monopolar spindle. Double mutant cdc31-2, mps1-1 cells also fail to arrest mitosis at 37 degrees C, despite having SPB structures similar to cdc31-2 single mutants as determined by EM analysis. Arrest of mitosis upon microtubule depolymerization by nocodazole is also conditionally absent in mps1 strains. This is observed in mps1 cells synchronized in S phase with hydroxyurea before exposure to nocodazole, indicating that failure of checkpoint function in mps1 cells is independent of SPB duplication failure. In contrast, hydroxyurea arrest and a number of other cdc mutant arrest phenotypes are unaffected by mps1 alleles. We propose that the essential MPS1 protein kinase functions both in SPB duplication and in a mitotic checkpoint monitoring spindle integrity.

title: Centrin-2 Is Required for Centriole Duplication in Mammalian Cells
abstract: Centrosomes are the favored microtubule-organizing framework of eukaryotic cells. Centrosomes contain a pair of centrioles that normally duplicate once during the cell cycle to give rise to two mitotic spindle poles, each containing one old and one new centriole. However, aside from their role as an anchor point for pericentriolar material and as basal bodies of flagella and cilia, the functional attributes of centrioles remain enigmatic.



Here, using RNA interference, we demonstrate that "knockdown" of centrin-2, a protein of centrioles, results in failure of centriole duplication during the cell cycle in HeLa cells. Following inhibition of centrin-2 synthesis, the preexisting pair of centrioles separate, and functional bipolar spindles form with only one centriole at each spindle pole. Centriole dilution results from the ensuing cell division, and daughter cells are "born" with only a single centriole. Remarkably, these unicentriolar daughter cells may complete a second and even third bipolar mitosis in which spindle microtubules converge onto unusually broad spindle poles and in which cell division results in daughter cells containing either one or no centrioles at all. Cells thus denuded of the mature or both centrioles fail to undergo cytokinesis in subsequent cell cycles, give rise to multinucleate products, and finally die.



These results demonstrate a requirement for centrin in centriole duplication and demonstrate that centrioles play a role in organizing spindle pole morphology and in the completion of cytokinesis.

title: Mps1 Phosphorylation Sites Regulate the Function of Centrin 2 in Centriole Assembly
abstract: We show that while Centrin2 is dispensable for centriole assembly, it is an Mps1 substrate that stimulates canonical and aberrant centriole assembly by two different Mps1-dependent mechanisms, HsSas-6–dependent and –independent. Centrin2 phosphorylation is also required for the ability of Mps1 to drive production of mature centrioles.

title: Sfi1p has conserved centrin-binding sites and an essential function in budding yeast spindle pole body duplication
abstract: Centrins are calmodulin-like proteins present in microtubule-organizing centers. The Saccharomyces cerevisiae centrin, Cdc31p, was functionally tagged with a single Z domain of protein A, and used in pull-down experiments to isolate Cdc31p-binding proteins. One of these, Sfi1p, localizes to the half-bridge of the spindle pole body (SPB), where Cdc31p is also localized. Temperature-sensitive mutants in SFI1 show a defect in SPB duplication and genetic interactions with cdc31-1. Sfi1p contains multiple internal repeats that are also present in a Schizosaccharomyces pombe protein, which also localizes to the SPB, and in several human proteins, one of which localizes close to the centriole region. Cdc31p binds directly to individual Sfi1 repeats in a 1:1 ratio, so a single molecule of Sfi1p binds multiple molecules of Cdc31p. The centrosomal human protein containing Sfi1 repeats also binds centrin in the repeat region, showing that this centrin-binding motif is conserved.

title: Regulation of spindle pole body assembly and cytokinesis by the centrin-binding protein Sfi1 in fission yeast
abstract: A previous model suggested doubling of Sfi1 as the first step of SPB assembly. Here it is shown that Sfi1 is gradually recruited to SPBs throughout the cell cycle. Conserved tryptophans in Sfi1 are required for its equal partitioning during mitosis, and unequal partitioning of Sfi1 underlies SPB assembly and mitotic defects in the next cell cycle.

title: γ-Tubulin complexes in microtubule nucleation and beyond
abstract: Tremendous progress has been made in understanding the functions of γ-tubulin and, in particular, its role in microtubule nucleation since the publication of its discovery in 1989. The structure of γ-tubulin has been determined, and the components of γ-tubulin complexes have been identified. Significant progress in understanding the structure of the γ-tubulin ring complex and its components has led to a persuasive model for how these complexes nucleate microtubule assembly. At the same time, data have accumulated that γ-tubulin has important but less well understood functions that are not simply a consequence of its function in microtubule nucleation. These include roles in the regulation of plus-end microtubule dynamics, gene regulation, and mitotic and cell cycle regulation. Finally, evidence is emerging that γ-tubulin mutations or alterations of γ-tubulin expression play an important role in certain types of cancer and in other diseases.

title: Centriole replication. A study of spermatogenesis in the snail Viviparus.
abstract: This paper describes the replication of centrioles during spermatogenesis in the Prosobranch snail, Viviparus malleatus Reeve. Sections for electron microscopy were cut from pieces of testis fixed in OsO4 and embedded in the polyester resin Vestopal W. Two kinds of spermatocytes are present. These give rise to typical uniflagellate sperm carrying the haploid number of 9 chromosomes, and atypical multiflagellate sperm with only one chromosome. Two centrioles are present in the youngest typical spermatocyte. Each is a hollow cylinder about 160 mµ in diameter and 330 mµ long. The wall consists of 9 sets of triplet fibers arranged in a characteristic pattern. Sometime before pachytene an immature centriole, or procentriole as it will be called, appears next to each of the mature centrioles. The procentriole resembles a mature centriole in most respects except length: it is more annular than tubular. The daughter procentriole lies with its axis perpendicular to that of its parent. It presumably grows to full size during the late prophase, although the maturation stages have not been observed with the electron microscope. It is suggested that centrioles possess a constant polarization. The distal end forms the flagellum or other centriole products, while the proximal end represents the procentriole and is concerned with replication. The four centrioles of prophase (two parents and two daughters) are distributed by the two meiotic divisions to the four typical spermatids, in which they function as the basal bodies of the flagella. Atypical spermatocytes at first contain two normal centrioles. Each of these becomes surrounded by a cluster of procentrioles, which progressively elongate during the late prophase. After two aberrant meiotic divisions the centriole clusters give rise to the basal bodies of the multiflagellate sperm. These facts are discussed in the light of the theory, first proposed by Pollister, that the supernumerary centrioles in the atypical cells are derived from the centromeres of degenerating chromosomes.

title: Mitosis in the fission yeast Schizosaccharomyces pombe: a comparative study with light and electron microscopy.
abstract: Mitosis in Schizosaccharomyces pombe has been followed in living cells by phase-contrast microscopy and studied in fixed and suitably stained preparations by light microscopy. Successful preservation of nuclear fine structure in this yeast, not previously achieved, has allowed us to confirm and extend the observations made with light microscopy. Without first arranging themselves on a metaphase plate, mitotic chromosomes become grouped in 2 clusters radiating, finger-like, from 2 points of attachment at opposite poles of an elongating nucleus. At these 2 sites electron microscopy reveals the presence of disk-shaped electron-dense organelles which we have called kinetochore equivalents (KCE). At mitosis the KCEs are connected across the nucleus by a narrow bundle of parallel microtubules which we refer to as the spindle. Integration of our observations has led us to propose that at mitosis the separation of the KCEs and their attached chromosomes is initiated by a differential expansion of the nuclear envelope restricted to the region between recently divided KCEs and that expansion of the nuclear envelope later becomes general, resulting in a marked elongation of the nucleus. Displacement of the nuclear contents to the ends of the elongated nucleus gives it the shape of a dumbbell. The elongation of the microtubule bundle keeps in step with the elongation of the nucleus but does not appear to be the cause of it. It may have the function of keeping the separated KCEs rigidly apart. During mitosis the nucleolus persists and stretches out within the unbroken envelope of the nucleus as it elongates. Towards the end of division equal amounts of nucleolar material are found in the rounded ends of the dumbbell-shaped nucleus. The break up of the dumbbell shape into daughter nuclei seems to involve the breaking of its tenuous middle part and a pivoting of its 2 ends in opposite directions. In the course of our work on mitosis we have become aware of features in the cytoplasm of growing S. pombe cells which are described here for the first time. The cells invariably contain several prominent vacuoles containing an extremely electron-dense material which stains metachromatically with toluidine blue and may be polyphosphate. The mitochondria are of special interest for 2 reasons. First, because they have unique mesosome-like membrane invaginations and secondly, because a mitochondrion is regularly associated with the single KCE by the side of the interphase nucleus, as well as with each one of the 2 KCEs that occupy opposite ends of the intranuclear spindle during mitosis.

title: Behavior of spindles and spindle plaques in the cell cycle and conjugation of Saccharomyces cerevisiae
abstract: The interdependence of spindle plaque with other aspects of cell division and conjugation in Saccharomyces cerevisiae has been investigated. Three forms of the spindle plaque appear sequentially before the formation of the complete, intranuclear spindle. The single plaque is present initially in the mitotic cycle; it becomes transformed into a satellite-bearing single plaque during the latter part of G1. Subsequently, plaque duplication yields the double plaque characteristic of the early phase of budding, which coincides with the period of chromosome replication (S). The eventual separation of these plaques to form a complete spindle, with a single plaque at each pole, is nearly coincident with the completion of S. The form of the plaque differs in two independent cases of G1 arrest: the single plaque is found in a cell in stationary arrest of growth, whereas a cell arrested by mating factors in preparation for conjugation contains a satellite-bearing single plaque. The latter form is retained during zygote formation, where it serves as the initial site of fusion of each prezygotic nuceus with the other. This fusion results in the formation of a single zygotic nucleus with a satellite-bearing single plaque, which is subsequently transformed into a double plaque as the zygote buds. The double plaque is situated adjacent to the site of bud emergence in both vegetative cells and zygotes.

