The presence of N-glycosylated proteins in cell nuclei

Glycan analysis of Lamin A/C protein at G2/M and S phases of the cell cycle

During mitosis, phosphorylation and dephosphorylation of lamins triggers the nuclear envelope disassembly/assembly. However, it hasn't been known whether lamin proteins undergo any modification other than phosphorylation during the cell cycle. Glycosylation of lamin proteins is one of the less studied post-translational modification. Glycosylation and phosphorylation compete for the same positions and interplay between two modifications generate a post-translational code in the cell. Based on this, we hypothesized that glycosylation of lamin A/C protein may be important in the regulation of the structural organization of the nuclear lamina during interphase and mitosis. We analysed the glycan units of lamin A/C protein in lung carcinoma cells synchronized at G2/M and S phases via CapLC-ESI-MS/MS. Besides, the outermost glycan units were determined using lectin blotting and gold-conjugated antibody and lectin staining. TEM studies also allowed us to observe the localization of glycosylated lamin A/C protein. With this study, we determined that lamin A/C protein shows O-glycosylation at G2/M and S phases of the cell cycle. In addition to O-GlcNAcylation and O-GalNAcylation, lamin A/C is found to be contain Gal, Fuc, Man, and Sia sugars at G2/M and S phases for the first time. Having found the glycan units of the lamin A/C protein suggests that glycosylation might have a role in the nuclear organization during the cell cycle.

Lectin-binding pattern of the Senegalese sole Solea senegalensis oogenesis

The glycoconjugate pattern of developing ovarian follicles in wild and cultured Senegalese sole Solea senegalensis was investigated by means of lectin histochemistry. Ovaries from cultured fish contained oocytes up to the late vitellogenic stage, whereas they reached the hydration stage in wild specimens. The follicular cells bound MAL II, SBA, HPA, DBA, Con A, KOH-sialidase (K-s)-WGA, GSA I-B(4) in the late vitellogenic stage, and in wild fish also SNA and K-s-PNA, whereas in the hydration stage SBA, HPA, DBA, and GSA I-B(4) only. The zona radiata reacted with SBA, HPA, DBA, Con A, and GSA I B(4) in the late vitellogenic stage and in cultured fish also with UEA I, whereas in the hydration stage it stained with SBA only. The cortical alveoli bound SBA, HPA, RCA(120) during the late vitellogenic stage, also SNA, PNA, K-s-PNA, GSA I-B(4) in cultured fish, DBA, and K-s-WGA in wild ones which stained with SBA, HPA, and GSA I-B(4) in the hydrated stage. The yolk reacted with Con A in the late vitellogenic oocytes, and also with MAL II, SNA, K-s-PNA, SBA, HPA, K-s-WGA, GSA I-B(4), UEA I in the hydrated ones. From perinucleolus to late vitellogenic stages, the oocyte nucleoplasm bound Con A, GSA I-B(4), GSA II, UEA I, and in wild fish also MAL II, SNA, LTA but only GSA I-B(4) reactivity in the early maturation stage. These findings demonstrate that the glycan pattern of fish ovarian follicles changes during the maturative stages and that it is affected by culture-rearing conditions.

PARP-1 expression in the mouse is controlled by an autoregulatory loop: PARP-1 binding to an upstream S/MAR element and to a novel recognition motif in its promoter suppresses transcription

