Immunological localization of a major karyoskeletal protein in nucleoli of oocytes and somatic cells of Xenopus laevis

Human anti-centromere sera recognise a 19.5 kD non-histone chromosomal protein from HeLa cells

Autoimmune sera from 18 scleroderma patients were found to give a centromere positive immunofluorescence response on formaldehyde fixed HeLa cells and on chromosome spreads. Immunoblotting experiments with a protein fraction enriched in HeLa chromosomal proteins revealed that the antigenic target common to all 18 sera is a polypeptide of 19.5 kD. This polypeptide, which is not one of the core histones, is not soluble under conditions which favour the release of nuclear ribonucleoprotein particles. Antigen specific purification of autoantibodies with subsequent immunofluorescence studies confirmed that the 19.5 kD antigen is restricted to cell cycle-dependent single or double spheres at the centromere of HeLa chromosomes. Two additional polypeptides of 23 kD and 25.5 kD immunoreactive with five of the 18 centromere positive sera are not located at the centromere region, suggesting that other autoantibody systems are present in these sera.

Identification and localization of a novel nucleolar protein of high molecular weight by a monoclonal antibody

A monoclonal murine antibody (No-114) is described which reacts specifically with a polypeptide of molecular weight (Mr) 180 000 present in low-speed nuclear pellets from oocytes and somatic cells of Xenopus laevis and X. borealis and in isolated amplified nucleoli. Two-dimensional gel electrophoresis has revealed the acidic nature of this polypeptide (isoelectric at pH of ca 4.2 in the presence of 9.5 M urea). A relatively large proportion of the protein is extracted at elevated ionic strength (i.e., at 0.4-0.5 M alkali salt) in a form sedimenting at approx. 7-8S, compatible with a monomeric state. It is also extracted by digestion with RNase but not with DNase. In immunofluorescence microscopy, antibody No-114 stains intensely nucleoli of oocytes and all somatic cells examined, including the residual nucleolar structure of Xenopus erythrocytes which are transcriptionally inactive. During mitosis the antigen does not remain associated with the nucleolar organizer regions (NOR) of chromosomes but is released and dispersed over the cytoplasm until telophase when it re-associates with the reforming interphase nucleoli. At higher resolution the immunofluorescent region is often resolved into a number of distinct subnucleolar components of varied size and shape. Immunoelectron microscopy using colloidal gold-coupled secondary antibodies reveals that the Mr 180 000 protein is confined to the dense fibrillar component of the nucleolus. This conclusion is also supported by its localization in the fibrillar part of segregated nucleoli of cells treated with actinomycin D. We conclude that nucleoli contain a prominent protein of Mr 180 000 which contributes to the general structure of the dense fibrillar component of the interphase nucleolus, independent of its specific transcriptional activity.

The synaptonemal complex as part of the nuclear matrix of the flour moth, Ephestia kuehniella

A nuclear matrix fraction was prepared from ovaries of the achiasmatic flour moth, Ephestia kuehniella, by removal of the chromatin, using detergent treatment of homogenized ovaries or dissected ovary tips followed by DNase digestion and high salt extraction. Removal of DNA and histones from the nuclei was demonstrated by Feulgen staining and polyacrylamide gel electrophoresis (PAGE), respectively. By light microscopy, ribbon-like structures similar in dimension to the synaptonemal complex were observed in the oocyte after digestion of the chromosomes. Electron microscopic examination of matrix preparations of pachytene cells showed a defined synaptonemal complex structure with both lateral and central elements. Such structures were not found in either the fully differentiated nurse cells or in follicle cells which were exposed to the same preparative technique concurrently. However, in early post-pachytene nurse cells the typical polycomplex structures, formed in these cells from the synaptonemal complex, were found in nuclear matrix preparations. The results suggest an association of synaptonemal complexes with the nuclear matrix.