title: Localization of Core Spindle Pole Body (SPB) Components during SPB Duplication in Saccharomyces cerevisiae
abstract: We have examined the process of spindle pole body (SPB) duplication in Saccharomyces cerevisiae by electron microscopy and found several stages. These include the assembly, probably from the satellite, of a large plaque-like structure, the duplication plaque, on the cytoplasmic face of the half-bridge and its insertion into the nuclear envelope. We analyzed the role of the main SPB components in the formation of these structures by identifying them from an SPB core fraction by mass spectrometry. Temperature-sensitive mutants for two of the components, Spc29p and Nud1p, were prepared to partly define their function. The composition of two of the intermediates in SPB duplication, the satellite and the duplication plaque, was examined by immunoelectron microscopy. Both contain cytoplasmic SPB components showing that duplication has already been partly achieved by the end of the preceding cell cycle when the satellite is formed. We show that by overexpression of SPB components the structure of the satellite can be changed and SPB duplication inhibited by disrupting the attachment of the plaque-like intermediate to the half-bridge. We present a model for SPB duplication where binding of SPB components to either end of the bridge structure ensures two separate SPBs.

title: Structural role of Sfi1p–centrin filaments in budding yeast spindle pole body duplication
abstract: Centrins are calmodulin-like proteins present in centrosomes and yeast spindle pole bodies (SPBs) and have essential functions in their duplication. The Saccharomyces cerevisiae centrin, Cdc31p, binds Sfi1p on multiple conserved repeats; both proteins localize to the SPB half-bridge, where the new SPB is assembled. The crystal structures of Sfi1p–centrin complexes containing several repeats show Sfi1p as an α helix with centrins wrapped around each repeat and similar centrin–centrin contacts between each repeat. Electron microscopy (EM) shadowing of an Sfi1p–centrin complex with 15 Sfi1 repeats and 15 centrins bound showed filaments 60 nm long, compatible with all the Sfi1 repeats as a continuous α helix. Immuno-EM localization of the Sfi1p N and C termini showed Sfi1p–centrin filaments spanning the length of the half-bridge with the Sfi1p N terminus at the SPB. This suggests a model for SPB duplication where the half-bridge doubles in length by association of the Sfi1p C termini, thereby providing a new Sfi1p N terminus to initiate SPB assembly.

title: Molecular Mechanisms that Restrict Yeast Centrosome Duplication to One Event per Cell Cycle
abstract: The spindle pole body (SPB) of budding yeast is the functional equivalent of the mammalian centrosome. Like the centrosome, the SPB duplicates once per cell cycle. The new SPB assembles adjacent to the mother SPB at a substructure called the bridge. The half-bridge, the bridge precursor, is a one-sided extension of the SPB central plaque layered on both sides of the nuclear envelope. Parallel Sfi1 molecules longitudinally span the half-bridge with their N termini embedded in the SPB central plaque, whereas their C termini mark the half-bridge distal end. In early G1, half-bridge elongation by antiparallel C-to-C dimerization of Sfi1 exposes free N-Sfi1 where the new SPB assembles. After SPB duplication, the dimerized Sfi1 is severed to allow spindle formation and SPB reduplication.



We show that Sfi1 C-terminal domain harbors phosphorylation sites for Cdk1 and the polo-like kinase Cdc5. Cdk1 and, to a lesser extent, Cdc5 inhibit SPB duplication as phosphomimetic sfi1 mutations lead to metaphase cells with a single SPB. In contrast, phosphoinhibitory sfi1 mutations in Cdk1 sites are lethal because cells fail to sever the bridge after SPB duplication. Moreover, Cdc14 dephosphorylates C-Sfi1 to prepare it for a new round of duplication, and the kinase Mps1 promotes Sfi1 extension in G1.



Positive (Cdc14) and negative (Cdk1 and Cdc5) SPB duplication signals are integrated at the level of the half-bridge component Sfi1. In addition, Mps1 activates Sfi1 duplication. Fluctuating activities of these regulators ensure one SPB duplication event per cell cycle.

title: Licensing of Yeast Centrosome Duplication Requires Phosphoregulation of Sfi1
abstract: Duplication of centrosomes once per cell cycle is essential for bipolar spindle formation and genome maintenance and is controlled in part by cyclin-dependent kinases (Cdks). Our study identifies Sfi1, a conserved component of centrosomes, as the first Cdk substrate required to restrict centrosome duplication to once per cell cycle. We found that reducing Cdk1 phosphorylation by changing Sfi1 phosphorylation sites to nonphosphorylatable residues leads to defects in separation of duplicated spindle pole bodies (SPBs, yeast centrosomes) and to inappropriate SPB reduplication during mitosis. These cells also display defects in bipolar spindle assembly, chromosome segregation, and growth. Our findings lead to a model whereby phosphoregulation of Sfi1 by Cdk1 has the dual function of promoting SPB separation for spindle formation and preventing premature SPB duplication. In addition, we provide evidence that the protein phosphatase Cdc14 has the converse role of activating licensing, likely via dephosphorylation of Sfi1.

title: Characterization of spindle pole body duplication reveals a regulatory role for nuclear pore complexes
abstract: The spindle pole body (SPB) of budding yeast duplicates once per cell cycle. In G1, the satellite, an SPB precursor, assembles next to the mother SPB (mSPB) on the cytoplasmic side of the nuclear envelope (NE). How the growing satellite subsequently inserts into the NE is an open question. To address this, we have uncoupled satellite growth from NE insertion. We show that the bridge structure that separates the mSPB from the satellite is a distance holder that prevents deleterious fusion of both structures. Binding of the &ggr;-tubulin receptor Spc110 to the central plaque from within the nucleus is important for NE insertion of the new SPB. Moreover, we provide evidence that a nuclear pore complex associates with the duplicating SPB and helps to insert the SPB into the NE. After SPB insertion, membrane-associated proteins including the conserved Ndc1 encircle the SPB and retain it within the NE. Thus, uncoupling SPB growth from NE insertion unmasks functions of the duplication machinery.

title: Kar1 binding to Sfi1 C-terminal regions anchors the SPB bridge to the nuclear envelope
abstract: Kar1 interacts with the C-terminal (CT) region of Sfi1 to tether the yeast centrosome’s bridge to the nuclear envelope; Sfi1-CT and C-terminal Cdc31-binding regions form the antiparallel Sfi1 overlap in the bridge.

title: Duplication of the Yeast Spindle Pole Body Once per Cell Cycle
abstract: The yeast spindle pole body (SPB) is the functional equivalent of the mammalian centrosome. Centrosomes and SPBs duplicate exactly once per cell cycle by mechanisms that use the mother structure as a platform for the assembly of the daughter. The conserved Sfi1 and centrin proteins are essential components of the SPB duplication process. Sfi1 is an elongated molecule that has, in its center, 20 to 23 binding sites for the Ca2+-binding protein centrin. In the yeast Saccharomyces cerevisiae, all Sfi1 N termini are in contact with the mother SPB whereas the free C termini are distal to it. During S phase and early mitosis, cyclin-dependent kinase 1 (Cdk1) phosphorylation of mainly serine residues in the Sfi1 C termini blocks the initiation of SPB duplication (“off” state). Upon anaphase onset, the phosphatase Cdc14 dephosphorylates Sfi1 (“on” state) to promote antiparallel and shifted incorporation of cytoplasmic Sfi1 molecules into the half-bridge layer, which thereby elongates into the bridge. The Sfi1 C termini of the two Sfi1 layers localize in the bridge center, whereas the N termini of the newly assembled Sfi1 molecules are distal to the mother SPB. These free Sfi1 N termini then assemble the new SPB in G1 phase. Recruitment of Sfi1 molecules into the anaphase SPB and bridge formation were also observed in Schizosaccharomyces pombe, suggesting that the Sfi1 bridge cycle is conserved between the two organisms. Thus, restricting SPB duplication to one event per cell cycle requires only an oscillation between Cdk1 kinase and Cdc14 phosphatase activities. This clockwork regulates the “on”/“off” state of the Sfi1-centrin receiver.

title: The yeast protein kinase Mps1p is required for assembly of the integral spindle pole body component Spc42p
abstract: Saccharomyces cerevisiae MPS1 encodes an essential protein kinase that has roles in spindle pole body (SPB) duplication and the spindle checkpoint. Previously characterized MPS1 mutants fail in both functions, leading to aberrant DNA segregation with lethal consequences. Here, we report the identification of a unique conditional allele, mps1–8, that is defective in SPB duplication but not the spindle checkpoint. The mutations in mps1-8 are in the noncatalytic region of MPS1, and analysis of the mutant protein indicates that Mps1-8p has wild-type kinase activity in vitro. A screen for dosage suppressors of the mps1-8 conditional growth phenotype identified the gene encoding the integral SPB component SPC42. Additional analysis revealed that mps1-8 exhibits synthetic growth defects when combined with certain mutant alleles of SPC42. An epitope-tagged version of Mps1p (Mps1p-myc) localizes to SPBs and kinetochores by immunofluorescence microscopy and immuno-EM analysis. This is consistent with the physical interaction we detect between Mps1p and Spc42p by coimmunoprecipitation. Spc42p is a substrate for Mps1p phosphorylation in vitro, and Spc42p phosphorylation is dependent on Mps1p in vivo. Finally, Spc42p assembly is abnormal in a mps1-1 mutant strain. We conclude that Mps1p regulates assembly of the integral SPB component Spc42p during SPB duplication.

