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?     

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 β-elimination, hydrazinolysis, peptide-N-glycosydase F (PNGase F), and TFMS reactions. The two preparations were analyzed by affinity chromatography on immobilized lectins [Ricinus communis-I 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 βGal residues, Galβ1–3 GalNAc, Man α1–3 Man, sialic acid α2–6 linked to Gal or GalNAc; and sialic acid α2–3 linked to Gal. However, only nuclear CBP70 have terminal GlcNAc and α-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. J. Cell. Biochem. 66:370–385, 1997. © 1997 Wiley-Liss, Inc.     

Evidence for a lactose-mediated association between two nuclear carbohydrate-binding proteins

Nuclear proteins were extracted in 2 M NaCl from membrane-depletednuclei isolated from HL60 cells. Extracted proteins were submittedto affinity chromatography columns containing immobilized glucose,galactose or lactose. The polypeptides present in the differenteluted fractions were resolved by SDS—PAGE and were eithersilver stained or analysed by immunoblotting with monoclonalor polyclonal antibodies, respectively, raised against the glucose-bindingprotein CBP67 and the galactose-binding proteins CBP35 and L14.The results presented here show that HL60 cell nuclei containCBP35 and a glucose-binding lectin of 70 kDa (CBP70). Thesedata account for the previously reported binding of neoglyco-proteinscontaining glucosyl and galactosyl residues to HL60 cell nuclei.Furthermore, the present study provides the new informationthat CBP35 can associate with CBP70 by interactions dependenton the binding of CBP35 to lactose, and the results of someaffinity chromatography experiments strongly suggest that CBP35and CBP70 associate by protein—protein interactions. Thepotential function of this lactose-mediated interaction is discussedwith respect to data recently reported by others showing thatCBP35 is involved in in vitro mRNA splicing and that lactoseinhibits the processing of the pre-RNA substrate. HL60 lectins nucleus protein—protein interactions     

Stable expression of functional CBP70 lectin during heat shock

CBP70 is a glycoslylated lectin that interacts through either glycan-lectin or protein-protein interactions. In addition, depending on its cellular localization, this lectin has different partners, for example, galectin-3, an 82-kDa ligand in the nucleus, or Bcl-2 in the cytoplasm. In this study, we observed the persistence of plurilocalized lectin CBP70 after two heat-shock treatments conducted either under mild conditions, i.e., incubating the cells for 1 h at 42 degrees C then for 1, 3, 5, 7, or 9 h at 37 degrees C, or harsh conditions, i.e., incubation at 42 degrees C for 1, 2, 4, 6, 8, or 10 h. By combining the information collected from biochemical, fluorocytometric, confocal, and affinity-chromatography analyses, we concluded that CBP70 persisted in HL60 cells and its N-acetylglucosamine-binding sites remained active after all the heat-shock treatments tested. These data and the previously published findings reviewed in this report concur in supporting the hypothesis that CBP70 could function as an organizer of multimeric assembly, leading to the formation of various complexes in different cellular compartments, according to the needs of the cell.     

Two laminin receptors with N-acetylglucosamine-binding specificity

Two glycoproteins, the first, CBP70 which has lectin properties, and the second, cbg72 which is a laminin-1 receptor, have been previously described. We investigated whether cbg72 could have lectin properties and whether CBP70 could have a laminin-receptor function. We observed that CBP70, like cbg72, is a laminin-binding protein. CBP70 interacts with laminin-1 in a carbohydrate-dependent fashion, but this interaction could also be a protein-protein interaction. In parallel, we showed that cbg72, as well as CBP70, is a lectin that recognizes glucose and N-acetylglucosamine in a calcium-dependent manner. Moreover, cross-immunoreactivity was observed between these two lectins using their respective antibodies. The resistance of the two lectins, cbg72 and CBP70, to Triton X-100 extraction, suggests that they potentially interact with cytoskeleton elements, since transmembrane proteins that interact with cytoskeleton elements are known to be resistant to such an extraction.     

Identification of two nuclear N-acetylglucosamine-binding proteins

Using neoglycoproteins, lectine that reconize different sugars, including N-acetylglucosamine residues, were previously detected in animal cell nuclei. We report herein the isolation of two N-acetylglucosamine-binding protein 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 point of 7.8. This latter protein corresponds to the glucose-binding protein (CBP70) previously isolated, based on the following similsrties:(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 classsification. CBP70 appeared to recognized glucose and n-acetylglucosamine; howeve, its affinity for N-acetylglucosamine was found to be twice that for glucose. The presence in the nucleus of two nuclear N-acetylglucosamine-binding protein and their potential ligands, such as O-N-acetylglucosamine glycoproteins, strongly argues for possible intranuclear glycoprotein-lectine interactions.     

