Integrating chromosome structure with function

Chromosome clustering in mitosis by the nuclear protein Ki-67

Ki-67 is highly expressed in proliferating cells, a characteristic that made the protein a very important proliferation marker widely used in the clinic. However, the molecular functions and properties of Ki-67 remained quite obscure for a long time. Only recently important discoveries have shed some light on its function and shown that Ki-67 has a major role in the formation of mitotic chromosome periphery compartment, it is associated with protein phosphatase one (PP1) and regulates chromatin function in interphase and mitosis. In this review, we discuss the role of Ki-67 during cell division. Specifically, we focus on the importance of Ki-67 in chromosome individualisation at mitotic entry (prometaphase) and its contribution to chromosome clustering and nuclear remodelling during mitotic exit.

Condensins and the evolution of torsion-mediated genome organization

At first glance, bacteria and eukaryotes appear to use different strategies to pack and organize their genomes. At the basal level, bacterial genome compaction relies on unconstrained, negative supercoils, whereas eukaryotic genomes are packaged into nucleosomes via constrained, negative supercoils. Here, I integrate the action of condensins, chromosome-packaging complexes conserved from bacteria to humans, into this picture, and discuss how torsional stress on DNA might have dual impacts on genome organization and function. A common theme is that organisms have evolved flexible and reversible strategies to pack their genomes while keeping them readily accessible to many activities such as gene expression.

Atomic force microscopy for imaging human metaphase chromosomes

The present study introduces the principle of atomic force microscopy (AFM) and reviews our results of human metaphase chromosomes obtained by AFM. AFM imaging of the chromosomes revealed that the chromatid arm was not uniform in structure but had ridges and grooves along its length, which was most prominent in the late metaphase. The arrangement of these ridges and grooves was roughly symmetrical with the counterpart of the paired sister chromatids. AFM imaging of banded chromosomes also showed that the ridges and grooves were related to the G/Q-positive and G/Q-negative bands, respectively. At high magnification, the chromatid was seen to be produced by the compaction of highly twisted chromatin fiber loops, and its compaction tended to be stronger in the ridged regions of the chromosomes than in the grooved regions. Our AFM studies also showed the presence of catenation of chromatin fibers between the ridged portions of the chromatid in the late metaphase. Thus, AFM is useful for obtaining the three-dimensional surface topography not only in ambient conditions but also in physiological liquid conditions, and is expected to be an attractive tool for investigating the structure of chromosomes.

The perichromosomal layer

In addition to genetic information, mitotic chromosomes transmit essential components for nuclear assembly and function in a new cell cycle. A specialized chromosome domain, called the perichromosomal layer, perichromosomal sheath, chromosomal coat, or chromosome surface domain, contains proteins required for a variety of cellular processes, including the synthesis of messenger RNA, assembly of ribosomes, repair of DNA double-strand breaks, telomere maintenance, and apoptosis regulation. The layer also contains many proteins of unknown function and is a major target in autoimmune disease. Perichromosomal proteins are found along the entire length of chromosomes, excluding centromeres, where sister chromatids are paired and spindle microtubules attach. Targeting of proteins to the perichromosomal layer occurs primarily during prophase, and they generally remain associated until telophase. During interphase, perichromosomal proteins localize to nucleoli, the nuclear envelope, nucleoplasm, heterochromatin, centromeres, telomeres, and/or the cytoplasm. It has been suggested that the perichromosomal layer may contribute to chromosome structure, as several of the associated proteins have functions in chromatin remodeling during interphase. We review the identified proteins associated with this chromosome domain and briefly discuss their known functions during interphase and mitosis.

Glycoproteins with N-acetylglucosamine and mannose residues in Chinese hamster metaphase chromosomes

Distribution of glycosylated proteins in Chinese hamster metaphase chromosomes was studied with fluorescein isothiocyanate conjugated lectins. Three substructural domains with distinct glycoprotein compositions were identified. In situ binding of the lectins Wheat germ agglutinin (WGA) and Datura stramonium agglutinin (DSA) showed that chromosomal proteins containing N-acetylglucosamine residues preferentially localize to the surface domain and the helically coiled substructure of chromatids. In Western blots, digoxigenin conjugated WGA and DSA bound to several chromosomal proteins with molecular weight ranges of 45-220 kD and 66-220 kD, respectively. Binding of Galanthus nivalis agglutinin revealed that mannosylated chromosomal proteins are enriched at the surface and G/Q band domains, and their molecular weights range from 97 to 200 kD. The carbohydrate side chain structure of these mannosylated proteins must be quite specific, as after binding of another mannose-specific lectin, Concanavalin A, only a faint fluorescence was observed in metaphase chromosomes and only one protein band of 185 kD was weakly stained in Western blots. These data suggest that chromosomal glycoproteins containing N-acetylglucosamine and mannose residues play a role in the higher order structural organization of metaphase chromosomes.