This work identifies central components of a feedback mechanism for the expression of mouse poly(ADP-ribose) polymerase-1 (PARP-1). Using the stress-induced duplex destabilization algorithm, multiple base-unpairing regions (BURs) could be localized in the 5' region of the mouse PARP-1 gene (muPARP-1). Some of these could be identified as scaffold/matrix-attachment regions (S/MARs), suggesting an S/MAR-mediated transcriptional regulation. PARP-1 binding to the most proximal element, S/MAR 1, and to three consensus motifs, AGGCC, in its own promoter (basepairs -956 to +100), could be traced by electrophoretic mobility-shift assay. The AGGCC-complementary GGCCT motif was detected by cis-diammine-dichloro platinum cross-linking and functionally characterized by the effects of site-directed mutagenesis on its performance in wild type (PARP-1(+/+)) and PARP-1 knockout cells (PARP-1(-/-)). Mutation of the central AGGCC tract at basepairs -554 to -550 prevented PARP-1/promoter interactions, whereby muPARP-1 expression became up-regulated. Transfection of a series of reporter gene constructs with or without S/MAR 1 (basepairs -1523 to -1007) and the more distant S/MAR 2 (basepairs -8373 to -6880), into PARP-1(+/+) as well as PARP-1(-/-) cells, revealed an additional, major level of muPARP-1 promoter down-regulation, triggered by PARP-1 binding to S/MAR 1. We conclude that S/MAR 1 represents an upstream control element that acts in conjunction with the muPARP-1 promoter. These interactions are part of a negative autoregulatory loop.

Chromatin supraorganization and extensibility in mouse hepatocytes with development and aging

BACKGROUND

Chromatin supraorganization and extensibility, which lead to the formation of extended chromatin fibers (ECF), are affected by starvation and refeeding in adult mouse hepatocytes. It is expected that they could also change with mouse development and aging.

METHODS

Methods used involved topochemistry, image analysis, microspectrophotometry, gravity action, and polarization microscopy.

RESULTS

Increased nuclear areas and Feulgen-DNA amounts with advancing hepatocyte polyploidy were found with development and aging. A slightly less packed chromatin with more heterogeneously distributed condensation levels was detected in young and old mice. Con-A responsiveness was almost absent in young mice but very deep in aged mice. ECFs formed from nuclei of adult and aged mice but not from nuclei of young mice. The frequency of ECF formation with the long lysis protocol increased with aging.

CONCLUSIONS

In young mice, a less packed chromatin state may be associated with more intense gene activity, thus increasing the DNA-nuclear matrix interactions, and inhibiting ECF formation. Reduced DNA-nuclear matrix interactions besides defects in heterochromatin formation may induce higher ECF formation and chromatin unpackaging in old mice. We suggest that differences in Con-A staining relate to different gene activity with advancing development and aging.

The Con-A-peroxidase method for tissue localization of glucosyl and mannosyl groups applied to mouse hepatocytes and chicken erythrocytes

A variation of the Concanavalin A (Con-A)-peroxidase labelling method originally described by Kiernan [Localization of alpha-D-glucosyl and alpha-D-mannosyl groups of mucosubstances with Concanavalin A and horseradish peroxidase. Histochemistry 1975;44:39-45] was applied to unsectioned cell preparations, with an emphasis on the nuclear localization of glycoproteins. Mouse liver imprints and chicken blood smears fixed in acetic acid-ethanol solution were studied. Modifications of the method included using increased Con-A concentration, and a range of pH values for the Con-A solutions. The strongest Con-A labelling of both erythrocytes and hepatocytes was obtained after incubation with Con-A at pH 6.5 and with Con-A concentrations at least two-fold greater than those used for tissue sections. These conditions may alter the Con-A conformation, enabling the lectin molecule to enter the cell nucleus and bind to nuclear glycoproteins, thus allowing their localization and quantification.

Improvement of gene delivery mediated by mannosylated dendrimer/alpha-cyclodextrin conjugates