Regulation of Lef-mediated transcription and p53-dependent pathway by associating beta-catenin with CBP/p300

CBP and its homologue p300 play significant roles in cell differentiation, cell cycle, and anti-oncogenesis. We demonstrated that beta-catenin, recently known as a potent oncogene, and CBP/p300 are associated through its CH3 region, which is a primary target of adenoviral oncoprotein E1A and various nuclear proteins, such as p53, cyclin E, and AP-1, and both are colocalized in the nuclear bodies. CBP/p300 potentiated Lef-mediated transactivation of beta-catenin, and E1A, a potent inhibitor of CBP/p300, repressed its transactivation. Furthermore, overexpression of stable beta-catenin mutant competitively suppressed the p53-dependent pathway. These may be a key mechanism of beta-catenin involved in oncogenic events underlying disruption of tumor suppressor function through CBP/p300.     

Endogenous lectins from cultured cells: subcellular localization of carbohydrate-binding protein 35 in 3T3 fibroblasts

In previous studies, a lectin designated as carbohydrate-binding protein 35 (CBP35) has been isolated from cultured 3T3 fibroblasts. In the present study, rabbit antibodies directed against CBP35 were used to analyze the subcellular distribution of CBP35 in 3T3 cells. Several lines of evidence indicate that CBP35 is found externally exposed at the cell surface: immunofluorescent staining of live 3T3 cells; agglutination of suspension of 3T3 fibroblasts by specific antibodies; and isolation, by immunoaffinity chromatography, of a Mr 35,000 component from cells surface-labeled with 125I. In addition to the plasma membrane, CBP35 could also be found intracellularly, as revealed by immunofluorescence studies of fixed and permeabilized 3T3 cells. The staining pattern showed the presence of CBP35 on the nucleus and in the cytoplasm. These results are consistent with the finding that among several subcellular fractions, CBP35 can be found by immunoblotting procedures in the nuclear pellet, the soluble fraction, and the plasma membrane fraction of the postnuclear supernatant.     

Dendritic LSm1/CBP80-mRNPs mark the early steps of transport commitment and translational control

Messenger RNA (mRNA) transport to neuronal dendrites is crucial for synaptic plasticity, but little is known of assembly or translational regulation of dendritic messenger ribonucleoproteins (mRNPs). Here we characterize a novel mRNP complex that is found in neuronal dendrites throughout the central nervous system and in some axonal processes of the spinal cord. The complex is characterized by the LSm1 protein, which so far has been implicated in mRNA degradation in nonneuronal cells. In brain, it associates with intact mRNAs. Interestingly, the LSm1-mRNPs contain the cap-binding protein CBP80 that associates with (pre)mRNAs in the nucleus, suggesting that the dendritic LSm1 complex has been assembled in the nucleus. In support of this notion, neuronal LSm1 is partially nuclear and inhibition of mRNA synthesis increases its nuclear localization. Importantly, CBP80 is also present in the dendrites and both LSm1 and CBP80 shift significantly into the spines upon stimulation of glutamergic receptors, suggesting that these mRNPs are translationally activated and contribute to the regulated local protein synthesis.     

Histone deacetylase 6 (HDAC6) deacetylates survivin for its nuclear export in breast cancer

Survivin is an oncogenic protein that is highly expressed in breast cancer and has a dual function that is dependent on its subcellular localization. In the cytosol, survivin blocks programmed cell death by inactivating caspase proteins; however, in the nucleus it facilitates cell division by regulating chromosomal movement and cytokinesis. In prior work, we showed that survivin is acetylated by CREB-binding protein (CBP), which restricts its localization to the nuclear compartment and thereby inhibits its anti-apoptotic function. Here, we identify histone deacetylase 6 (HDAC6) as responsible for abrogating CBP-mediated survivin acetylation in the estrogen receptor (ER)-positive breast cancer cell line, MCF-7. HDAC6 directly binds survivin, an interaction that is enhanced by CBP. In quiescent breast cancer cells in culture and in malignant tissue sections from ER+ breast tumors, HDAC6 localizes to a perinuclear region of the cell, undergoing transport to the nucleus following CBP activation where it then deacetylates survivin. Genetically modified mouse embryonic fibroblasts that lack mhdac6 localize survivin predominantly to the nuclear compartment, whereas wild-type mouse embryonic fibroblasts localize survivin to distinct cytoplasmic structures. Together, these data imply that HDAC6 deacetylates survivin to regulate its nuclear export, a feature that may provide a novel target for patients with ER+ breast cancer.     