Cell and Molecular Biology of Nucleolar Assembly and Disassembly

Nucleoli disassemble in prophase of the metazoan mitotic cycle, and they begin their reassembly (nucleologenesis) in late anaphase?early telophase. Nucleolar disassembly and reassembly were obvious to the early cytologists of the eighteenth and nineteenth centuries, and although this has lead to a plethora of literature describing these events, our understanding of the molecular mechanisms regulating nucleolar assembly and disassembly has expanded immensely just within the last 10-15 years. We briefly survey the findings of nineteenth-century cytologists on nucleolar assembly and disassembly, followed by the work of Heitz and McClintock on nucleolar organizers. A primer review of nucleolar structure and functions precedes detailed descriptions of modern molecular and microscopic studies of nucleolar assembly and disassembly. Nucleologenesis is concurrent with the reinitiation of rDNA transcription in telophase. The perichromosomal sheath, prenucleolar bodies, and nucleolar-derived foci serve as repositories for nucleolar processing components used in the previous interphase. Disassembly of the perichromosomal sheath along with the dynamic movements and compositional changes of the prenucleolar bodies and nucleolus-derived foci coincide with reactivation of rDNA synthesis within the chromosomal nucleolar organizers during telophase. Nucleologenesis is considered in various model organisms to provide breadth to our understanding. Nucleolar disassembly occurs at the onset of mitosis primarily as a result of the mitosis-specific phosphorylation of Pol I transcription factors and processing components. Although we have learned much regarding nucleolar assembly and disassembly, many questions still remain, and these questions are as vibrant for us today as early questions were for nineteenth- and early twentieth-century cytologists.

Isolation and characterization of a new nucleolar protein, Nrap, that is conserved from yeast to humans

BACKGROUND

The nucleolus is the site of rRNA synthesis and processing in eukaryotic cells, but its composition remains poorly understood.

RESULTS

We have identified a novel nucleolar RNA-associated protein (Nrap) which is highly conserved from yeast (Saccharomyces cerevisiae) to human, with homologues in mouse, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Schizosaccharomyces pombe, and other species. In the mouse, we have found that Nrap is ubiquitously expressed and is specifically localized in the nucleolus. We have also identified splice variants in human and mouse, and defined the intron-exon structure of the human Nrap gene. Nrap is inherited into daughter nuclei by associating with the condensed chromosomes during mitosis. RNase treatment of permeabilized cells indicated that the nucleolar localization of Nrap is RNA dependent. The effects of actinomycin D, cycloheximide and 5,6-dichloro-beta-d-ribofuranosyl-benzimidazole on Nrap expression and distribution in cultured cells suggest that Nrap is associated with the pre-rRNA transcript.

CONCLUSIONS

Nrap is a large nucleolar protein (of more than 1000 amino acids), and is a new class of protein with new structural and functional motifs. Nrap appears to be associated with ribosome biogenesis by interacting with pre-rRNA primary transcript.

Integrating centrosome structure with protein composition and function in animal cells

The centrosome found in animal cells is a complex and dynamic organelle that functions as the major microtubule organizing center. Structural studies over the past several decades have defined the primary structural features of the centrosome but recent studies are now beginning to reveal structural detail previously unknown. Concurrent with these studies has been an explosion in the identification of the proteins that reside within the centrosome. Our growing understanding of how protein composition integrates with centrosome structure and hence with function is the focus of this review.

Essay on the nucleoli survey by the alpha- and beta-satellite DNA probes of the acrocentric chromosomes in mitogen-stimulated human lymphocytes

The two constitutive heterochromatin (alpha- and beta-satellite DNA) probes of human acrocentric chromosomes were assayed separately to label the nucleoli in the phytohemagglutinin (PHA)-stimulated human lymphocytes. Fluorescent in situ hybridisation (FISH) results have shown that: a) whole (100%) signal-nucleoli overlapping was obtained with both heterochromatin probes in maximally activated nuclei (MANs); b) partial overlapping was observed in non-activated or slightly activated nuclei; c) random signal-nucleolus overlapping (background level) was found to be approximately 6% by the NOR-irrelevant euchromatic probe (D5S23); d) Yq-nucleolus association in the MANs was found to be approximately 97% without the subtraction of the background level. We concluded that: a) acrocentric alpha- or beta-satellite DNA probes may be used as nucleolar markers only in the MANs and not in slightly activated or non-activated nuclei; b) the distances between rDNA loci and alpha-/beta-satellite DNA on human acrocentrics are short enough to permit their observation on the same nucleolus.

Subcellular trafficking of the nuclear receptor COUP-TF in the early embryonic cell cycle

The nuclear receptor SpCOUP-TF is the highly conserved sea urchin homologue of the COUP family of transcription factors. Previous results from our laboratory demonstrated that SpCOUP-TF transcripts are localized in the egg and asymmetrically distributed in the early embryonic blastomeres (A. Vlahou et al., 1996, Development 122, 521-526). To examine the subcellular localization of SpCOUP-TF protein, polyclonal antibodies were separately raised against the divergent N-terminus as well as the conserved DNA-binding and ligand-binding domains. Immunohistochemical analyses suggest that SpCOUP-TF is a maternal protein residing in the cytoplasm of the unfertilized egg. After fertilization, and as soon as the two-cell-stage embryo, most of the receptor translocates from the cytoplasm to the cell nuclei. During the rapid embryonic cell division, SpCOUP-TF was found to shuttle from the interphase nuclear periphery to the condensed chromosomes in mitosis, in a cell-cycle-dependent manner. In an attempt to confirm these observations, the subcellular localization of myc-tagged human COUP-TF I introduced into the sea urchin embryo by RNA injection of fertilized eggs was examined. The pattern of human COUP-TF I subcellular localization, detected with a monoclonal myc antibody, recapitulated the essential features described for the endogenous SpCOUP-TF trafficking. Replacement of the N-terminus of the human receptor with the unique sea urchin N-terminus enhanced its localization to the nuclear rim during interphase. Deletion of the DNA-binding domain of human COUP-TF I resulted in loss of all aspects of nuclear periphery and chromosomal localization. Taken together these data suggest that SpCOUP-TF transcriptional activity is keyed on a cell-cycle-dependent mechanism that regulates chromosomal protein traffic.