Binding of cytoskeletal proteins by monoclonal anti-DNA lupus autoantibodies

Monoclonal anti-DNA antibodies produced by hybridomas derived from MRL-lpr/lpr mice and human lupus patients were found to bind to the cytoskeleton of mink lung cells. When tested by indirect immunofluorescence, 17/29 human monoclonal anti-DNA antibodies reacted with the cytoskeleton; 4 of the 29 also produce antinuclear reactions with epithelial cells. The cytoskeletal staining was not inhibited by prior treatment of the cells with DNase, but it was completely blocked by prior incubation of the monoclonal antibodies with DNA and other nucleic acids. The ability of the polynucleotides to inhibit the cytoskeletal staining corresponded to their ability to bind to the antibodies in competitive immunoassays. An (Fab')2 preparation of a monoclonal antibody bound to the cytoskeleton as well as the whole immunoglobulin. The effect of colcemid on the staining pattern, the blocking effect of a monoclonal antivimentin antibody, and results with nitrocellulose blots of cellular proteins indicated that the cytoskeletal protein to which the antibodies bound was vimentin.

Localization of RNA polymerase I in interphase cells and mitotic chromosomes by light and electron microscopic immunocytochemistry

Rabbit antibodies to RNA polymerase I from a rat hepatoma have been used to localize the enzyme in a variety of cells at the light and electron microscopic level. In interphase cells the immunofluorescence pattern indicated that polymerase I is contained exclusively within the nucleolus. That this fluorescence, which appeared punctated rather than uniform, represented transcriptional complexes of RNA polymerase I and rRNA genes was suggested by the observation that it was enhanced in regenerating liver and in a hepatoma and was markedly diminished in cells treated with actinomycin D. Electron microscopic immunolocalization using gold-coupled second antibodies showed that transcribed rRNA genes are located in, and probably confined to, the fibrillar centers of the nucleolus. In contrast, the surrounding dense fibrillar component, previously thought to be the site of nascent pre-rRNA, did not contain detectable amounts of polymerase I. During mitosis, polymerase I molecules were detected by immunofluorescence microscopy at the chromosomal nucleolus organizer region, indicating that a considerable quantity of the enzyme remains bound to the rRNA genes. From this we conclude that rRNA genes loaded with polymerase I molecules are transmitted from one cell generation to the next one and that factors other than the polymerase itself are involved in the modulation of transcription of DNA containing rRNA genes during the cell cycle.

Electron microscopic immunolocalization of a karyoskeletal protein of molecular weight 145 000 in nucleoli and perinucleolar bodies of Xenopus laevis

Amplified nucleoli of Xenopus laevis oocytes contain a major karyoskeletal protein of Mr 145 000 insoluble in low- and high-salt buffer as well as in non-denaturing detergents. Electron microscopic localization on native and high-salt extracted nucleoli using specific murine antibodies against this polypeptide and gold-coupled antibodies for visualization reveals that the Mr 145 000 protein is located in coils of filaments of ca 4 nm diameter. In addition, this protein occurs in the medusoid filament bodies (MFBs) present in the nucleolar cortex and free in the nucleoplasm. In somatic cells of tissues and in A6 kidney epithelial cells grown in vitro the Mr 145 000 polypeptide or an immunologically related protein is also organized in coiled aggregates of filaments 4-12 nm in diameter present both in the periphery of nucleoli and free in the nucleoplasm. We discuss a possible role of this protein as a karyoskeletal support involved in the storage and transport of preribosomal particles.