NLS-RARα蛋白在重组腺病毒Ad-NE感染的NB4中定位的验证

该实验主要验证重组腺病毒Ad．NE感染NB4细胞后,NLS-RARa蛋白的表达及其定位。用重组腺病毒Ad-NE感染NB4细胞,检测感染效率,分别用RT-PCR和Westernblot法在mRNA水平和蛋白水平验证转染成功：提取转染成功的NB4细胞的核蛋白,Westernblot法检测细胞核中NLS—RARα蛋白的表达;FITC—AnnexinV／DAPI双染色免疫荧光法检测转染成功的NB4细胞NLS-RARα的表达及定位;FITC—AnnexinV／PI双染色激光共聚焦法检测转染成功的NB4细胞中PNLS-RARα的表达及定位。结果显示,重组腺病毒Ad—NE和阴性对照腺病毒Ad-KZ对NB4细胞的感染效率可达70％～80％。RT-PCR和Westernblot结果显示,感染了重组腺病毒Ad—NLS-RAR的NB4细胞成功表龇基因和NE蛋白,且有NLS．RARa的蛋白表达。用细胞免疫荧光法、激光共聚焦法检测出已感染的NB4细胞中NLS—RARer蛋白的表达,并推测其主要定位于胞核。综上所述,该文成功用重组腺病毒Ad-NE感染NB4细胞,并用Westernblot法、免疫荧光法、激光共聚焦法验证了NLS-RARα蛋白的存在并推测其定位,为进一步研究急性早幼粒细胞白血病的早期诊断及复发监测提供了新的思路。     

Metallic prions

Prion diseases, also referred to as transmissible spongiform encephalopathies, are characterized by the deposition of an abnormal isoform of the prion protein in the brain. However, this aggregated, fibrillar, amyloid protein, termed PrPSc, is an altered conformer of a normal brain glycoprotein, PrPc. Understanding the nature of the normal cellular isoform of the prion protein is considered essential to understanding the conversion process that generates PrPSc. To this end much work has focused on elucidation of the normal function and activity of PrPc. Substantial evidence supports the notion that PrPc is a copper-binding protein. In conversion to the abnormal isoform, this Cu-binding activity is lost. Instead, there are some suggestions that the protein might bind other metals such as Mn or Zn. PrPc functions currently under investigation include the possibility that the protein is involved in signal transduction, cell adhesion, Cu transport and resistance to oxidative stress. Of these possibilities, only a role in Cu transport and its action as an antioxidant take into consideration PrPc's Cu-binding capacity. There are also more published data supporting these two functions. There is strong evidence that during the course of prion disease, there is a loss of function of the prion protein. This manifests as a change in metal balance in the brain and other organs and substantial oxidative damage throughout the brain. Thus prions and metals have become tightly linked in the quest to understand the nature of transmissible spongiform encephalopathies.     

Mechanism of glia-neuron cell-fate switch in the Drosophila thoracic neuroblast 6-4 lineage

During development of the Drosophila central nervous system, neuroblast 6-4 in the thoracic segment (NB6-4T) divides asymmetrically into a medially located glial precursor cell and a laterally located neuronal precursor cell. In this study, to understand the molecular basis for this glia-neuron cell-fate decision, we examined the effects of some known mutations on the NB6-4T lineage. First, we found that prospero (pros) mutations led to a loss of expression of Glial cells missing, which is essential to trigger glial differentiation, in the NB6-4T lineage. In wild-type embryos, Pros protein was localized at the medial cell cortex of dividing NB6-4T and segregated to the nucleus of the glial precursor cell. miranda and inscuteable mutations altered the behavior of Pros, resulting in failure to correctly switch the glial and neuronal fates. Our results suggested that NB6-4T used the same molecular machinery in the asymmetric cell division as other neuroblasts in cell divisions producing ganglion mother cells. Furthermore, we showed that outside the NB6-4T lineage most glial cells appeared independently of Pros.     