title: Cdc28/Cdk1 regulates spindle pole body duplication through phosphorylation of Spc42 and Mps1.
abstract: Duplication of the Saccharomyces cerevisiae spindle pole body (SPB) once per cell cycle is essential for bipolar spindle formation and accurate chromosome segregation during mitosis. We have investigated the role that the major yeast cyclin-dependent kinase Cdc28/Cdk1 plays in assembly of a core SPB component, Spc42, to better understand how SPB duplication is coordinated with cell cycle progression. Cdc28 is required for SPB duplication and Spc42 assembly, and we found that Cdc28 directly phosphorylates Spc42 to promote its assembly into the SPB. The Mps1 kinase, previously shown to regulate Spc42 phosphorylation and assembly, is also a Cdc28 substrate, and Cdc28 phosphorylation of Mps1 is needed to maintain wild-type levels of Mps1 in cells. Analysis of nonphosphorylatable mutants in SPC42 and MPS1 indicates that direct Spc42 phosphorylation and indirect regulation of Spc42 through Mps1 are two overlapping pathways by which Cdc28 regulates Spc42 assembly and SPB duplication during the cell cycle.

title: Nuclear envelope insertion of spindle pole bodies and nuclear pore complexes
abstract: The defining feature of eukaryotic cells is the double lipid bilayer of the nuclear envelope (NE) that serves as a physical barrier separating the genome from the cytosol. Nuclear pore complexes (NPCs) are embedded in the NE to facilitate transport of proteins and other macromolecules into and out of the nucleus. In fungi and early embryos where the NE does not completely breakdown during mitosis, microtubule-organizing centers such as the spindle pole body (SPB) must also be inserted into the NE to facilitate organization of the mitotic spindle. Several recent papers have shed light on the mechanism by which SPB complexes are inserted into the NE. An unexpected link between the SPB and NPCs suggests that assembly of these NE complexes is tightly coordinated. We review the findings of these reports in light of our current knowledge of SPB, NPC and NE structure, assembly and function.

title: Poring over pores: nuclear pore complex insertion into the nuclear envelope.
abstract: The nuclear boundary is formed by the nuclear envelope (NE), a double membrane system that establishes a selective barrier between the nucleoplasm and the cytoplasm. Its barrier characteristics are determined by nuclear pore complexes (NPCs); huge protein assemblies that guard nucleocytoplasmic compartmentalization. In this review, we discuss the biogenesis of NPCs during interphase of the cell cycle. This process requires a mechanistically enigmatic fusion step between the inner and the outer nuclear membrane. We focus on the principle of membrane pore formation in the NE and consider existing paradigms of other cellular membrane remodeling events. The emerging roles of transmembrane proteins and membrane-shaping factors in NPC biogenesis are discussed.

title: Viral membrane fusion
abstract: Infection by viruses having lipid-bilayer envelopes proceeds through fusion of the viral membrane with a membrane of the target cell. Viral 'fusion proteins' facilitate this process. They vary greatly in structure, but all seem to have a common mechanism of action, in which a ligand-triggered, large-scale conformational change in the fusion protein is coupled to apposition and merger of the two bilayers. We describe three examples—the influenza virus hemagglutinin, the flavivirus E protein and the vesicular stomatitis virus G protein—in some detail, to illustrate the ways in which different structures have evolved to implement this common mechanism. Fusion inhibitors can be effective antiviral agents.

title: A simple, versatile method for GFP-based super-resolution microscopy via nanobodies
abstract: We developed a method to use any GFP-tagged construct in single-molecule super-resolution microscopy. By targeting GFP with small, high-affinity antibodies coupled to organic dyes, we achieved nanometer spatial resolution and minimal linkage error when analyzing microtubules, living neurons and yeast cells. We show that in combination with libraries encoding GFP-tagged proteins, virtually any known protein can immediately be used in super-resolution microscopy and that simplified labeling schemes allow high-throughput super-resolution imaging.

title: Structured illumination with particle averaging reveals novel roles for yeast centrosome components during duplication
abstract: Duplication of the yeast centrosome (called the spindle pole body, SPB) is thought to occur through a series of discrete steps that culminate in insertion of the new SPB into the nuclear envelope (NE). To better understand this process, we developed a novel two-color structured illumination microscopy with single-particle averaging (SPA-SIM) approach to study the localization of all 18 SPB components during duplication using endogenously expressed fluorescent protein derivatives. The increased resolution and quantitative intensity information obtained using this method allowed us to demonstrate that SPB duplication begins by formation of an asymmetric Sfi1 filament at mitotic exit followed by Mps1-dependent assembly of a Spc29- and Spc42-dependent complex at its tip. Our observation that proteins involved in membrane insertion, such as Mps2, Bbp1, and Ndc1, also accumulate at the new SPB early in duplication suggests that SPB assembly and NE insertion are coupled events during SPB formation in wild-type cells.

title: The Saccharomyces cerevisiae spindle pole body (SPB) component Nbp1p is required for SPB membrane insertion and interacts with the integral membrane proteins Ndc1p and Mps2p.
abstract: The spindle pole body (SPB) in Saccharomyces cerevisiae functions to nucleate and organize spindle microtubules, and it is embedded in the nuclear envelope throughout the yeast life cycle. However, the mechanism of membrane insertion of the SPB has not been elucidated. Ndc1p is an integral membrane protein that localizes to SPBs, and it is required for insertion of the SPB into the nuclear envelope during SPB duplication. To better understand the function of Ndc1p, we performed a dosage suppressor screen using the ndc1-39 temperature-sensitive allele. We identified an essential SPB component, Nbp1p. NBP1 shows genetic interactions with several SPB genes in addition to NDC1, and two-hybrid analysis revealed that Nbp1p binds to Ndc1p. Furthermore, Nbp1p is in the Mps2p-Bbp1p complex in the SPB. Immunoelectron microscopy confirmed that Nbp1p localizes to the SPB, suggesting a function at this location. Consistent with this hypothesis, nbp1-td (a degron allele) cells fail in SPB duplication upon depletion of Nbp1p. Importantly, these cells exhibit a "dead" SPB phenotype, similar to cells mutant in MPS2, NDC1, or BBP1. These results demonstrate that Nbp1p is a SPB component that acts in SPB duplication at the point of SPB insertion into the nuclear envelope.

title: NDC1: a nuclear periphery component required for yeast spindle pole body duplication
abstract: The spindle pole body (SPB) of Saccharomyces cerevisiae serves as the centrosome in this organism, undergoing duplication early in the cell cycle to generate the two poles of the mitotic spindle. The conditional lethal mutation ndc1-1 has previously been shown to cause asymmetric segregation, wherein all the chromosomes go to one pole of the mitotic spindle (Thomas, J. H., and D. Botstein. . Examination by electron microscopy of mutant cells subjected to the nonpermissive temperature reveals a defect in SPB duplication. Although duplication is seen to occur, the nascent SPB fails to undergo insertion into the nuclear envelope. The parental SPB remains functional, organizing a monopolar spindle to which all the chromosomes are presumably attached. Order-of-function experiments reveal that the NDC1 function is required in G1 after alpha-factor arrest but before the arrest caused by cdc34. Molecular analysis shows that the NDC1 gene is essential and that it encodes a 656 amino acid protein (74 kD) with six or seven putative transmembrane domains. This evidence for membrane association is further supported by immunofluorescent localization of the NDC1 product to the vicinity of the nuclear envelope. These findings suggest that the NDC1 protein acts within the nuclear envelope to mediate insertion of the nascent SPB.

title: MPS1 and MPS2: novel yeast genes defining distinct steps of spindle pole body duplication
abstract: It is crucial to the eucaryotic cell cycle that the centrosome undergo precise duplication to generate the two poles of the mitotic spindle. In the budding yeast Saccharomyces cerevisiae, centrosomal functions are provided by the spindle pole body (SPB), which is duplicated at the time of bud emergence in G1 of the cell cycle. Genetic control of this process has previously been revealed by the characterization of mutants in CDC31 and KAR1, which prevent SPB duplication and lead to formation of a monopolar spindle. Newly isolated mutations described here (mps1 and mps2, for monopolar spindle) similarly cause monopolar mitosis but their underlying effects on SPB duplication are unique. The MPS1 gene is found by electron microscopy to be essential for proper formation of the site at which the new SPB normally arises adjacent to the existing one. By contrast, a mutation in MPS2 permits duplication to proceed, but the newly formed SPB is structurally defective and unable to serve as a functional spindle pole. Distinct temporal requirements for the CDC31, MPS1, and MPS2 gene functions during the SPB duplication cycle further demonstrate the individual roles of these genes in the morphogenetic pathway.

title: The Bbp1p–Mps2p complex connects the SPB to the nuclear envelope and is essential for SPB duplication
abstract: In budding yeast, microtubules are organized by the spindle pole body (SPB), which is embedded in the nuclear envelope via its central plaque structure. Here, we describe the identification of BBP1 in a suppressor screen with a conditional lethal allele of SPC29. Bbp1p was detected at the central plaque periphery of the SPB and bbp1‐1 cells were found to be defective in SPB duplication. bbp1‐1 cells extend their satellite into a duplication plaque like wild‐type cells; however, this duplication plaque then fails to insert properly into the nuclear envelope and does not assemble a functional inner plaque. This function in SPB duplication is probably fulfilled by a stable complex of Bbp1p and Mps2p, a nuclear envelope protein that is also essential for duplication plaque insertion. In addition, we found that Bbp1p interacts with Spc29p and the half‐bridge component Kar1p. These interactions are likely to play a role in connecting the SPB with the nuclear envelope and the central plaque with the half‐bridge.