A structural concept for nucleoli of Dictyostelium discoideum deduced from dissociation studies

We aimed to establish whether there is a matrix structure in the nucleolus to which the ribosomal DNA (rDNA) is strongly attached. To detect artifacts that might occur during the harsh histone extraction procedures frequently used for matrix preparation, we dissociated nucleoli of Dictyostelium discoideum with a range of NaCl or heparin concentrations. With heparin treatment significant amounts of rDNA were solubilized into the dissociating solution. When the residual nucleoli were digested with Eco RI, none of the Eco RI fragments of the rDNA remained preferentially bound to the residual nucleoli, indicating that there is no matrix attached to a specific site on the rDNA. When residual nucleoli were examined by electron microscopy, a correlation was found between the extent of solubilization of rDNA, the loss of nucleosomes, and, in heparin-treated nucleoli, the loss of ribonucleoprotein-bound components. These results suggest that the rDNA is released from the nucleoli as soon as nucleosomes have been dissociated and transcription complexes disrupted. Electron microscopy also showed that the NaCl concentration required for dissociation of nucleosomes was higher when divalent cations (Ca2+, Mg2+, Cu2+) were used during the isolation or the treatment of the nucleoli prior to dissociation in high salt. Furthermore, the residual, high-salt-resistant structures were much larger when nucleoli were pretreated with divalent cations or when they were purified in the presence of Ca2+ than when they were purified in its absence. Hence divalent cations, which induce chromatin condensation, prevented nucleolar dissociation whereas treatment with chelating agents, which loosen chromatin compaction, led to much smaller residual matrixlike structures. Nucleoli could be dissociated with heparin to a larger extent than with NaCl so that in Ca2+-free preparations no residual nucleolar matrixlike structures could be detected. Our results suggest that the nucleolar "matrix" seen in the electron microscope is due to incomplete dissociation of the nucleolar material. We propose that in nucleoli of Dictyostelium the rDNA is not attached to a tightly binding matrix structure, but that nucleoli are stabilized by side-to-side contacts between chromatin fibers and transcription complexes.

Alterations in nuclear anatomy by chemical modification of proteins in isolated rat liver nuclei

Whole rat liver nuclei were treated with citraconic anhydride, a reagent specific for primary amines. Dramatic changes were observed in nuclear morphology and light scattering properties. An analysis for DNA and RNA content suggested that DNA was released from the nuclei with a short half-time, approximately 2-4s demonstrating a biphasic release profile. RNA was similarly released but with a monophasic profile. Analysis of SDS-PAGE gels of modified nuclei demonstrated a progressive enrichment of nuclear matrix (lamins) polypeptides with extent of modification. H1 histone was quantitatively lost as a function of modification reagent concentration, while approx. 50% of the nucleosomal histones cosedimented with DNA- and RNA-free nuclei. Modification in the presence of 2 mM EGTA released all the DNA and RNA [less than or equal to 1% remaining) while retaining structures characteristic of nuclear matrix, nucleoli, and ribonucleoprotein (predominantly hnRNA group A and B). These nucleic acid-deficient structures have been termed nuclear fossils to differentiate them from high salt detergent-prepared empty nuclear sacks, nuclear remnants, or nuclear scaffolds. Modification in the presence of 2% Triton X-100 results in structures similar to the nuclear fossils (EGTA treatment), but missing the double bilayer and a 51K polypeptide that is a major component of the other structures. The use of chemical modification on the nucleus provides an experimental approach for examining the role of ionic interactions in controlling nuclear structure. Citraconylation may thus serve two functions: (a) as a protein-specific perturbant of nuclei capable of simply and rapidly preparing a range of structural variants for the analysis of nuclear interactions; (b) offer a paradigm for control of nucleic acid-polypeptide interactions based on post-translational alterations in protein charge.

Identification of human Sm and (U1) RNP antigens by immunoblotting

When HeLa nuclear extracts or ribonucleoproteins (RNPs) from rat liver nuclei were used as antigens, a monospecific anti-(U1)RNP serum recognized in each preparation only 1 polypeptide of 68 or 70 kilodalton (kd) respectively. With a serum of combined anti-Sm/(U1)RNP specificity, HeLa nuclear extracts showed 3 additional antigenic polypeptides of 29, 28, and 16 kd, whereas only 2 additional polypeptides of 27 and 16 kd were observed in rat liver RNPs. However, no antigenic reaction at 68/70 kd was detected with a monospecific anti-Sm serum, indicating that the 68/70 kd antigen is specific for anti-(U1)RNP antibodies. When commercially available ENA extract was used as antigen source only weak immunostaining in the range 70-40 kd and at 16 kd was seen. Elution experiments with anti-Sm antibodies bound to their specific polypeptides demonstrated that neither protein degradation nor cross-reaction was responsible for recognition of the 29/28 and 16 kd antigens by this serum, and that in fact 2 different autoantibody systems are involved.