The purpose of this study is to evaluate in vitro and in vivo gene delivery efficiency of polyamidoamine (PAMAM) starburst dendrimer (generation 2, G2) conjugate with alpha-cyclodextrin (alpha-CDE conjugate (G2)) bearing mannose (Man-alpha-CDE conjugates) with the various degrees of substitution of the mannose moiety (DSM) as a novel non-viral vector in a variety of cells. Man-alpha-CDE conjugates (DSM 3.3 and 4.9) were found to have much higher gene transfer activity than dendrimer, alpha-CDE conjugate and Man-alpha-CDE conjugates (DSM 1.1 and 8.3) in various cells, which are independent of the expression of cell surface mannose receptors. Cellular association of pDNA complexes with dendrimer, alpha-CDE conjugate and Man-alpha-CDE conjugate (DSM 3.3) and their cytotoxic effects differed only very slightly. Surface plasmon resonance study demonstrated that the specific binding activity of Man-alpha-CDE conjugates to concanavalin A was not very strong. Much more conjugation of the mannose moiety to alpha-CDE conjugates provided unfavorable physicochemical properties of pDNA complexes for gene transfer, e.g. the low interaction with pDNA, the low enzymatic stability of pDNA and the lack of pDNA compaction. Man-alpha-CDE conjugate (DSM 3.3) provided gene transfer activity higher than dendrimer and alpha-CDE conjugate in kidney 12 h after intravenous injection in mice. These results suggest the potential use of Man-alpha-CDE conjugate (DSM 3.3) as a non-viral vector.

Nestin is a potential mediator of malignancy in human neuroblastoma cells

Amplification of the N-myc proto-oncogene signifies aggressive behavior in human neuroblastoma. Likewise, overexpression of the intermediate filament nestin, a neuroectodermal stem cell marker, is linked to increased aggressiveness in several nervous system tumors. We investigated the interaction of these two proteins in human neuroblastoma cells. Neuroblastic cell variants with high levels of N-Myc protein have significantly higher nestin protein levels than non-amplified cell lines, suggesting that the transcription factor N-Myc may regulate nestin expression. Stable transfection of a nestin antisense sequence into neuroblastic, N-myc-amplified, LA1-55n cells results in a 2-fold reduction in nestin protein without altering N-Myc expression. However, cell functions attributed to N-Myc (growth rate, anchorage-independent growth, and motility) all decrease significantly. Transfection studies that modulate N-Myc levels also result in commensurate changes in nestin mRNA and protein amounts as well as in cell proliferation and motility. Thus, nestin appears to be downstream of and regulated by N-Myc. Gel mobility shift assays show that N-Myc binds specifically to E-box sequences in the regulatory second intron of the nestin gene and nuclear run-off studies show that increases in N-Myc protein up-regulate nestin transcription rate. Subcellular fractionation and immunoblot studies indicate that nestin is present in the nucleus as well as in the cytoplasm of neuroblastoma cell lines. Finally, DNA cross-linking experiments show that nestin binds DNA in N-myc-amplified N-type cell lines. Thus, nestin may be one mediator of N-myc-associated tumor aggressiveness of human neuroblastoma.

Interaction of EBV latent origin of replication with the nuclear matrix: identification of S/MAR sequences and protein components

During latency, Epstein Barr virus (EBV) genome, as an episome, is attached to the nuclear matrix (NM) via the latent origin of replication ori P. Within this element, we have found that a region, 580 bp long, encompassing the replicator DS element, shows the strongest affinity for the NM. In addition, by cross-linking with cis-diamminedichloroplatinum, we have identified two NM proteins with an apparent molecular weight of 85 and 60 kDa that, with high affinity and specificity, bind ori P. These proteins are not induced by EBV infection, but their interaction with ori P is lost upon induction of EBV lytic cycle. These data strongly suggest that the binding of ori P to specific components of the NM is required for EBV latent replication.

Immunoprecipitation of DNA-protein complexes cross-linked by cis-diamminedichloroplatinum

For the study of in vitro and in vivo DNA-protein interactions, cross-linking reactions driven by UV or formaldehyde have been frequently used, followed by standard protocols of immunoprecipitation and analysis of the DNA isolated from the complexes. Here we present a basically modified method to analyze the DNA-protein cross-linked complexes obtained by an alternative cross-linking reagent. The innovations presented here include cross-linking by cis-diamminedichloroplatinum II, a fast method to isolate DNA-protein complexes using gel-filtration chromatography, and a modified procedure to obtain specific immunocomplexes that can be analyzed either for DNA or for protein content. The application of this method to two nuclear proteins from chicken liver nuclei is described.