title: Saccharomyces cerevisiae MPS2 encodes a membrane protein localized at the spindle pole body and the nuclear envelope.
abstract: The MPS2 (monopolar spindle two) gene is one of several genes required for the proper execution of spindle pole body (SPB) duplication in the budding yeast Saccharomyces cerevisiae (). We report here that the MPS2 gene encodes an essential 44-kDa protein with two putative coiled-coil regions and a hydrophobic sequence. Although MPS2 is required for normal mitotic growth, some null strains can survive; these survivors exhibit slow growth and abnormal ploidy. The MPS2 protein was tagged with nine copies of the myc epitope, and biochemical fractionation experiments show that it is an integral membrane protein. Visualization of a green fluorescent protein (GFP) Mps2p fusion protein in living cells and indirect immunofluorescence microscopy of 9xmyc-Mps2p revealed a perinuclear localization with one or two brighter foci of staining corresponding to the SPB. Additionally, immunoelectron microscopy shows that GFP-Mps2p localizes to the SPB. Our analysis suggests that Mps2p is required as a component of the SPB for insertion of the nascent SPB into the nuclear envelope.

title: A ternary membrane protein complex anchors the spindle pole body in the nuclear envelope in budding yeast
abstract: In budding yeast (Saccharomyces cerevisiae) the multilayered spindle pole body (SPB) is embedded in the nuclear envelope (NE) at fusion sites of the inner and outer nuclear membrane. The SPB is built from 18 different proteins, including the three integral membrane proteins Mps3, Ndc1, and Mps2. These membrane proteins play an essential role in the insertion of the new SPB into the NE. How the huge core structure of the SPB is anchored in the NE has not been investigated thoroughly until now. The present model suggests that the NE protein Mps2 interacts via Bbp1 with Spc29, one of the coiled-coil proteins forming the central plaque of the SPB. To test this model, we purified and reconstituted the Mps2-Bbp1 complex from yeast and incorporated the complex into liposomes. We also demonstrated that Mps2-Bbp1 directly interacts with Mps3 and Ndc1. We then purified Spc29 and reconstituted the ternary Mps2-Bbp1-Spc29 complex, proving that Bbp1 can simultaneously interact with Mps2 and Spc29 and in this way link the central plaque of the SPB to the nuclear envelope. Interestingly, Bbp1 induced oligomerization of Spc29, which may represent an early step in SPB duplication. Together, this analysis provides important insights into the interaction network that inserts the new SPB into the NE and indicates that the Mps2-Bbp1 complex is the central unit of the SPB membrane anchor.

title: Targeting of Nbp1 to the inner nuclear membrane is essential for spindle pole body duplication
abstract: Spindle pole bodies (SPBs), like nuclear pore complexes, are embedded in the nuclear envelope (NE) at sites of fusion of the inner and outer nuclear membranes. A network of interacting proteins is required to insert a cytoplasmic SPB precursor into the NE. A central player of this network is Nbp1 that interacts with the conserved integral membrane protein Ndc1. Here, we establish that Nbp1 is a monotopic membrane protein that is essential for SPB insertion at the inner face of the NE. In vitro and in vivo studies identified an N‐terminal amphipathic α‐helix of Nbp1 as a membrane‐binding element, with crucial functions in SPB duplication. The karyopherin Kap123 binds to a nuclear localization sequence next to this amphipathic α‐helix and prevents unspecific tethering of Nbp1 to membranes. After transport into the nucleus, Nbp1 binds to the inner nuclear membrane. These data define the targeting pathway of a SPB component and suggest that the amphipathic α‐helix of Nbp1 is important for SPB insertion into the NE from within the nucleus.

title: Amphipathic helices and membrane curvature
abstract: Numerous data have been collected on lipid‐binding amphipathic helices involved in membrane‐remodeling machineries and vesicular transport. Here we describe how, with regard to lipid composition, the physicochemical features of some amphipathic helices explain their ability to recognize membrane curvature or to participate in membrane remodeling. We propose that sensing highly‐curved membranes requires that the polar and hydrophobic faces of the helix do not cooperate in lipid binding. A more detailed description of the interaction between amphipathic helices and lipids is however needed; notably to explain how new helices contribute to detection of modest changes in curvature or even negative curvature.

title: Mitosis in the fission yeast Schizosaccharomyces pombe as revealed by freeze-substitution electron microscopy.
abstract: Nuclear division in Schizosaccharomyces pombe has been studied in transmission electron micrographs of sections of cells fixed by a method of freeze-substitution. We have found cytoplasmic microtubules in the vicinity of the spindle pole bodies and two kinds of microtubules, short discontinuous ones and long, parallel ones in the intranuclear mitotic spindle. For most of the time taken by nuclear division the spindle pole bodies face each other squarely across the nuclear space but early in mitosis they briefly appear twisted out of alignment with each other, thereby imparting a sigmoidal shape to the bundle of spindle microtubules extending between them. This configuration is interpreted as indicating active participation of the spindle in the initial elongation of the dividing nucleus. It is proposed that mitosis is accompanied by the shortening of chromosomal microtubules simultaneously with the elongation of the central pole-to-pole bundle of microtubules of the intranuclear spindle. Daughter nuclei are separated by the sliding apart of interdigitating microtubules of the spindle at telophase. Some of the latter bear dense knobs at their ends.

title: Spindle pole body duplication in fission yeast occurs at the G1/S boundary but maturation is blocked until exit from S by an event downstream of cdc10+.
abstract: The regulation and timing of spindle pole body (SPB) duplication and maturation in fission yeast was examined by transmission electron microscopy. When cells are arrested at G1 by nitrogen starvation, the SPB is unduplicated. On release from G1, the SPBs were duplicated after 1-2 h. In cells arrested at S by hydroxyurea, SPBs are duplicated but not mature. In G1 arrest/release experiments with cdc2.33 cells at the restrictive temperature, SPBs remained single, whereas in cells at the permissive temperature, SPBs were duplicated. In cdc10 mutant cells, the SPBs seem not only to be duplicated but also to undergo partial maturation, including invagination of the nuclear envelope underneath the SPB. There may be an S-phase-specific inhibitor of SPB maturation whose expression is under control of cdc10(+). This model was examined by induction of overreplication of the genome by overexpression of rum1p or cdc18p. In cdc18p-overexpressing cells, the SPBs are duplicated but not mature, suggesting that cdc18p is one component of this feedback mechanism. In contrast, cells overexpressing rum1p have large, deformed SPBs accompanied by other features of maturation and duplication. We propose a feedback mechanism for maturation of the SPB that is coupled with exit from S to trigger morphological changes.

title: Monitoring SPB biogenesis in fission yeast with high resolution and quantitative fluorescent microscopy.
abstract: Like centrosomes, yeast spindle pole bodies (SPBs) undergo a tightly controlled duplication cycle in order to restrict their number to one or two per cell and promote the assembly of a bipolar spindle at mitotic entry. This conservative duplication cycle is tightly coordinated with cell cycle progression although the mechanisms that ensure this coordination remain largely unknown. In this chapter, we describe simple high resolution microscopy- and quantitative light microscopy-based methods that allow to monitor SPB biogenesis in fission yeast and may be useful to study the molecular pathways controlling the successive phases of the duplication cycle.

title: Molecular model of fission yeast centrosome assembly determined by superresolution imaging
abstract: Microtubule-organizing centers (MTOCs), known as centrosomes in animals and spindle pole bodies (SPBs) in fungi, are important for the faithful distribution of chromosomes between daughter cells during mitosis as well as for other cellular functions. The cytoplasmic duplication cycle and regulation of the Schizosaccharomyces pombe SPB is analogous to centrosomes, making it an ideal model to study MTOC assembly. Here, we use superresolution structured illumination microscopy with single-particle averaging to localize 14 S. pombe SPB components and regulators, determining both the relationship of proteins to each other within the SPB and how each protein is assembled into a new structure during SPB duplication. These data enabled us to build the first comprehensive molecular model of the S. pombe SPB, resulting in structural and functional insights not ascertained through investigations of individual subunits, including functional similarities between Ppc89 and the budding yeast SPB scaffold Spc42, distribution of Sad1 to a ring-like structure and multiple modes of Mto1 recruitment.

title: Cell cycle control of spindle pole body duplication and splitting by Sfi1 and Cdc31 in fission yeast
abstract: Spindle pole biogenesis and segregation are tightly coordinated to produce a bipolar mitotic spindle. In yeasts, the spindle pole body (SPB) half‐bridge composed of Sfi1 and Cdc31 duplicates to promote the biogenesis of a second SPB. Sfi1 accumulates at the half‐bridge in two phases in Schizosaccharomyces pombe, from anaphase to early septation and throughout G2 phase. We found that the function of Sfi1–Cdc31 in SPB duplication is accomplished before septation ends and G2 accumulation starts. Thus, Sfi1 early accumulation at mitotic exit might correspond to half‐bridge duplication. We further show that Cdc31 phosphorylation on serine 15 in a Cdk1 (encoded by cdc2) consensus site is required for the dissociation of a significant pool of Sfi1 from the bridge and timely segregation of SPBs at mitotic onset. This suggests that the Cdc31 N‐terminus modulates the stability of Sfi1–Cdc31 arrays in fission yeast, and impacts on the timing of establishment of spindle bipolarity.