Human chromosome structure studied by scanning force microscopy after an enzymatic digestion of the covering cell material

In standard preparations, metaphase human chromosomes are covered by a cell material film composed mainly of proteins and RNA. This film (approximately 30 nm thickness) hides the chromosome structure to the tip of a scanning force microscope. In this work, a mild enzymatic treatment is applied to remove the cell material film. After treatment, the individual chromatin fibers at the surface were resolved. Furthermore, the chromosome shows a thickness modulation, in which thicker/thinner regions could be associated with G/R bands. Finally, the topography of the chromosomes in solution is presented. The chromosome volume swelled about five-fold and chromatin packaging in bands and coils was observed.

Selective cleaning of the cell debris in human chromosome preparations studied by scanning force microscopy

The chromosome structure is one of most challenging biological structures to be discovered. Most evidence about the structure comes from optical microscopy. Scanning force microscopy (SFM) can achieve molecular resolution and allows imaging in liquids. However, little information about the chromosome structure has been revealed by SFM. In this work, a mild enzymatic treatment is applied to the chromosomes to remove selectively the RNA and proteins coming from the cell. The resulting SFM images indicate that a protein film with embedded RNA molecules covers chromosomes in standard cytogenetic preparations. The thickness of the protein layer is 15-35 nm and the RNA adheres preferentially to the chromosome surface. The cell material film results in a quite smooth chromosome surface without evidence of any structural detail. After treatment, the chromosome was cleaned from cell residues and individual chromatin fibers at the surface were resolved. Furthermore, insights about the higher order structure of the chromosome can be inferred.

Characterization of a novel nucleolar protein that transiently associates with the condensed chromosomes in mitotic cells

We report the isolation and characterization of a murine gene encoding a conserved mammalian nucleolar protein. The protein, called Tsg118, has a predicted molecular mass of 59.4 kDa and a high content of basic amino acids. A homologous human gene was localized to chromosome 16p12.3. The Tsg118 protein is predominantly expressed in proliferating somatic cells and in male germ cells. Indirect immunofluorescence microscopy analysis using an affinity-purified anti-Tsg118 serum shows colocalization of Tsg118 and a known nucleolar protein, fibrillarin, to the dense fibrillar component of the nucleolus. The nucleolar localization of the Tsg118 protein appears to be temporally restricted to the interphase stages of the somatic cell cycle and to the meiotic phase of spermatogenesis. We find that the Tsg118 protein localizes to the nucleolus in both proliferating and serum-starved cells. Interestingly, as the nucleolar signal disappears in mitotic cells, the Tsg118 protein instead becomes associated with the surface of the condensed chromosomes.

Elasticity measurements show the existence of thin rigid cores inside mitotic chromosomes

Chromosome condensation is one of the most critical steps during cell division. However, the structure of condensed mitotic chromosomes is poorly understood. In this paper we describe a new approach based on elasticity measurements for studying the structure of in vitro assembled mitotic chromosomes in Xenopus egg extract. The approach is based on a unique combination of measurements of both longitudinal deformability and bending rigidity of whole chromosomes. By using specially designed micropipettes, the chromosome force-extension curve was determined. Analysis of the curvature fluctuation spectrum allowed for the measurement of chromosome bending ridigity. The relationship between the values of these two parameters is very specific: the measured chromosome flexibility was found to be 2,000 times lower than the flexibility calculated from the experimentally determined Young modulus. This requires the chromosome structure to be formed of one or a few thin rigid elastic axes surrounded by a soft envelope. The properties of these axes are well-described by models developed for the elasticity of titin-like molecules. Additionally, the deformability of in vitro assembled chromosomes was found to be very similar to that of native somatic chromosomes, thus demonstrating the existence of an essentially identical structure.

Autoantibodies to components of the mitotic apparatus

Autoantibodies directed to a variety of cellular antigens and organelles are a feature of autoimmune diseases. They have proven useful in a clinical setting to establish diagnosis, estimate prognosis, follow disease progression, alter therapy, and initiate new investigations. Cellular and molecular biologists have used autoantibodies as probes to identify molecules involved in key cellular processes. One of the most interesting sets of autoantibodies are those that target antigens within the mitotic apparatus (MA). The MA includes chromosomes, spindle microtubules and centrosomes. The identification, localization, function, and clinical relevance of MA autoantigens is the focus of this review.