Rad53 homologue forkhead-associated kinase A (FhkA) and Ca<Superscript>2+</Superscript>-binding protein 4a (CBP4a) are nucleolar proteins that differentially redistribute during mitosis in <Emphasis Type="Italic">Dictyostelium</Emphasis>

Background

During mitosis most nucleolar proteins redistribute to other locales providing an opportunity to study the relationship between nucleolar protein localization and function. Dictyostelium is a model organism for the study of several fundamental biological processes and human diseases but only two nucleolar proteins have been studied during mitosis: NumA1 and Snf12. Both of them are linked to the cell cycle. To acquire a better understanding of nucleolar protein localization and dynamics in Dictyostelium we studied the nucleolar localization of two additional proteins during mitosis: Snf12-linked forkhead-associated kinase A (FhkA), which is involved in the cell cycle, and Ca²⁺-binding protein 4a (CBP4a), which is a binding partner of NumA1.

Methods

Polyclonal antibodies were produced in-house. Cells were fixed and probed with either anti-FhkA or anti-CBP4a in order to determine cellular localization during interphase and throughout the stages of mitosis. Colocalization with DAPI nuclear stain allowed us to determine the location of the nucleus and nucleolus while colocalization with anti-α-tubulin allowed us to determine the cell cycle stage.

Results

Here we verify two novel nucleolar proteins, Rad53 homologue FhkA which localized around the edge of the nucleolus and CBP4a which was detected throughout the entire nucleolus. Treatment with the Ca²⁺ chelator BAPTA (5?mM) showed that the nucleolar localization of CBP4a is Ca²⁺-dependent. In response to actinomycin D (0.05?mg/mL) CBP4a disappeared from the nucleolus while FhkA protruded from the nucleus, eventually pinching off as cytoplasmic circles. FhkA and CBP4a redistributed differently during mitosis. FhkA redistributed throughout the entire cell and at the nuclear envelope region from prometaphase through telophase. In contrast, during prometaphase CBP4a relocated to many large, discrete “CBP4a islands” throughout the nucleoplasm. Two larger “CBP4a islands” were also detected specifically at the metaphase plate region.

Conclusions

FhkA and CBP4a represent the sixth and seventh nucleolar proteins that have been verified to date in Dictyostelium and the third and fourth studied during mitosis. The protein-specific distributions of all of these nucleolar proteins during interphase and mitosis provide unique insight into nucleolar protein dynamics in this model organism setting the stage for future work.

     

NMR structure and regulated expression in APL cell of human SH3BGRL3

SH3 domain binding glutamic acid-rich protein like 3 (SH3BGRL3) is the new member of thioredoxin (TRX) super family, whose posttranslational modified form was identified as tumor necrosis factor alpha (TNF-alpha) inhibitory protein, TIP-B1. In this paper, we determined its solution structure by multi-dimensional nuclear magnetic resonance spectroscopy. The overall structure of human SH3BGRL3 conformed to a TRX-like fold. To understand its function in vivo, the upregulated expression in acute promyelocytic leukemia cell line NB4 at both mRNA and protein level was elucidated. Immunofluorescence and immunohistochemistry staining with monoclonal antibody against SH3BGRL3 demonstrated that it was a cytoplasmic protein in both NB4 cell and human tissues. These results, as a whole, indicate that SH3BGRL3 may function as a regulator in all-trans retinoic acid-induced pathway.     

Nitric oxide-induced nuclear GAPDH activates p300/CBP and mediates apoptosis

Besides its role in glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) initiates a cell death cascade. Diverse apoptotic stimuli activate inducible nitric oxide synthase (iNOS) or neuronal NOS (nNOS), with the generated nitric oxide (NO) S-nitrosylating GAPDH, abolishing its catalytic activity and conferring on it the ability to bind to Siah1, an E3-ubiquitin-ligase with a nuclear localization signal (NLS). The GAPDH-Siah1 protein complex, in turn, translocates to the nucleus and mediates cell death; these processes are blocked by procedures that interfere with GAPDH-Siah1 binding. Nuclear events induced by GAPDH to kill cells have been obscure. Here we show that nuclear GAPDH is acetylated at Lys 160 by the acetyltransferase p300/CREB binding protein (CBP) through direct protein interaction, which in turn stimulates the acetylation and catalytic activity of p300/CBP. Consequently, downstream targets of p300/CBP, such as p53 (Refs 10,11,12,13,14,15), are activated and cause cell death. A dominant-negative mutant GAPDH with the substitution of Lys 160 to Arg (GAPDH-K160R) prevents activation of p300/CBP, blocks induction of apoptotic genes and decreases cell death. Our findings reveal a pathway in which NO-induced nuclear GAPDH mediates cell death through p300/CBP.