title: Fission yeast cdc31p is a component of the half-bridge and controls SPB duplication.
abstract: The fission yeast spindle pole body (SPB) is a nucleus-associated organelle that duplicates once each cell cycle during interphase. Duplicated SPBs serve as the poles of an intranuclear mitotic spindle after their insertion into the nuclear envelope in mitosis (Ding et al., ). Here, we report the identification and characterization of Schizosaccharomyces pombe cdc31p, a member of the conserved calcium-binding centrin/CDC31 family. Immunofluorescence and immunoelectron microscopy show that cdc31p is a SPB component localized at the half-bridge structure of the SPB. cdc31 is an essential gene and Deltacdc31 cells and cdc31 conditional mutant cells arrest in mitosis with a monopolar mitotic spindle organized from a single SPB. EM analysis demonstrates that mutant cdc31 cells fail to duplicate the SPB. In addition, cdc31p exhibits genetic interactions with the SPB component sad1p and is required for sad1p localization. Finally, cdc31 mutant can undergo single or multiple rounds of septation before the exit from mitosis, suggesting that cdc31p activity or SPB duplication may be required for the proper coordination between the exit from mitosis and the initiation of septation.

title: Ppc89 links multiple proteins, including the septation initiation network, to the core of the fission yeast spindle-pole body.
abstract: The spindle-pole body (SPB), the yeast analog of the centrosome, serves as the major microtubule (MT) organizing center in the yeast cell. In addition to this central function, the SPB organizes and concentrates proteins required for proper coordination between the nuclear-division cycle and cytokinesis. For example, the Schizosaccharomyces pombe septation-initiation network (SIN), which is responsible for initiating actomyosin ring constriction and septation, is assembled at the SPB through its two scaffolding components, Sid4 and Cdc11. In an effort to identify novel SIN interactors, we purified Cdc11 and identified by mass spectrometry a previously uncharacterized protein associated with it, Ppc89. Ppc89 localizes constitutively to the SPB and interacts directly with Sid4. A fusion between the N-terminal 300 amino acids of Sid4 and a SPB targeting domain of Ppc89 supplies the essential function of Sid4 in anchoring the SIN. ppc89Delta cells are inviable and exhibit defects in SPB integrity, and hence in spindle formation, chromosome segregation, and SIN localization. Ppc89 overproduction is lethal, resulting primarily in a G2 arrest accompanied by massive enlargement of the SPB and increased SPB MT nucleation. These results suggest a fundamental role for Ppc89 in organization of the S. pombe SPB.

title: cut11(+): A gene required for cell cycle-dependent spindle pole body anchoring in the nuclear envelope and bipolar spindle formation in Schizosaccharomyces pombe.
abstract: The "cut" mutants of Schizosaccharomyces pombe are defective in spindle formation and/or chromosome segregation, but they proceed through the cell cycle, resulting in lethality. Analysis of temperature-sensitive alleles of cut11(+) suggests that this gene is required for the formation of a functional bipolar spindle. Defective spindle structure was revealed with fluorescent probes for tubulin and DNA. Three-dimensional reconstruction of mutant spindles by serial sectioning and electron microscopy showed that the spindle pole bodies (SPBs) either failed to complete normal duplication or were free floating in the nucleoplasm. Localization of Cut11p tagged with the green fluorescent protein showed punctate nuclear envelope staining throughout the cell cycle and SPBs staining from early prophase to mid anaphase. This SPB localization correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope. Immunoelectron microscopy confirmed the localization of Cut11p to mitotic SPBs and nuclear pore complexes. Cloning and sequencing showed that cut11(+) encodes a novel protein with seven putative membrane-spanning domains and homology to the Saccharomyces cerevisiae gene NDC1. These data suggest that Cut11p associates with nuclear pore complexes and mitotic SPBs as an anchor in the nuclear envelope; this role is essential for mitosis.

title: The fission yeast SPB component Cut12 links bipolar spindle formation to mitotic control.
abstract: During fission yeast mitosis, the duplicated spindle pole bodies (SPBs) nucleate microtubule arrays that interdigitate to form the mitotic spindle. cut12.1 mutants form a monopolar mitotic spindle, chromosome segregation fails, and the mutant undergoes a lethal cytokinesis. The cut12(+) gene encodes a novel 62-kD protein with two predicted coiled coil regions, and one consensus phosphorylation site for p34(cdc2) and two for MAP kinase. Cut12 is localized to the SPB throughout the cell cycle, predominantly around the inner face of the interphase SPB, adjacent to the nucleus. cut12(+) is allelic to stf1(+); stf1.1 is a gain-of-function mutation bypassing the requirement for the Cdc25 tyrosine phosphatase, which normally dephosphorylates and activates the p34(cdc2)/cyclin B kinase to promote the onset of mitosis. Expressing a cut12(+) cDNA carrying the stf1.1 mutation also suppressed cdc25.22. The spindle defect in cut12.1 is exacerbated by the cdc25.22 mutation, and stf1.1 cells formed defective spindles in a cdc25.22 background at high temperatures. We propose that Cut12 may be a regulator or substrate of the p34(cdc2) mitotic kinase.

title: The spindle pole body plays a key role in controlling mitotic commitment in the fission yeast Schizosaccharomyces pombe.
abstract: Commitment to mitosis is regulated by a conserved protein kinase complex called MPF (mitosis-promoting factor). MPF activation triggers a positive-feedback loop that further promotes the activity of its activating phosphatase Cdc25 and is assumed to down-regulate the MPF-inhibitory kinase Wee1. Four protein kinases contribute to this amplification loop: MPF itself, Polo kinase, MAPK (mitogen-activated protein kinase) and Greatwall kinase. The fission yeast SPB (spindle pole body) component Cut12 plays a critical role in modulating mitotic commitment. In this review, I discuss the relationship between Cut12 and the fission yeast Polo kinase Plo1 in mitotic control. These results indicate that commitment to mitosis is co-ordinated by control networks on the spindle pole. I then describe how the Cut12/Plo1 control network links growth control signalling from TOR (target of rapamycin) and MAPK networks to the activation of MPF to regulate the timing of cell division.

title: The S. pombe mitotic regulator Cut12 promotes spindle pole body activation and integration into the nuclear envelope
abstract: The fission yeast spindle pole body (SPB) comprises a cytoplasmic structure that is separated from an ill-defined nuclear component by the nuclear envelope. Upon mitotic commitment, the nuclear envelope separating these domains disperses as the two SPBs integrate into a hole that forms in the nuclear envelope. The SPB component Cut12 is linked to cell cycle control, as dominant cut12.s11 mutations suppress the mitotic commitment defect of cdc25.22 cells and elevated Cdc25 levels suppress the monopolar spindle phenotype of cut12.1 loss of function mutations. We show that the cut12.1 monopolar phenotype arises from a failure to activate and integrate the new SPB into the nuclear envelope. The activation of the old SPB was frequently delayed, and its integration into the nuclear envelope was defective, resulting in leakage of the nucleoplasm into the cytoplasm through large gaps in the nuclear envelope. We propose that these activation/integration defects arise from a local deficiency in mitosis-promoting factor activation at the new SPB.

title: The product of the spindle formation gene sad1+ associates with the fission yeast spindle pole body and is essential for viability
abstract: Spindle formation in fission yeast occurs by the interdigitation of two microtubule arrays extending from duplicated spindle pole bodies which span the nuclear membrane. By screening a bank of temperature-sensitive mutants by anti-tubulin immunofluorescence microscopy, we previously identified the sad1.1 mutation (Hagan, I., and M. Yanagida. 1990. . Here we describe the isolation and characterization of the sad1+ gene. We show that the sad1.1 mutation affected both spindle formation and function. The sad1+ gene is a novel essential gene that encodes a protein with a predicted molecular mass of 58 kD. Deletion of the gene was lethal resulting in identical phenotypes to the sad1.1 mutation. Sequence analysis predicted a potential membrane-spanning domain and an acidic amino terminus. Sad1 protein migrated as two bands of 82 and 84 kD on SDS-PAGE, considerably slower than its predicted mobility, and was exclusively associated with the spindle pole body (SPB) throughout the mitotic and meiotic cycles. Microtubule integrity was not required for Sad1 association with the SPB. Upon the differentiation of the SPB in metaphase of meiosis II, Sad1-staining patterns similarly changed from a dot to a crescent supporting an integral role in SPB function. Moderate overexpression of Sad1 led to association with the nuclear periphery. As Sad1 was not detected in the cytoplasmic microtubule-organizing centers activated at the end of anaphase or kinetochores, we suggest that Sad1 is not a general component of microtubule-interacting structures per se, but is an essential mitotic component that associates with the SPB but is not required for microtubule nucleation. Sad1 may play a role in SPB structure, such as maintaining a functional interface with the nuclear membrane or in providing an anchor for the attachment of microtubule motor proteins.

title: Two-hybrid search for proteins that interact with Sad1 and Kms1, two membrane-bound components of the spindle pole body in fission yeast
abstract: In interphase cells of fission yeast, the spindle pole body (SPB) is thought to be connected with chromosomal centromeres by an as yet unknown mechanism that spans the nuclear membrane. To elucidate this mechanism, we performed two-hybrid screens for proteins that interact with Kms1 and Sad1, which are constitutive membrane-bound components of the SPB that interact with each other. Seven and 26 genes were identified whose products potentially interact with Kms1 and Sad1, respectively. With the exception of Dlc1 (a homolog of the 14-kDa dynein light chain), all of the Kms1 interactors also interacted with Sad1. Among the genes identified were the previously known genes rhp9+/ crb2+, cut6+, ags1+/ mok1+, gst3+, kms2+, and sid4+. The products of kms2+ and sid4+ localize to the SPB. The novel genes were characterized by constructing disruption mutations and by localization of the gene products. Two of them, putative homologues of budding yeast UFE1 (which encodes a t-SNARE) and SFH1 (an essential component of a chromatin-remodeling complex), were essential for viability. Two further genes, which were only conditionally essential, genetically interact with sad1+. One of these was named sif1+ (for Sad1-interacting factor) and is required for proper septum formation at high temperature. Cells in which this gene was overexpressed displayed a wee -like phenotype. The product of the other gene, apm1+, is very similar to the medium chain of an adaptor protein complex in clathrin-coated vesicles. Apm1 appears to be required for SPB separation and spindle formation, and tended to accumulate at the SPB when it was overproduced. It was functionally distinct from its homologues Apm2 and Apm4. Other novel genes identified in this study included one for a nucleoporin and genes encoding novel membrane-bound proteins that were genetically related to Sad1. We found that none of the newly identified genes tested were necessary for centromere/telomere clustering.