Molecular cloning of metaphase chromosome protein 1 (MCP1), a novel human autoantigen that associates with condensed chromosomes during mitosis

Systemic lupus erythematosus autoantibodies were used to identify and to characterize new human chromosome-associated proteins. Previous immunolocalization studies in human and murine tissue culture cells showed that some of these monoclonal antibodies recognize nuclear antigens that associate with condensed chromosomes during mitosis. One antibody was selected for screening a human HeLa S3 cDNA expression library, and cDNAs that code for an antigen of 31-33 kDa were isolated. Immunological, biochemical and cell fractionation data indicate that the 31- to 33-kDa antigen corresponds to the chromosome-associated protein recognized by the original monoclonal antibody. Sequence analysis shows that we isolated a novel human gene. Immunolocalization to human tissue culture cells shows that during interphase the antigen is dispersed in the nucleus and that during mitosis it associates exclusively with condensed chromosomes. A similar pattern of localization was also observed in mouse fibroblasts, suggesting that the antigen is conserved among different species. Finally, we show that part of the antigen remains bound to the scaffold/matrix component, even after high salt extraction.

Effects of anti-fibrillarin antibodies on building of functional nucleoli at the end of mitosis

During mitosis some nuclear complexes are relocalized at the chromosome periphery and are then reintegrated into the re-forming nuclei in late telophase. To address questions concerning translocation from the chromosome periphery to nuclei, the dynamics of one nucleolar perichromosomal protein which is involved in the ribosomal RNA processing machinery, fibrillarin, was followed. In the same cells, the onset of the RNA polymerase I (RNA pol I) activity and translocation of fibrillarin were simultaneously investigated. In PtK1 cells, RNA pol I transcription was first detected at anaphase B. At the same mitotic stage, fibrillarin formed foci of increasing size around the chromosomes, these foci then gathered into prenucleolar bodies (PNBs) and later PNBs were targeted into the newly formed nucleoli. Electron microscopy studies enabled the visualization of the PNBs forming the dense fibrillar component (DFC) of new nucleoli. Anti-fibrillarin antibodies microinjected at different periods of mitosis blocked fibrillarin translocation at different steps, i.e. the formation of large foci, foci gathering in PNBs or PNB targeting into nucleoli, and thereby modified the ultrastructural organization of the nucleoli as well as of the PNBs. In addition, antibody-bound fibrillarin seemed localized with blocks of condensed chromatin in early G1 nuclei. It has been found that blocking fibrillarin translocation reduced or inhibited RNA pol I transcription. It is postulated that when translocation of proteins belonging to the processing machinery is inhibited or diminished, a negative feed-back effect is induced on nucleolar reassembly and transcriptional activity.

Mapping elasticity of rehydrated metaphase chromosomes by scanning force microscopy

Scanning force microscopy was used for mapping the viscoelastic properties of metaphase chromosomes. These properties were probed by scanning with various imaging forces and subsequent calculation of the difference image. The procedure allows a mapping of the viscoelastic behavior expressed as force-dependent indentation of the local surface feature and results in an image with material contrast. The approach is demonstrated on rehydrated metaphase chromosomes, which were spread and air-dried before rehydration in aqueous buffer. The rehydration resulted in a swelling of the chromosome structure and was accompanied by drastic changes in the viscoelastic properties. For comparisons, force-distance curves on metaphase chromosomes were accumulated; these curves were also used for the calculation of the stiffness curve. The demonstrated approach of mapping viscoelasticity by differential scanning force microscopy allows the detection of domains with varying mechanical properties in biomolecules such as chromosomes.

Relative distribution of rDNA and proteins of the RNA polymerase I transcription machinery at chromosomal NORs

Using confocal and immunofluorescence microscopy the relative distribution of the ribosomal chromatin and some proteins of the RNA polymerase I transcription machinery such as upstream binding factor (UBF), RNA polymerase I and DNA topoisomerase I was analyzed on chromosomal nucleolus organizer regions (NORs) of PtK1 cells. Staining with various DNA fluorochromes revealed that the ribosomal chromatin may be found at the axial region of the NOR and also at lateral expansions around the axis that can also be detected by in situ hybridization. It was observed that the transcription machinery shows a crescent-shaped distribution around the axial ribosomal chromatin at the NOR of metaphase and anaphase chromatids. An ultrastructural analysis of serially sectioned NORs supports this crescent-shape organization. Taking into account previous and present results and the loop/scaffold model of chromosome structure, we propose a model of NOR organization. The model proposes that ribosomal genes that were inactive in the preceding interphase would be present as condensed short Q-loops occupying the axial region of the NOR. Ribosomal genes previously active during interphase would be undercondensed as large R-loops associated with the transcription machinery, which is distributed in a crescent-shaped fashion around the previously active ribosomal DNA.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Location of the HIV-1 Rev protein during mitosis: inactivation of the nuclear export signal alters the pathway for postmitotic reentry into nucleoli

The HIV-1 Rev protein localizes predominantly to the nucleolus of HIV-1-infected or Rev-expressing cells. The subcellular location of Rev during mitotic nucleolar disintegration was examined at various stages of mitosis in synchronized Rev-expressing CMT3 cells. During early prophase Rev was predominantly located in disintegrating nucleoli and began to accumulate at the peripheral regions of chromosomes in late prophase, eventually distributing uniformly on all chromosomes in prometaphase. In anaphase Rev remained associated with the perichromosomal regions, but significant amounts of Rev were also seen in numerous nucleolus-derived foci. The movement of Rev from disintegrating nucleoli to perichromosomal regions and foci was similar to that of nonribosomal nucleolar proteins, including fibrillarin, nucleolin, protein B23 and p52 of the granular component. During telophase Rev remained associated with perichromosomal regions and mitotic foci until the nuclear envelope started to reform. When nuclear envelope formation was complete in late telophase, nonribosomal nucleolar proteins were present in prenucleolar bodies (PNBs) which were eventually incorporated into nucleoli; at the same time, Rev was excluded from nuclei. In contrast, a trans-dominant negative Rev protein containing an inactive nuclear export signal reentered nuclei by the nonribosomal nucleolar protein pathway in late telophase, associating with PNBs and reformed nucleoli. Rev protein reentry into postmitotic nuclei was delayed until early G1 phase, but before the arrival of ribosomal protein S6. Thus, Rev behaves like a nonribosomal nucleolar protein through mitosis until early telophase; however, its nuclear reentry seems to require reestablishment of both a nuclear import system and active nucleoli.