DNA-protein cross-linking in nuclei of immature and mature chicken erythrocytes

DNA-protein cross-linkages were formed in isolated nuclei from immature and mature chicken erythrocytes by reaction with cis-diammine dichloroplatinum. On the basis of electrophoretic behaviour, the most abundant proteins involved in the cross-linking appeared to be present also in preparations of nuclear matrix. The maturation of the erythrocyte, which is accompanied by transcriptional inactivation, leads to a decrease in the amount of DNA-interacting proteins, to a loss of proteins capable of a specific recognition of DNA sequences and, unexpectedly, to the appearence of some new DNA-protein interactions. At least three cross-linked proteins were found predominantly or exclusively in nuclei of immature cells, and three others in those of mature ones. The three DNA-bound proteins, typical of mature erythrocytes, were not found among the components of a high-salt preparation of nuclear matrix. The results obtained suggest that, in addition to the well-known histone H5 and MENT protein, these newly identified DNA-bound proteins contribute to the formation of the condensed, inactive chromatin characteristic of mature erythrocyte.

Glycosylated nuclear lectin CBP70 also associated with endoplasmic reticulum and the Golgi apparatus: does the "classic pathway" of glycosylation also apply to nuclear glycoproteins?

The subcellular plurilocalization of some lectins (galectin-1, galectin-3, galectin-10, calreticulin, etc.) is an intriguing problem, implying different partners according to their localization, and involvement in a variety of cellular activities. For example, the well-known lectin, galectin-3, a lactose-binding protein, can act inside the nucleus in splicing events, and at the plasma membrane in adhesion, and it was demonstrated that galectin-3 interacts in the cytoplasm with Bcl-2, an antiapoptotic protein. Some years ago, our group isolated a nuclear lectin CBP70, capable of recognizing N-acetylglucosamine residues. This lectin, first isolated from the nucleus of HL60 cells, was also localized in the cytoplasm. It has been demonstrated that CBP70 is a glycosylated lectin, with different types of glycosylation, comparing cytoplasmic and nuclear forms. In this article, we have studied the localization of CBP70 in undifferentiated HL60 cells by electron microscopy, immunofluorescence analysis, and subcellular fractionation. The results obtained clearly demonstrated that CBP70 is a plurilocalized lectin that is found in the nucleus, at the endoplasmic reticulum, the Golgi apparatus, and mitochondria, but not at the plasma membrane. Because CBP70, a nuclear glycoprotein, was found to be associated also with the endoplasmic reticulum and the Golgi apparatus where the glycosylation take place, it raised the question: where does the glycosylation of nuclear proteins occur?

Nuclear localization of PAPS synthetase 1: a sulfate activation pathway in the nucleus of eukaryotic cells

Sulfation is a major modification of many molecules in eukaryotes that is dependent on the enzymatic synthesis of an activated sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). While sulfate activation has long been assumed to occur in the cytosol, we show in this study that human PAPS synthetase 1 (PAPSS1), a bifunctional ATP sulfurylase/adenosine 5'-phosphosulfate (APS) kinase enzyme sufficient for PAPS synthesis, accumulates in the nucleus of mammalian cells. Nuclear targeting of the enzyme is mediated by its APS kinase domain and requires a catalytically dispensable 21 amino acid sequence at the amino terminus. Human PAPSS1 and Drosophila melanogaster PAPSS localize to the nucleus in yeast and relieve the methionine auxotrophy of ATP sulfurylase- or APS kinase-deficient strains, suggesting that PAPSS1 is fully functional in vivo when targeted to the nucleus. A second PAPS synthetase gene, designated PAPSS2, has recently been described, mutations of which are responsible for abnormal skeletal development in human spondyloepimetaphyseal dysplasia and murine brachymorphism. We found that PAPSS2, which localizes to the cytoplasm when ectopically expressed in mammalian cells, is relocated to the nucleus when coexpressed with PAPSS1. Taken together, these results indicate that a sulfation pathway might exist in the nucleus of eukaryotic cells. -Besset, S., Vincourt, J.-B., Amalric, F., Girard, J.-P. Nuclear localization of PAPS synthetase 1: a sulfate activation pathway in the nucleus of eukaryotic cells.