title: The KASH protein Kms2 coordinates mitotic remodeling of the spindle pole body
abstract: Defects in the biogenesis of the spindle pole body (SPB), the yeast centrosome equivalent, can lead to monopolar spindles and mitotic catastrophe. The KASH domain protein Kms2 and the SUN domain protein Sad1 colocalize within the nuclear envelope at the site of SPB attachment during interphase and at the spindle poles during mitosis in Schizosaccharomyces pombe. We show that Kms2 interacts with the essential SPB components Cut12 and Pcp1 and the Polo kinase Plo1. Depletion of Kms2 delays mitotic entry and leads to defects in the insertion of the SPB into the nuclear envelope, disrupting stable bipolar spindle formation. These effects are mediated in part by a delay in the recruitment of Plo1 to the SPB at mitotic entry. Plo1 activity supports mitotic SPB remodeling by driving a burst of incorporation of Cut12 and Pcp1. Thus, a fission yeast SUN–KASH complex plays an important role in supporting the remodeling of the SPB at mitotic entry.

title: Cell cycle-dependent specific positioning and clustering of centromeres and telomeres in fission yeast
abstract: Fluorescence in situ hybridization (FISH) shows that fission yeast centromeres and telomeres make up specific spatial arrangements in the nucleus. Their positioning and clustering are cell cycle regulated. In G2, centromeres cluster adjacent to the spindle pole body (SPB), while in mitosis, their association with each other and with the SPB is disrupted. Similarly, telomeres cluster at the nuclear periphery in G2 and their associations are disrupted in mitosis. Mitotic centromeres interact with the spindle. They remain undivided until the spindle reaches a critical length, then separate and move towards the poles. This demonstrated, for the first time, that anaphase A occurs in fission yeast. The mode of anaphase A and B is similar to that of higher eukaryotes. In nda3 and cut7 mutants defective in tubulin of a kinesin-related motor, cells are blocked in early stages of mitosis due to the absence of the spindle, and centromeres dissociate but remain close to the SPB, whereas in a metaphase-arrested nuc2 mutant, they reside at the middle of the spindle. FISH is therefore a powerful tool for analyzing mitotic chromosome movement and disjunction using various mutants. Surprisingly, in top2 defective in DNA topoisomerase II, while most chromatid DNAs remain undivided, sister centromeres are separated. Significance of this finding is discussed. In contrast, most chromatid DNAs are separated but telomeric DNAs are not in cut1 mutant. In cut1, the dependence of SPB duplication on the completion of mitosis is abolished. In crm1 mutant cells defective in higher-order chromosome organization, the interphase arrangements of centromeres and telomeres are disrupted.

title: Telomere-led premeiotic chromosome movement in fission yeast.
abstract: The movement of chromosomes that precedes meiosis was observed in living cells of fission yeast by fluorescence microscopy. Further analysis by in situ hybridization revealed that the telomeres remain clustered at the leading end of premeiotic chromosome movement, unlike mitotic chromosome movement in which the centromere leads. Once meiotic chromosome segregation starts, however, centromeres resume the leading position in chromosome movement, as they do in mitosis. Although the movement of the telomere first has not been observed before, the clustering of telomeres is reminiscent of the bouquet structure of meiotic-prophase chromosomes observed in higher eukaryotes, which suggests that telomeres perform specific functions required for premeiotic chromosomal events generally in eukaryotes.

title: Meiotic Proteins Bqt1 and Bqt2 Tether Telomeres to Form the Bouquet Arrangement of Chromosomes
abstract: In many organisms, meiotic chromosomes are bundled at their telomeres to form a "bouquet" arrangement. The bouquet formation plays an important role in homologous chromosome pairing and therefore progression of meiosis. As meiotic telomere clustering occurs in response to mating pheromone signaling in fission yeast, we looked for factors essential for bouquet formation among genes induced under mating pheromone signaling. This genome-wide search identified two proteins, Bqt1 and Bqt2, that connect telomeres to the spindle-pole body (SPB; the centrosome equivalent in fungi). Neither Bqt1 nor Bqt2 alone functions as a connector, but together the two proteins form a bridge between Rap1 (a telomere protein) and Sad1 (an SPB protein). Significantly, when both Bqt1 and Bqt2 are ectopically expressed in mitotic cells, they also form a bridge between Rap1 and Sad1. Thus, a complex including Bqt1 and Bqt2 is essential for connecting telomeres to the SPB.

title: Mitotic Nuclear Envelope Breakdown and Spindle Nucleation Are Controlled by Interphase Contacts between Centromeres and the Nuclear Envelope.
abstract: Faithful genome propagation requires coordination between nuclear envelope (NE) breakdown, spindle formation, and chromosomal events. The conserved linker of nucleoskeleton and cytoskeleton (LINC) complex connects fission yeast centromeres and the centrosome, across the NE, during interphase. During meiosis, LINC connects the centrosome with telomeres rather than centromeres. We previously showed that loss of telomere-LINC contacts compromises meiotic spindle formation. Here, we define the precise events regulated by telomere-LINC contacts and address the analogous possibility that centromeres regulate mitotic spindle formation. We develop conditionally inactivated LINC complexes in which the conserved SUN-domain protein Sad1 remains stable but severs interphase centromere-LINC contacts. Strikingly, the loss of such contacts abolishes spindle formation. We pinpoint the defect to a failure in the partial NE breakdown required for centrosome insertion into the NE, a step analogous to mammalian NE breakdown. Thus, interphase chromosome-LINC contacts constitute a cell-cycle control device linking nucleoplasmic and cytoplasmic events.

title: Brr6 drives the Schizosaccharomyces pombe spindle pole body nuclear envelope insertion/extrusion cycle
abstract: The fission yeast interphase spindle pole body (SPB) is a bipartite structure in which a bulky cytoplasmic domain is separated from a nuclear component by the nuclear envelope. During mitosis, the SPB is incorporated into a fenestra that forms within the envelope during mitotic commitment. Closure of this fenestra during anaphase B/mitotic exit returns the cytoplasmic component to the cytoplasmic face of an intact interphase nuclear envelope. Here we show that Brr6 is transiently recruited to SPBs at both SPB insertion and extrusion. Brr6 is required for both SPB insertion and nuclear envelope integrity during anaphase B/mitotic exit. Genetic interactions with apq12 and defective sterol assimilation suggest that Brr6 may alter envelope composition at SPBs to promote SPB insertion and extrusion. The restriction of the Brr6 domain to eukaryotes that use a polar fenestra in an otherwise closed mitosis suggests a conserved role in fenestration to enable a single microtubule organizing center to nucleate both cytoplasmic and nuclear microtubules on opposing sides of the nuclear envelope.

title: Architecture of the Xenopus nuclear pore complex revealed by three- dimensional cryo-electron microscopy
abstract: The nuclear pore complex spans the nuclear envelope and functions as a macromolecular transporter in the ATP-dependent process of nucleocytoplasmic transport. In this report, we present three dimensional (3D) structures for both membrane-associated and detergent- extracted Xenopus NPCs, imaged in frozen buffers by cryo-electron microscopy. A comparison of the differing configurations present in the 3D maps suggests that the spokes may possess an intrinsic conformational flexibility. When combined with recent data from a 3D map of negatively stained NPCs (Hinshaw, J. E., B. O. Carragher, and R. A. Milligan. ), these observations suggest a minimal domain model for the spoke-ring complex which may account for the observed plasticity of this assembly. Moreover, lumenal domains in adjacent spokes are interconnected by radial arm dimers, forming a lumenal ring that may be responsible for anchoring the NPC within the nuclear envelope pore. Importantly, the NPC transporter is visualized as a centrally tapered cylinder that spans the entire width of the NPC, in a direction normal to the nuclear envelope. The central positioning, tripartite structure, and hollow nature of the transporter suggests that it may form a macromolecular transport channel, with a globular gating domain at each end. Finally, the packing of the transporter within the spokes creates a set of eight internal channels that may be responsible, in part, for the diffusion of ions and small molecules across the nuclear envelope.

title: Super-resolution imaging visualizes the eightfold symmetry of gp210 proteins around the nuclear pore complex and resolves the central channel with nanometer resolution
abstract: One of the most complex molecular machines of cells is the nuclear pore complex (NPC), which controls all trafficking of molecules in and out of the nucleus. Because of their importance for cellular processes such as gene expression and cytoskeleton organization, the structure of NPCs has been studied extensively during the last few decades, mainly by electron microscopy. We have used super-resolution imaging by direct stochastic optical reconstruction microscopy (dSTORM) to investigate the structure of NPCs in isolated Xenopus laevis oocyte nuclear envelopes, with a lateral resolution of ~15 nm. By generating accumulated super-resolved images of hundreds of NPCs we determined the diameter of the central NPC channel to be 41±7 nm and demonstrate that the integral membrane protein gp210 is distributed in an eightfold radial symmetry. Two-color dSTORM experiments emphasize the highly symmetric NPCs as ideal model structures to control the quality of corrections to chromatic aberration and to test the capability and reliability of super-resolution imaging methods.