Morphology and behaviour of silver-stained chromatid cores in mitotic chromosomes analysed by whole mount electron microscopy

Using silver staining and the whole mount electron microscopy technique of squashed chromosomes, we studied the substructural organization and behaviour of chromatid cores in mitotic chromosomes of spermatogonia of the grasshopper Oedaleus infernalis during mitosis. It was found that the formation of mitotic chromatid cores takes place during the transition from prophase to prometaphase. Each chromosome contains two compact chromatid cores which are surrounded by a halo of dispersed argyrophilic material emanating radially from the cores. In early metaphase the chromatid core usually appears as an extended, slender network running longitudinally through the entire length of the chromatid, while in late metaphase the core frequently has a spiral appearance. In addition, our results revealed the existence of interconnections between sister chromatid cores along their entire length, as a result of which sister chromatid cores appear as a single interconnected core network in mitotic metaphase chromosomes. At this stage the core occupies a lateral position in each chromatid. However, during the transition from metaphase to anaphase, the interconnections are gradually released to allow the individualization of sister chromatid cores and the segregation of chromosomes. The core comes to occupy a central position in each segregated chromatid. These findings demonstrate the presence of an intrinsic interconnected core network within metaphase chromosomes which could be involved in the maintenance and segregation of chromosomes during mitosis.

Changes in chromosomal ultrastructure during the cell cycle

The surface structure of mitotic barley and rye chromosomes was studied by high-resolution scanning electron microscopy. Chromosomes with various degrees of chromatin condensation were prepared from untreated meristematic tissue of root tips. At lower magnifications the highly condensed chromosomes in metaphase and anaphase showed a compact structure with a smooth surface. The condensation starts from the centromeric region and the chromatids are often discernible in the still uncondensed telomeric region. Decondensation begins at the telomeric region during telophase. Parallel arrangement of fibres is a characteristic feature predominately seen in prophase and telophase chromosomes. Chromatin structures that resemble tiles on a roof or braided strands were often observed. Prophase and telophase chromosomes are particularly suitable for further studies of chromatin arrangement and organization in plant chromosomes.

Nuclear matrix proteins as structural and functional components of the mitotic apparatus

The eukaryotic nucleus is a membrane-enclosed compartment containing the genome and associated organelles supported by a complex matrix of nonhistone proteins. Identified as the nuclear matrix, this component maintains spatial order and provides the structural framework needed for DNA replication, RNA synthesis and processing, nuclear transport, and steroid hormone action. During mitosis, the nucleoskeleton and associated chromatin is efficiently dismantled, packaged, partitioned, and subsequently reassembled into daughter nuclei. The dramatic dissolution of the nucleus is accompanied by the assembly of a mitotic apparatus required to facilitate the complex events associated with nuclear division. Until recently, little was known about the fate or disposition of nuclear matrix proteins during mitosis. The availability of specific molecular probes and imaging techniques, including confocal microscopy and improved immunoelectron microscopy using resinless sections and related procedures, has enabled investigators to identify and map the distribution of nuclear matrix proteins throughout the cell cycle. This chapter will review the structure, function, and distribution of the protein NuMA (nuclear matrix mitotic apparatus) and other nuclear matrix proteins that depart the nucleus during the interphase/mitosis transition to become structural and functional components within specific domains of the mitotic apparatus.

Characterization of a cell cycle-dependent nuclear autoantigen

Analysis of reactivity to nuclear antigens in autoimmune sera revealed a serum that produced a previously undescribed cell cycle-dependent immunofluorescence staining pattern. By indirect immunofluorescence using HEp-2 cells as substrate, the serum generated a speckled and nucleolar pleomorphic staining pattern. This characteristic immunofluorescence pattern was detected in different cell lines from various species indicating that the antigen was highly conserved. This serum immunoprecipitated a 85 kDa protein using an extract from [35S]-labeled HeLa cells. Indirect immunofluorescence of proliferating mouse 3T3 cells displayed the characteristic pleomorphic staining which was not observed in serum-starved cells. Resting human and mouse peripheral blood lymphocytes were negative in immunofluorescence while mitogen-stimulated lymphocytes were positive. Germinal centers of mice two weeks after immunization with 2-phenyl-oxazolone showed speckled immunofluorescence staining in the dark zones whereas unimmunized mice were completely negative. Cell synchronization experiments showed a characteristic sequence of locations of the antigen during the cell cycle. In G1, cells were completely negative. In late G1, G1/S and S phase, speckles were visible. In early G2, speckles were visible, and later in G2, the nucleoli were positive. During mitosis chromosomes were stained. Further characterization of this antibody specificity and cloning of cDNA are in progress.