High mobility group proteins of amphibian oocytes: a large storage pool of a soluble high mobility group-1-like protein and involvement in transcriptional events

Oocytes of several amphibian species (Xenopus laevis, Rana temporaria, and Pleurodeles waltlii) contained a relatively large pool of nonchromatin-bound, soluble high mobility group (HMG) protein with properties similar to those of calf thymus proteins HMG-1 and HMG-2 (protein HMG-A; A, amphibian). About half of this soluble HMG-A was located in the nuclear sap, the other half was recovered in enucleated ooplasms. This protein was identified by its mobility on one- and two-dimensional gel electrophoresis, by binding of antibodies to calf thymus HMG-1 to polypeptides electrophoretically separated and blotted on nitrocellulose paper, and by tryptic peptide mapping of radioiodinated polypeptides. Most, if not all, of the HMG-A in the soluble nuclear protein fraction, preparatively defined as supernatant obtained after centrifugation at 100,000 g for 1 h, was in free monomeric form, apparently not bound to other proteins. On gel filtration it eluted with a mean peak corresponding to an apparent molecular weight of approximately 25,000; on sucrose gradient centrifugation it appeared with a very low S value (2-3 S), and on isoelectric focusing it appeared in fractions ranging from pH approximately 7 to 9. This soluble HMG-A was retained on DEAE-Sephacel but could be eluted already at moderate salt concentrations (0.2 M KCl). In oocytes of various stages of oogenesis HMG-A was accumulated in the nucleus up to concentrations of approximately 14 ng per nucleus (in Xenopus), corresponding to approximately 0.2 mg/ml, similar to those of the nucleosomal core histones. This nuclear concentration is also demonstrated using immunofluorescence microscopy. When antibodies to bovine HMG-1 were microinjected into nuclei of living oocytes of Pleurodeles the lateral loops of the lampbrush chromosomes gradually retracted and the whole chromosomes condensed. As shown using electron microscopy of spread chromatin from such injected oocyte nuclei, this process of loop retraction was accompanied by the appearance of variously-sized and irregularly-spaced gaps within transcriptional units of chromosomal loops but not of nucleoli, indicating that the transcription of non-nucleolar genes was specifically inhibited by this treatment and hence involved an HMG-1-like protein. These data show that proteins of the HMG-1 and -2 category, which are usually chromatin-bound components, can exist, at least in amphibian oocytes, in a free soluble monomeric form, apparently not bound to other molecules. The possible role of this large oocyte pool of soluble HMG-A in early embryogenesis is discussed as well as the possible existence of soluble HMG proteins in other cells.

Biochemical and immunological characterization of desmoplakins I and II, the major polypeptides of the desmosomal plaque