title: Quantifying Nucleoporin Stoichiometry Inside Single Nuclear Pore Complexes In vivo
abstract: The nuclear pore complex (NPC) is one of the largest supramolecular structures in eukaryotic cells. Its octagonal ring-scaffold perforates the nuclear envelope and features a unique molecular machinery that regulates nucleocytoplasmic transport. NPCs are composed of ~30 different nucleoporins (Nups), averaged at 8, 16 or 32 copies per NPC. This estimate has not been confirmed for individual NPCs in living cells due to the inherent difficulty of counting proteins inside single supramolecular complexes. Here we used single-molecule SPEED microscopy to directly count the copy-number of twenty-four different Nups within individual NPCs of live yeast, and found agreement as well as significant deviation from previous estimates. As expected, we counted 8 copies of four peripheral Nups and 16 copies of fourteen scaffold Nups. Unexpectedly, we counted a maximum of 16 copies of Nsp1 and Nic96, rather than 32 as previously estimated; and found only 10–15 copies of six other Nups, rather than 8 or 16 copies as expected. This in situ molecular-counting technology can test structure-function models of NPCs and other supramolecular structures in cells.

title: POM152 is an integral protein of the pore membrane domain of the yeast nuclear envelope
abstract: We have identified a concanavalin A-reactive glycoprotein of 150 kD that coenriches with isolated yeast nuclear pore complexes. Molecular cloning and sequencing of this protein revealed a single canonical transmembrane segment. Epitope tagging and localization by both immunofluorescence and immunoelectron microscopy confirmed that it is a pore membrane protein. The protein was termed POM152 (for pore membrane protein of 152 kD) on the basis of its location and cDNA-deduced molecular mass. POM152 is likely to be a type II membrane protein with its NH2-terminal region (175 residues) and its COOH-terminal region (1,142 residues) positioned on the pore side and cisternal side of the pore membrane, respectively. The proposed cisternally exposed domain contains eight repetitive motifs of approximately 24 residues. Surprisingly, POM152 deletion mutants were viable and their growth rate was indistinguishable from that of wild-type cells at temperatures between 17 and 37 degrees C. However, overproduction of POM152 inhibited cell growth. When expressed in mouse 3T3 cells, POM152 was found to be localized to the pore membrane, suggesting a conserved sorting pathway between yeast and mammals.

title: Saccharomyces cerevisiae Ndc1p Is a Shared Component of Nuclear Pore Complexes and Spindle Pole Bodies
abstract: We report a novel connection between nuclear pore complexes (NPCs) and spindle pole bodies (SPBs) revealed by our studies of the Saccharomyces cerevisiae NDC1 gene. Although both NPCs and SPBs are embedded in the nuclear envelope (NE) in yeast, their known functions are quite distinct. Previous work demonstrated that NDC1 function is required for proper SPB duplication (Winey, M., M.A. Hoyt, C. Chan, L. Goetsch, D. Botstein, and B. Byers. 1993. ). Here, we show that Ndc1p is a membrane protein of the NE that localizes to both NPCs and SPBs. Indirect immunofluorescence microscopy shows that Ndc1p displays punctate, nuclear peripheral localization that colocalizes with a known NPC component, Nup49p. Additionally, distinct spots of Ndc1p localization colocalize with a known SPB component, Spc42p. Immunoelectron microscopy shows that Ndc1p localizes to the regions of NPCs and SPBs that interact with the NE. The NPCs in ndc1-1 mutant cells appear to function normally at the nonpermissive temperature. Finally, we have found that a deletion of POM152, which encodes an abundant but nonessential nucleoporin, suppresses the SPB duplication defect associated with a mutation in the NDC1 gene. We show that Ndc1p is a shared component of NPCs and SPBs and propose a shared function in the assembly of these organelles into the NE.

title: The Integral Membrane Protein Pom34p Functionally Links Nucleoporin Subcomplexes
abstract: Here we have examined the function of Pom34p, a novel membrane protein in Saccharomyces cerevisiae, localized to nuclear pore complexes (NPCs). Membrane topology analysis revealed that Pom34p is a double-pass transmembrane protein with both the amino (N) and carboxy (C) termini positioned on the cytosolic/pore face. The network of genetic interactions between POM34 and genes encoding other nucleoporins was established and showed specific links between Pom34p function and Nup170p, Nup188p, Nup59p, Gle2p, Nup159p, and Nup82p. The transmembrane domains of Pom34p in addition to either the N- or C-terminal region were necessary for its function in different double mutants. We further characterized the pom34ΔN nup188Δ mutant and found it to be perturbed in both NPC structure and function. Mislocalization of a subset of nucleoporins harboring phenylalanine–glycine repeats was observed, and nuclear import capacity for the Kap104p and Kap121p pathways was inhibited. In contrast, the pom34Δ pom152Δ double mutant was viable at all temperatures and showed no such defects. Interestingly, POM152 overexpression suppressed the synthetic lethality of pom34Δ nup170Δ and pom34Δ nup59Δ mutants. We speculate that multiple integral membrane proteins, either within the nuclear pore domain or in the nuclear envelope, execute coordinated roles in NPC structure and function.

title: The molecular architecture of the nuclear pore complex
abstract: Nuclear pore complexes (NPCs) are proteinaceous assemblies of approximately 50 MDa that selectively transport cargoes across the nuclear envelope. To determine the molecular architecture of the yeast NPC, we collected a diverse set of biophysical and proteomic data, and developed a method for using these data to localize the NPC’s 456 constituent proteins (see the accompanying paper). Our structure reveals that half of the NPC is made up of a core scaffold, which is structurally analogous to vesicle-coating complexes. This scaffold forms an interlaced network that coats the entire curved surface of the nuclear envelope membrane within which the NPC is embedded. The selective barrier for transport is formed by large numbers of proteins with disordered regions that line the inner face of the scaffold. The NPC consists of only a few structural modules that resemble each other in terms of the configuration of their homologous constituents, the most striking of these being a 16-fold repetition of ‘columns’. These findings provide clues to the evolutionary origins of the NPC.

title: Role of the Ndc1 interaction network in yeast nuclear pore complex assembly and maintenance
abstract: The nuclear pore complex (NPC) mediates all nucleocytoplasmic transport, yet its structure and biogenesis remain poorly understood. In this study, we have functionally characterized interaction partners of the yeast transmembrane nucleoporin Ndc1. Ndc1 forms a distinct complex with the transmembrane proteins Pom152 and Pom34 and two alternative complexes with the soluble nucleoporins Nup53 and Nup59, which in turn bind to Nup170 and Nup157. The transmembrane and soluble Ndc1-binding partners have redundant functions at the NPC, and disruption of both groups of interactions causes defects in Ndc1 targeting and in NPC structure accompanied by significant pore dilation. Using photoconvertible fluorescent protein fusions, we further show that the depletion of Pom34 in cells that lack NUP53 and NUP59 blocks new NPC assembly and leads to the reversible accumulation of newly made nucleoporins in cytoplasmic foci. Therefore, Ndc1 together with its interaction partners are collectively essential for the biosynthesis and structural integrity of yeast NPCs.

title: Nuclear pore complexes exceeding eightfold rotational symmetry.
abstract: Nuclear pore complexes are rotationally symmetric structures that span the nuclear envelope and provide channels for nucleocytoplasmic traffic. These large complexes normally consist of eight spokes arranged around a central channel, although, occasionally, 9- and 10-fold nuclear pore complexes are found in preparations of Xenopus oocyte macronuclei. Here we examine these unusual nuclear pore complexes by negative stain electron microscopy and image analysis and compare the results with data previously obtained from 8-fold structures. The details in two-dimensional and three-dimensional maps indicate that the substructure of the spoke is the same in 8-, 9- and 10-fold nuclear pore complexes: therefore, the spoke is likely an immutable structural component. In all three variant forms, the spacing between adjacent annular subunits, which surround the central channel, is identical. Distances between spokes at higher radius decrease in the 9- and 10-fold nuclear pore complexes. These data imply that the most important connections holding the nuclear pore complex together are those between adjacent annular subunits and that these interactions may play a predominant role in nuclear pore complex assembly. Circumferential connections mediated by ring subunits and radial arms presumably further stabilize the structure and are flexible enough to accommodate additional spokes.

title: Nuclear Pores Form de Novo from Both Sides of the Nuclear Envelope
abstract: Nuclear pore complexes are multiprotein channels that span the double lipid bilayer of the nuclear envelope. How new pores are inserted into the intact nuclear envelope of proliferating and differentiating eukaryotic cells is unknown. We found that the Nup107-160 complex was incorporated into assembly sites in the nuclear envelope from both the nucleoplasmic and the cytoplasmic sides. Nuclear pore insertion required the generation of Ran guanosine triphosphate in the nuclear and cytoplasmic compartments. Newly formed nuclear pore complexes did not contain structural components of preexisting pores, suggesting that they can form de novo.

title: Nuclear pore assembly proceeds by an inside-out extrusion of the nuclear envelope
abstract: The nuclear pore complex (NPC) mediates nucleocytoplasmic transport through the nuclear envelope. How the NPC assembles into this double membrane boundary has remained enigmatic. Here, we captured temporally staged assembly intermediates by correlating live cell imaging with high-resolution electron tomography and super-resolution microscopy. Intermediates were dome-shaped evaginations of the inner nuclear membrane (INM), that grew in diameter and depth until they fused with the flat outer nuclear membrane. Live and super-resolved fluorescence microscopy revealed the molecular maturation of the intermediates, which initially contained the nuclear and cytoplasmic ring component Nup107, and only later the cytoplasmic filament component Nup358. EM particle averaging showed that the evagination base was surrounded by an 8-fold rotationally symmetric ring structure from the beginning and that a growing mushroom-shaped density was continuously associated with the deforming membrane. Quantitative structural analysis revealed that interphase NPC assembly proceeds by an asymmetric inside-out extrusion of the INM.