Estrogen regulates the association of intermediate filament proteins with nuclear DNA in human breast cancer cells

In a previous study we showed that the levels of the intermediate filament proteins, cytokeratins 8, 18, and 19, in the nuclear matrix-intermediate filament (NM-IF) fraction from the hormone-dependent and estrogen receptor (ER)-positive human breast cancer cell line T-47D5 were regulated by estrogens. In contrast, estrogens did not regulate the cytokeratins in the NM-IF fraction of the hormone-independent and ER-positive cell line, T5-PRF. In this study, human breast cancer cells were treated with cis-diamminedichloroplatinum to cross-link protein to nuclear DNA in situ, and proteins bound to DNA were isolated. We show that cytokeratins 8, 18, and 19 of T-47D5 and T5-PRF were associated with nuclear DNA in situ. The levels of the cytokeratins 8, 18, and 19 bound to nuclear DNA or associated with the cytoskeleton of T-47D5 human breast cancer cells decreased when estrogens were depleted or the pure antiestrogen ICI 164,384 was added. In contrast, the cytokeratin levels associated with nuclear DNA or cytoskeleton were not significantly affected by estrogen withdrawal or antiestrogen administration in T5-PRF cells. These observations suggest that estrogen regulates the organization of nuclear DNA by rearrangement of the cytokeratin filament network in hormone-dependent, ER-positive human breast cancer cells and that this regulation is lost in hormone-independent, ER-positive breast cancer cells.

Differentiation-specific nuclear matrix proteins cross-linked to DNA by cis-diammine dichloroplatinum

DNA-protein cross-linkages were performed in intact undifferentiated and differentiated-HL60 cells by the action of cis-diammine dichloroplatinum. Total nuclear matrix proteins and DNA cross-linked nuclear matrix proteins were resolved by two-dimensional gel electrophoresis. The comparison of the electrophoretic patterns allowed the identification of a set of differentiation-induced nuclear matrix proteins cross-linked to DNA. One of these proteins binds cloned histone SAR sequences. Our results outline an experimental strategy for isolating and characterizing nuclear matrix components that may play a fundamental role in the overall control and coordination of gene expression during differentiation.

CBP70, a glycosylated nuclear lectin

Some years ago, a lectin designated CBP70 that recognized glucose (Glc) but had a stronger affinity for N-acetylglucosamine (GlcNAc), was first isolated from HL60 cell nuclei. Recently, a cytoplasmic form of this lectin was described, and one 82 kDa nuclear ligand was characterized for the nuclear CBP70. In the present study, the use of Pronase digestion and the trifluoromethanesulphonic acid (TFMS) procedure strongly suggest that the nuclear and the cytoplasmic CBP70 have a same 23 kDa polypeptide backbone and, consequently, could be the same protein. In order to know the protein better and to obtain the best recombinant possible in the future, the post-translational modification of the nuclear and cytoplasmic CBP70 was analyzed in terms of glycosylation. Severals lines of evidence indicate that both forms of CBP70 are N- and O-glycosylated. Surprisingly, this glycosylation pattern differs between the two forms, as revealed by beta-elimination, hydrazinolysis, peptide-N-glycosydase F (PNGase F), and TFMS reactions. The two preparations were analyzed by affinity chromatography on immobilized lectins [Ricinus communis-l agglutinin (RCA-I), Arachis hypogaea agglutinin (PNA), Galanthus nivalis agglutinin (GNA), and wheat germ agglutinin (WGA)] and by lectin-blotting analysis Sambucus nigra agglutinin (SNA), Maackia amurensis agglutinin (MAA), Lotus tetragonolobus (Lotus), succinylated-WGA, and Psathyrella velutina agglutinin (PVA)]. Both forms of CBP70 have the following sugar moities: terminal beta Gal residues, Gal beta 1-3 GalNAc, Man alpha 1-3 Man, sialic acid alpha 2-6 linked to Gal or GalNAc; and sialic acid alpha 2-3 linked to Gal. However, only nuclear CBP70 have terminal GlcNAc and alpha-L-fucose residues. All these data are consistent with the fact that different glycosylation pattern found for each form of CBP70 might act as a complementary signal for cellular targeting.