A chromomeric model for nuclear and chromosome structure

The basic structural elements of chromatin and chromosomes are reviewed. Then a model involving only three architectural motifs, nucleosomes, chromatin loops and transcription factories/chromomeres, is presented. Loops are tied through transcription factors and RNA polymerases to factories during interphase and to the remnants of those factories, chromomeres, during mitosis. On entry into mitosis, increased adhesiveness between nucleosomes and between factories drives a ‘sticky-end’ aggregation to the most compact and stable structure, a cylinder of nucleosomes around an axial chromomeric core.

The C terminus of mitosin is essential for its nuclear localization, centromere/kinetochore targeting, and dimerization

Mitosin is a novel 350-kDa nuclear phosphoprotein that dramatically relocates from the evenly nuclear distribution in S phase to the centromere/kinetochore and mitotic apparatus in M phase. The dynamic relocalization of mitosin is accompanied by the phosphorylation of itself, suggesting that mitosin plays a role in mitotic progression. The molecular basis of nuclear localization and targeting of mitosin to the centromere/kinetochore were characterized using a set of epitope-tagged deletion mutants. The data indicate that the extreme C terminus (amino acids 2,487-3,113) of mitosin has both an independent centromere/kinetochore targeting domain and an unusually spaced bipartite nuclear localization signal. Moreover, the same centromere/kinetochore targeting domain was shown to be essential for the ability of mitosin to bind to itself or other putative mitosin-associated proteins through use of the yeast two-hybrid system. These results suggest that the C terminus of the mitosin is essential for its role in influencing cell cycle progression.

Topographic changes in a heterochromatic chromosome block in humans (15P) during formation of the nucleolus

Fluorescence in situ hybridization and multispectral confocal laser scanning microscopy revealed a highly dynamic nucleolar-associated chromosome 15 satellite III heterochromatin cluster in humans. This nucleolar-associated DNA was highly decondensed at the metaphase plate compared with its topography at interphase and appeared to act as a centre for the post-mitotic reorganization of the nucleolus. Our data showed unexpected trans-mitotic changes in the topography of this nucleolar-associated satellite III DNA that suggest that this locus-specific heterochromatin superstructure may be involved in nucleolar organization.

Synaptonemal complex proteins: occurrence, epitope mapping and chromosome disjunction

We have used polyclonal antibodies against fusion proteins produced from cDNA fragments of a meiotic chromosome core protein, Cor1, and a protein present only in the synapsed portions of the cores, Syn1, to detect the occurrence and the locations of these proteins in rodent meiotic prophase chromosomes. The 234 amino acid Cor1 protein is present in early unpaired cores, in the lateral domains of the synaptonemal complex and in the chromosome cores when they separate at diplotene. A novel observation showed the presence of Cor1 axial to the metaphase I chromosomes and substantial amounts of Cor1 in association with pairs of sister centromeres. The centromere-associated Cor1 protein becomes dissociated from the centromeres at anaphase II and it is not found in mitotic metaphase centromeres. The extended presence of Cor1 suggests that it may have a role in chromosome disjunction by fastening chiasmata at metaphase I and by joining sister kinetochores, which ensures co-segregation at anaphase I. Two-colour immunofluorescence of Cor1 and Syn1 demonstrates that synapsis between homologous cores is initiated at few sites but advances rapidly relative to the establishment of new initiation sites. If the rapid advance of synapsis deters additional initiation sites between pairs of homologues, it may provide a mechanism for positive recombination interference. Immunogold epitope mapping of antibodies to four Syn1 fusion proteins places the amino terminus of Syn1 towards the centre of the synaptonemal complex while the carboxyl terminus extends well into the lateral domain of the synaptonemal complex. The Syn1 fusion proteins have a non-specific DNA binding capacity. Immunogold labelling of Cor1 antigens indicates that the lateral domain of the synaptonemal complex is about twice as wide as the apparent width of lateral elements when stained with electron-dense metal ions. Electron microscopy of shadow-cast surface-spread SCs confirms the greater width of the lateral domain. The implication of these dimensions is that the proteins that comprise the synaptic domain overlap with the protein constituents of the lateral domains of the synaptonemal complex more than was apparent from earlier observations. This arrangement suggests that direct interactions might be expected between some of the synaptonemal complex proteins.

The telochore: a telomeric differentiation of the chromosome axis

We have analysed by means of silver staining the structure of the chromosome axis at the telomeres of meiotic chromosomes in three different grasshopper species. At metaphase I the chromatid axes run the length of the chromatids although they do not reach the chromosome ends. The axes of sister chromatids are associated and show a round differentiation at their distal ends that we have named the 'telochore'. Telochores never contact the chromosome ends: there is always some chromatin beyond them. In late metaphase I bivalents with a distal chiasma, anaphase I and metaphase II half-bivalents and anaphase II chromatids, the axes clearly possess one telochore in each chromosome end. These results seem to indicate that telochores are differentiations of the distal ends of chromatids. We discuss the possible structural significance of telochores according to the current scaffold/radial loop model of chromatin organization of eukaryotic metaphase chromosomes. Additionally, we suggest the possible functional role of the telochore as a nucleoprotein domain forming a protective cap for telomeric DNA.