title: Brr6 and Brl1 locate to nuclear pore complex assembly sites to promote their biogenesis
abstract: The paralogous Brr6 and Brl1 are conserved integral membrane proteins of the nuclear envelope (NE) with an unclear role in nuclear pore complex (NPC) biogenesis. Here, we analyzed double-degron mutants of Brr6/Brl1 to understand this function. Depletion of Brr6 and Brl1 caused defects in NPC biogenesis, whereas the already assembled NPCs remained unaffected. This NPC biogenesis defect was not accompanied by a change in lipid composition. However, Brl1 interacted with Ndc1 and Nup188 by immunoprecipitation, and with transmembrane and outer and inner ring NPC components by split yellow fluorescent protein analysis, indicating a direct role in NPC biogenesis. Consistently, we found that Brr6 and Brl1 associated with a subpopulation of NPCs and emerging NPC assembly sites. Moreover, BRL1 overexpression affected NE morphology without a change in lipid composition and completely suppressed the nuclear pore biogenesis defect of ; and gle2&Dgr; cells. We propose that Brr6 and Brl1 transiently associate with NPC assembly sites where they promote NPC biogenesis.

title: A comprehensive two-hybrid analysis to explore the yeast protein interactome
abstract: Protein–protein interactions play crucial roles in the execution of various biological functions. Accordingly, their comprehensive description would contribute considerably to the functional interpretation of fully sequenced genomes, which are flooded with novel genes of unpredictable functions. We previously developed a system to examine two-hybrid interactions in all possible combinations between the ≈6,000 proteins of the budding yeast Saccharomyces cerevisiae. Here we have completed the comprehensive analysis using this system to identify 4,549 two-hybrid interactions among 3,278 proteins. Unexpectedly, these data do not largely overlap with those obtained by the other project [Uetz, P., et al. (2000) Nature (London) 403, 623–627] and hence have substantially expanded our knowledge on the protein interaction space or interactome of the yeast. Cumulative connection of these binary interactions generates a single huge network linking the vast majority of the proteins. Bioinformatics-aided selection of biologically relevant interactions highlights various intriguing subnetworks. They include, for instance, the one that had successfully foreseen the involvement of a novel protein in spindle pole body function as well as the one that may uncover a hitherto unidentified multiprotein complex potentially participating in the process of vesicular transport. Our data would thus significantly expand and improve the protein interaction map for the exploration of genome functions that eventually leads to thorough understanding of the cell as a molecular system.

title: A global genetic interaction network maps a wiring diagram of cellular function
abstract: Genetic interactions occur when mutations in two or more genes combine to generate an unexpected phenotype. An extreme negative or synthetic lethal genetic interaction occurs when two mutations, neither lethal individually, combine to cause cell death. Conversely, positive genetic interactions occur when two mutations produce a phenotype that is less severe than expected. Genetic interactions identify functional relationships between genes and can be harnessed for biological discovery and therapeutic target identification. They may also explain a considerable component of the undiscovered genetics associated with human diseases. Here, we describe construction and analysis of a comprehensive genetic interaction network for a eukaryotic cell.  Genome sequencing projects are providing an unprecedented view of genetic variation. However, our ability to interpret genetic information to predict inherited phenotypes remains limited, in large part due to the extensive buffering of genomes, making most individual eukaryotic genes dispensable for life. To explore the extent to which genetic interactions reveal cellular function and contribute to complex phenotypes, and to discover the general principles of genetic networks, we used automated yeast genetics to construct a global genetic interaction network.  We tested most of the ~6000 genes in the yeast Saccharomyces cerevisiae for all possible pairwise genetic interactions, identifying nearly 1 million interactions, including ~550,000 negative and ~350,000 positive interactions, spanning ~90% of all yeast genes. Essential genes were network hubs, displaying five times as many interactions as nonessential genes. The set of genetic interactions or the genetic interaction profile for a gene provides a quantitative measure of function, and a global network based on genetic interaction profile similarity revealed a hierarchy of modules reflecting the functional architecture of a cell. Negative interactions connected functionally related genes, mapped core bioprocesses, and identified pleiotropic genes, whereas positive interactions often mapped general regulatory connections associated with defects in cell cycle progression or cellular proteostasis. Importantly, the global network illustrates how coherent sets of negative or positive genetic interactions connect protein complex and pathways to map a functional wiring diagram of the cell.  A global genetic interaction network highlights the functional organization of a cell and provides a resource for predicting gene and pathway function. This network emphasizes the prevalence of genetic interactions and their potential to compound phenotypes associated with single mutations. Negative genetic interactions tend to connect functionally related genes and thus may be predicted using alternative functional information. Although less functionally informative, positive interactions may provide insights into general mechanisms of genetic suppression or resiliency. We anticipate that the ordered topology of the global genetic network, in which genetic interactions connect coherently within and between protein complexes and pathways, may be exploited to decipher genotype-to-phenotype relationships. A global network of genetic interaction profile similarities. (Left) Genes with similar genetic interaction profiles are connected in a global network, such that genes exhibiting more similar profiles are located closer to each other, whereas genes with less similar profiles are positioned farther apart. (Right) Spatial analysis of functional enrichment was used to identify and color network regions enriched for similar Gene Ontology bioprocess terms. We generated a global genetic interaction network for Saccharomyces cerevisiae, constructing more than 23 million double mutants, identifying about 550,000 negative and about 350,000 positive genetic interactions. This comprehensive network maps genetic interactions for essential gene pairs, highlighting essential genes as densely connected hubs. Genetic interaction profiles enabled assembly of a hierarchical model of cell function, including modules corresponding to protein complexes and pathways, biological processes, and cellular compartments. Negative interactions connected functionally related genes, mapped core bioprocesses, and identified pleiotropic genes, whereas positive interactions often mapped general regulatory connections among gene pairs, rather than shared functionality. The global network illustrates how coherent sets of genetic interactions connect protein complex and pathway modules to map a functional wiring diagram of the cell.

title: The nuclear pore complex–associated protein, Mlp2p, binds to the yeast spindle pole body and promotes its efficient assembly
abstract: The two yeast proteins Mlp1p and Mlp2p (homologues of the vertebrate protein Tpr) are filamentous proteins attached to the nuclear face of nuclear pore complexes. Here we perform a proteomic analysis, which reveals that the two Mlps have strikingly different interacting partners, testifying to their different roles within the cell. We find that Mlp2p binds directly to Spc110p, Spc42p, and Spc29p, which are three core components of the spindle pole body (SPB), the nuclear envelope–associated yeast spindle organizer. We further show that SPB function is compromised in mlp2 mutants. Cells lacking Mlp2p form significantly smaller SPBs, accumulate aberrant SPB component-containing structures inside the nucleus, and have stochastic failures of cell division. In addition, depletion of Mlp2p is synthetically lethal with mutants impaired in SPB assembly. Based on these data, we propose that Mlp2p links the SPB to the peripheral Mlp assembly, and that this linkage is required for efficient incorporation of components into the SPB.

title: The SESA network links duplication of the yeast centrosome with the protein translation machinery.
abstract: The yeast spindle pole body (SPB), the functional equivalent of mammalian centrosome, duplicates in G1/S phase of the cell cycle and then becomes inserted into the nuclear envelope. Here we describe a link between SPB duplication and targeted translation control. When insertion of the newly formed SPB into the nuclear envelope fails, the SESA network comprising the GYF domain protein Smy2, the translation inhibitor Eap1, the mRNA-binding protein Scp160 and the Asc1 protein, specifically inhibits initiation of translation of POM34 mRNA that encodes an integral membrane protein of the nuclear pore complex, while having no impact on other mRNAs. In response to SESA, POM34 mRNA accumulates in the cytoplasm and is not targeted to the ER for cotranslational translocation of the protein. Reduced level of Pom34 is sufficient to restore viability of mutants with defects in SPB duplication. We suggest that the SESA network provides a mechanism by which cells can regulate the translation of specific mRNAs. This regulation is used to coordinate competing events in the nuclear envelope.

title: Changes in the Nuclear Envelope Environment Affect Spindle Pole Body Duplication in Saccharomyces cerevisiae
abstract: The Saccharomyces cerevisiae nuclear membrane is part of a complex nuclear envelope environment also containing chromatin, integral and peripheral membrane proteins, and large structures such as nuclear pore complexes (NPCs) and the spindle pole body. To study how properties of the nuclear membrane affect nuclear envelope processes, we altered the nuclear membrane by deleting the SPO7 gene. We found that spo7Δ cells were sickened by the mutation of genes coding for spindle pole body components and that spo7Δ was synthetically lethal with mutations in the SUN domain gene MPS3. Mps3p is required for spindle pole body duplication and for a variety of other nuclear envelope processes. In spo7Δ cells, the spindle pole body defect of mps3 mutants was exacerbated, suggesting that nuclear membrane composition affects spindle pole body function. The synthetic lethality between spo7Δ and mps3 mutants was suppressed by deletion of specific nucleoporin genes. In fact, these gene deletions bypassed the requirement for Mps3p entirely, suggesting that under certain conditions spindle pole body duplication can occur via an Mps3p-independent pathway. These data point to an antagonistic relationship between nuclear pore complexes and the spindle pole body. We propose a model whereby nuclear pore complexes either compete with the spindle pole body for insertion into the nuclear membrane or affect spindle pole body duplication by altering the nuclear envelope environment.

title: Functional Characterization of Pneumocystis carinii brl1 by Transspecies Complementation Analysis
abstract: Pneumocystis jirovecii is a fungus which causes severe opportunistic infections in immunocompromised humans. The brl1 gene of P. carinii infecting rats was identified and characterized by using bioinformatics in conjunction with functional complementation in Saccharomyces cerevisiae and Schizosaccharomyces pombe. The ectopic expression of this gene rescues null alleles of essential nuclear membrane proteins of the Brr6/Brl1 family in both yeasts.