Lectin binding sites on CD34+ human haematopoietic stem cells and lymphocytes from peripheral blood: an ultrastructural post-embedding study

This study was performed to obtain a better insight into the glycosylation pattern of human CD34+ haematopoietic stem cells and lymphocytes from peripheral blood using an ultrastructural post-embedding technique. Lectins applied were derived from Canavalia ensiformis (Con A), Triticum vulgare (WGA), Lycopersicon esculentum (LEA), Limulus polyphemus (LPA), Ulex europaeus-I (UEA-I), Bauhinia purpurea (BPA), Glycine max (SBA), Helix pomatia (HPA), Arachis hypogaea (PNA) and Erythrina cristagalli (ECA). Our results showed almost identical staining patterns with both CD34+ cells and mature lymphocytes from peripheral blood. Con A displayed a prominent reactivity with the nuclear envelope and a weak staining of the plasma membrane. As demonstrated by an elaborate lectin double-labelling technique, WGA revealed an opposite staining pattern. Following neuraminidase treatment of sections, BPA, PNA and SBA exhibited a prominent staining of the plasma membrane in CD34+ cells and lymphocytes as well. Membrane reactivity with HPA was restricted to the majority of lymphocytes, presumably T-lymphocytes. Infrequently occurring dense cytoplasmic (lysosomal) bodies were reactive with a variety of lectins, and a weak diffuse nuclear labelling was observable with LPA, UEA-I, WGA and Con A. It is tempting to speculate that carbohydrate moieties on plasma membranes may be involved in the complex mechanisms characterizing cell-to-cell interactions (adhesion) and particularly in the so-called phenomenon of homing.

Comparison of DNA-protein interactions in intact nuclei from avian liver and erythrocytes: a cross-linking study

DNA-protein cross-linkages were formed in intact nuclei of chicken erythrocytes and liver cells by the action of cis-diammine dichloroplatinum (II). Most cross-linked proteins were components of the nuclear matrix, and their heterogeneity reflected the different complexity of liver and erythrocytes matrices, respectively. Some basic proteins, including histones, were also cross-linked, particularly in erythrocyte nuclei. South-Western blotting revealed that a variety of proteins isolated from the cross-linked liver nuclei recognized DNA specifically. In this group of proteins two relatively abundant, acidic, species of 38 and 66 kDa, respectively, might represent novel DNA-binding proteins from the nuclear matrix. In the case of erythrocytes, only the basic proteins showed a DNA-recognition capacity, and among them there were some unidentified species, absent from liver. Lamin B2 was cross-linked but was unable to recognize DNA, and the same was true for other abundant, cross-linked proteins from both types of nuclei. This led to the hypothesis that for some DNA-nuclear matrix interactions the aggregation typical of matrix proteins is essential for the specificity of DNA recognition. Hybridization analysis of the DNA isolated from the cross-linked complexes showed that SARs (scaffold attachment regions) and telomeric sequences were well represented in the cross-linked fragments, that the cross-linked DNA of liver was partially different from that of erythrocytes and that two defined SAR sequences were found to be present only in the cross-linked DNA. These results are in agreement with the present views on DNA-nuclear matrix interactions, which are usually studied on isolated nuclear matrices or purified proteins. Instead, our results provide experimental evidence obtained directly from intact nuclei.