Unusual chromosome structure of fission yeast DNA in mouse cells

Chromosomes from the fission yeast Schizosaccharomyces pombe have been introduced into mouse cells by protoplast fusion. In most cell lines the yeast DNA integrates into a single site within a mouse chromosome and results in striking chromosome morphology at metaphase. Both light and electron microscopy show that the yeast chromosome region is narrower than the flanking mouse DNA. Regions of the yeast insert stain less intensely with propidium iodide than surrounding DNA and bear a morphological resemblance to fragile sites. We investigate the composition of the yeast transgenomes and the modification and chromatin structure of this yeast DNA in mouse cells. We suggest that the underlying basis for the structure we see lies above the level of DNA modification and nucleosome assembly, and may reflect the attachment of the yeast DNA to the rodent cell nucleoskeleton. The yeast integrant replicates late in S phase at a time when G bands of the mouse chromosomes are being replicated, and participates in sister chromatid exchanges at a high frequency. We discuss the implications of these studies to the understanding of how chromatin folding relates to metaphase chromosome morphology and how large stretches of foreign DNA behave when introduced into mammalian cells.

The chromosome periphery during mitosis

A complex structure, visible by electron microscopy, surrounds each chromosome during mitosis. The organization of this structure is distinct from that of the chromosomes and the cytoplasm. It forms a perichromosomal layer that can be isolated together with the chromosomes. This layer covers the chromosomes except in centromeric regions. The perichromosomal layer includes nuclear and nucleolar proteins as well as ribonucleoproteins (RNPs). The list of proteins and RNAs identified includes nuclear matrix proteins (perichromin, peripherin), nucleolar proteins (perichro-monucleolin, Ki-67 antigen, B23 protein, fibrillarin, p103, p52), ribosomal proteins (S1) and snRNAs (U3 RNAs). Only limited information is available about how and when the perichromosomal layer is formed. During early prophase, the proteins extend from the nucleoli towards the periphery of the nucleus. Thin cordon-like structures reach the nuclear envelope delimiting areas in which chromosomes condense. At telophase, the proteins are associated with the part of the chromosomes remaining condensed and accumulate in newly formed nucleoli in regions where chromatin is already decondensed. The perichromosomal layer contains several different classes of proteins and RNPs and it has been attributed various roles: (1) in chromosome organization, (2) as a barrier around the chromosomes, (3) involvement in compartmentation of the cells in prophase and telophase and (4) a binding site for chromosomal passenger proteins necessary to the early process of nuclear assembly.

The structure of human S-phase chromosome fibres

Recent in situ hybridization studies suggested that within the range of 0.1-1.0 Mb, human interphase chromosomes follow a random walk model (i.e. they behave as flexible polymers without major constraints). However, chromosome structure may differ in the G1, S, and G2 phases, and phase-specific constraints may be masked if the chromosome analysis does not discriminate between the phases. Therefore, using confocal microscopy, we examined the structure of S-phase chromosomes labelled with 5-iododeoxyuridine after prolonged treatment with 5-fluorodeoxyuridine. In the S-phase, labelled 0.32 mu chromosome fibres mostly appear as semi-circles with an average diameter of 0.83 +/- 0.03 mu. These semi-circles are joined together to form different 3D structures, and two semicircles frequently adopt s- or omega-like conformations involving about 2.5 mu of the chromosome contour length (L). Morphometric analysis of the S-phase fibres suggests that our data fit both the random flexible polymer model and also a model in which two constrained semi-circles are attached to each other by a flexible joint, thus eliminating constraints at long distances (L more than 2 mu).

Male gametes and fertilization in angiosperms

Double fertilization appears to have evolved as a product of change directly related to an accelerated rate and timing of reproduction. In this review, the focus is on the angiosperm male gametophyte, where changes include a reduction in the number of mitoses, establishment of the male germ unit and involvement of both members of the pair of sperm cells in reproduction. The organization of the generative cell during mitosis indicates that there may be basic similarities between this process in plant and animal cells. The microtubular organization of generative cells alters after isoiation. However, mitosis in Allamanda, proceeds as usual during in vitro culture. The presence of actin microfilaments within generative cells has previously been shown in Rhododendron and here we provide further evidence that actin microfilaments are indeed present in generative cells. Two different kinds of intermediate-filament-like systems (IFS) are present in the generative cells of Allamanda: one in the cytoplasm and the other closely associated with the surface domain of chromosomes, both identified by the use of monoclonal antibodies. This is the first report of an IFS existing in the vegetative nucleus of pollen. Two alternate views have been proposed for the involvement of sperm cells in double fertilization of angiosperms. First, the chance hypothesis assumes that sperm fusions with the egg and central cell are random interactions. Second, the specific receptor hypothesis proposes that one of the pair of sperm (the true male gamete) is destined to fuse specifically with the egg. Support for this latter view has come from demonstrations of sperm cell dimorphism, both in size and content of mitochondria and plastids. The production of monoclonal antibodies which bind to surface domains on the reproductive cells of higher and lower plants, and specifically to the cytoplasm of generative and sperm cells also suggest that directed fertilization occurs. Recently, the existence of translatable mRNA pools within the generative and sperm cells indicates that, with the use of recent technological advances such as the polymerase chain reaction, the potential exists to identify male gamete-specific genes. Contents Summary 679 I. Introduction 680 III. A cell biological perspective 681 IV. Two hypotheses for double fertilization 687 V. Isolation of living sperm from flowering plants 687 VI. Sperm surface antigens of plants 688 VII. Molecular characterization 690 VIII. Conclusions 691 Acknowledgements 691 References 692.