Epithelial cells contain complexes of cytokeratin filaments (tonofilaments) with specific domains of the plasma membrane that appear as symmetric junctions, i.e. desmosomes, or as asymmetric hemi-desmosomes. These regions of filament-membrane-attachment are characterized by 14 to 20 nm thick dense plaques (desmosomal plaque). In isolated desmosome-tonofilament complexes or other desmosomal fractions from various stratified squamous epithelia (e.g. bovine muzzle epidermis and tongue mucosa) desmosomal plaque structures are recognized and show a relatively high resistance to various extraction buffers and detergents. Such fractions enriched in desmosomal plaque material are also enriched in two prominent polypeptide bands of apparent molecular weights 250,000 (desmoplakin I) and 215,000 (desmoplakin II) which appear, on two-dimensional gel electrophoresis, as two distinct polypeptides isoelectric near neutral pH. These two polypeptides are present in almost equimolar amounts and each of them appears as a series of isoelectric variants, including some labeled by [32P]phosphate in tissue slices. The two desmoplakin polypeptides are closely related as shown by tryptic peptide map analysis and are different from keratin-like proteins and other major polypeptides of desmosome-rich fractions. Guinea pig antibodies raised against desmoplakins and specific for these proteins do not cross-react with other desmosomal antigen(s) or constituents of other types of junctions. Using desmoplakin antibodies we have identified desmoplakins as the major constituents of the desmosomal plaques present in epithelial and myocardiac cells of diverse species. The significance of this group of cell type-specific membrane-associated cytoskeletal proteins and their possible cytoskeletal functions are discussed.

Immunocytochemical identification of epithelium-derived human tumors with antibodies to desmosomal plaque proteins

Epithelial cells contain desmosomes, special intercellular junctions providing sites of membrane attachment for intermediate-sized filaments of the cytokeratin type (tonofilaments). Such sites of anchorage of tonofilaments appear as dense plaques on the cytoplasmic side of the desmosomal membrane. We have isolated desmosome-enriched fractions from bovine snout epidermis and tongue mucosa and have characterized the major protein associated with the desmosomal plaque. This protein occurs in equimolar amounts of two polypeptides of Mr 250,000 (desmoplakin I) and Mr 215,000 (desmoplakin II) which are chemically and immunologically related. Antibodies raised against desmoplakins allow the identification and localization of this protein in epithelial cells grown in tissues or in vitro and show crossreaction in species as diverse as man, mouse, and chicken. Using immunolocalization at the light and electron microscope levels, we show that these antibodies bind specifically to desmosomal plaques. Antibodies to desmoplakins have been used successfully for detection of desmosomal proteins in a broad variety of epithelium-derived human tumors, including primary carcinomas and their metastases, irrespective of the morphology of the specific tumor. Nonepithelial tumors examined have been negative. We propose to use antibodies to desmoplakins and to cytokeratins in pathological diagnosis as two independent markers for the positive immunocytochemical identification and classification of epithelium derived tumors.

Desmoplakins of epithelial and myocardial desmosomes are immunologically and biochemically related

Guinea pig antibodies against desmoplakins from bovine muzzle epidermis showed specific reaction in several epithelial tissues with desmoplakin I (Mr 250,000) and desmoplakin II (Mr 215,000). By immunofluorescence microscopy, prominent punctate staining was observed in various lines of cultured epithelial cells, revealing desmosomal junctions at sites of established cell-to-cell contacts as well as hemidesmosomes and internalized desmosome-derived membrane domains. On frozen tissue sections punctate staining was observed along plasma membranes of epithelial cells, and electron microscopy using the immunoperoxidase technique revealed that the antibodies were specifically localized at the plaques associated with desmosomes and hemidesmosomes. Of a large number of non-epithelial cells examined positive staining was only observed on desmosome-like junctions of myocardial cells and Purkinje fiber cells. In both epithelial and myocardial tissues the antibodies showed a broad range of cross-reactivity between diverse vertebrate species such as man, cow, rodent, and chicken, indicating that desmoplakins contain determinants strongly conserved during evolution. When binding of these antibodies to cytoskeletal polypeptides separated by gel electrophoresis and blotted on nitrocellulose paper sheets was examined, specific reaction was noted with desmoplakin I and, to a variable degree, also desmoplakin II from various epithelial cells. Reaction was also observed with a myocardial polypeptide from bovine and human hearts which had a similar Mr value (250,000) and isoelectric pH range as desmoplakin I. We conclude that desmoplakins are the major proteins present in the desmosomal plaques of both epithelial and myocardial cells and that the desmoplakin polypeptides present in these two different cell types are very similar, if not identical.