Identification of two nuclear N-acetylglucosamine-binding proteins

Using neoglycoproteins, lectins that recognize different sugars, including N-acetylglucosamine residues, were previously detected in animal cell nuclei. We report herein the isolation of two N-acetylglucosamine-binding proteins from HL60 cell nuclei: i) a 22 kDa polypeptide (CBP22) with an isoelectric point of 4.5 was isolated for the first time and ii) a 70 kDa polypeptide with an isoelectric point of 7.8. This latter protein corresponds to the glucose-binding protein (CBP70) previously isolated, based on the following similarities: i) they have the same molecular mass, ii) they have the same isoelectric point, iii) they are recognized by antibodies raised against CBP70, and iv) both are lectins from the C group of Drickamer's classification. CBP70 appeared to recognize glucose and N-acetylglucosamine; however, its affinity for N-acetylglucosamine was found to be twice that for glucose. The presence in the nucleus of two nuclear N-acetylglucosamine-binding proteins and their potential ligands, such as O-N-acetylglucosamine glycoproteins, strongly argues for possible intranuclear glycoprotein-lectin interactions.

Lectin-binding glycoproteins in nuclear fractions from hamster liver and Kirkman-Robbins hepatoma

As a further step toward characterizing the major nuclear glycoproteins from hamster liver and Kirkman-Robbins hepatoma (Lipińska A. and Gaczyński M. Int. J. Biochem. 4, 1385-1390, 1992) its intranuclear localization was studied. The glycoprotein patterns of examined nuclear fractions of hamster liver and hepatoma revealed some cell specificity observed especially in nuclear matrix preparations. Our results show the extensive presence of envelope glycoproteins in the nuclear matrix.

Evidence for the presence of complex high-molecular mass N-linked oligosaccharides in intranuclear glycoproteins from HeLa cells

Nonhistone proteins were extracted in 0.4 M NaCl from membrane-depleted nuclei of HeLa cells grown in the presence or the absence of [5,6-3H]fucose. Control experiments strongly suggest that most extracted proteins were indeed nuclear components. Several proteins, present in the 0.4 M NaCl nuclear extract, with M(r) ranging from 35,000 to 115,000 were identified on Western blots as fucosylated glycoproteins owing to their binding to the fucose-specific lectin, Ulex europeus agglutinin I. Results of experiments involving mild alkaline treatment and peptide N-glycosidase F digestion showed that the carbohydrate moieties of these fucosylated nuclear glycoproteins were N-linked to the polypeptide backbone. Analysis of the N-glycans revealed the presence of two populations of sialylated oligosaccharides on the basis of their relative molecular masses. The sensitivity of the high-M(r) oligosaccharides to endo-beta-galactosidase and their incorporation of [3H]glucosamine suggest that they could contain repeating N-acetyllactosamine units. [3H]Fucose incorporated into nuclei was confined to the nucleoli, as judged by autoradiography of sections cut through cells grown in the presence of [3H]fucose. Electron microscopy autoradiography showed that the fibrillar centers were never labeled, while silver grains were observed on the dense and the granular components of nucleoli. Taking into account of these data most nuclear fucosylated glycoproteins extracted in 0.4 M NaCl might be nucleolar ribonucleoproteins.

Crosslinking of nuclear proteins to DNA by cis-diamminedichloroplatinum in intact cells. Involvement of nuclear matrix proteins

In order to detect the nuclear matrix proteins involved in DNA binding, avoiding possible artifacts derived from the disruption of nuclei, proteins were crosslinked to DNA by the action of cis-diamminedichloroplatinum on intact chicken liver cells and analyzed by two-dimensional gel electrophoresis. At least eleven species of crosslinked proteins were found to derive from the nuclear matrix prepared from the same cell type, and five of these were found also among the proteins crosslinked to DNA in intact liver cells from ox and pig. This subset of common proteins, conserved in different animal species, is likely to have a fundamental role for the anchorage of DNA to the nuclear matrix.