The analysis of 40 kDa nuclear protein, p40, in interphase cells and mitotic cells

We previously reported that the monoclonal antibody M108 recognized a 40 kDa protein both in the nucleus and the cytoplasm. This nuclear 40 kDa antigen was located in the nuclear envelope in interphase cells and in the perichromosomal region during mitosis. Now, we have analyzed this nuclear 40 kDa protein (p40) further, through morphological and biochemical approaches. At the beginning of mitosis, the perinuclear p40 detached from the nuclear envelope and moved to surround the condensing chromatin, while in the late stage of mitosis, the perichromosomal p40 moved back to the reassembled nuclear envelope. Most of the perichromosomal p40 on the metaphase chromosome was solubilized only by DNase I treatment, not by either high salt or detergent treatment. On the other hand, the perinuclear p40 was not solubilized by DNase1 alone, or high salt detergent alone. Sequential treatments with DNase I and high salt detergent were required to extract p40 in interphase nuclei. These results suggest that p40 was associated both with the nuclear envelope and chromatin DNA in interphase nuclei, while it bound only to chromatin DNA in mitosis.

CENP-F is a .ca 400 kDa kinetochore protein that exhibits a cell-cycle dependent localization

We have identified a novel .ca 400 kDa cell-cycle dependent kinetochore associated protein in human cells, designated CENP-F, using human autoimmune serum. Immunofluorescence staining using the native serum, affinity purified antibodies, or antibodies raised against a cloned portion of CENP-F first reveals CENP-F homogeneously distributed throughout the nucleus of HeLa cells in the G2 stage of the cell cycle. Progression into prophase is accompanied by the localization of CENP-F to all the kinetochore regions of the karyotype. Kinetochore association is maintained throughout metaphase, but at the onset of anaphase CENP-F is no longer detected in association with the kinetochore but is found at the spindle mid-zone. By telophase, it is concentrated into a narrow band on either side of the midbody. Studies of the interaction of CENP-F with the kinetochore indicate that this protein associates with the kinetochore independent of tubulin and dissociation is dependent on events connected with the onset of anaphase. Nuclease digestion studies and immunoelectron-microscopy indicate that CENP-F is localized to the kinetochore plates and specifically to the outer surface of the outer kinetochore plate. The distribution of CENP-F closely parallels that of another high molecular weight kinetochore associated protein, CENP-E. Comparative studies indicate that there are antibodies in the CENP-F reactive autoimmune serum that recognize determinants present in the central helical rod domain of CENP-E. Immune depletion experiments confirm that CENP-F exhibits the distribution pattern in cells that was seen with the native autoimmune serum.

Maternal age effect: the enigma of Down syndrome and other trisomic conditions

Aneuploidy is the most frequently observed chromosome abnormality in human liveborn, abortuses and oocytes. The only etiological factor that has been established is advanced maternal age for the occurrence of trisomies, particularly trisomy 21 which causes Down syndrome. The maternal age effect remains an enigma. Recent molecular data bearing on this question are reviewed as are the hypotheses that have been proposed linking nondisjunction and maternal age. Rationale is presented for a compromised microcirculation hypothesis that explains the cause of nondisjunction and why its occurrence changes with maternal age from menarche to menopause. It takes into account two facts: (1) 95% of Down syndrome children receive their extra chromosome from their mother, and in 80% or more of these the nondisjunction occurred in the first meiotic division, which is completed in the ovary. (2) The ovarian follicle containing the primary oocyte has no internal circulation. The hypothesis proposes that aneuploid oocytes arise from a concatenation of events. It begins with hormonal imbalance that causes a less-than-optimal microvasculature to develop around the maturing and mature follicles. The resulting decrease in the size of the perifollicular capillary bed reduces the volume of blood flow through the area, leading to an oxygen deficit and a concomitant increase inside the follicle of carbon dioxide and anaerobic products, such as lactic acid. This in turn causes a decrease in the intracellular pH of the oocyte that diminishes the size of the spindle, with consequent displacement and nondisjunction of a chromosome. The compromised microcirculation hypothesis explains the occurrence of aneuploidy in primary and secondary oocytes, sperm precursor cells, tumor and embryonic cells. It also explains why women of all reproductive ages may have a Down syndrome child.

MSA-36: a chromosomal and mitotic spindle-associated protein

We have identified a novel M(r) 36,000 protein (MSA-36) that has a complex cell cycle dependent distribution. This protein is first detected in interphase nuclei just prior to the onset of chromosome condensation. MSA-36 is found along condensing chromosomes and is a component of the centromere through metaphase. At anaphase, this protein is no longer detected in association with the chromosomes but appears at the forming stembodies and subsequently within the intercellular bridge at either side of the midbody. At the completion of cell division, the amount of MSA-36 in the bridge appears to decline concurrent with the appearance of this protein briefly within the reforming nucleus. To investigate whether MSA-36 is an active component of the chromosome or a passive passenger protein, we studied the behaviour of this protein in cells exhibiting premature chromosome condensation and in cells during and following recovery from mitotic arrest. These studies suggest that MSA-36 is not essential for a variety of major chromosome-associated events.