Identification and purification of a stress associated nuclear carbohydrate binding protein (M(r) 33,000) from rat liver by application of a new photoreactive carbohydrate probe

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Sugar-dependent nuclear import of glycosylated proteins in living cells

The nuclear import of proteins larger than Mr 40,000 depends on the presence of a nuclear localization signal (NLS) corresponding either to a short peptide sequence or to defined sugars. The sugar-dependent nuclear import was previously evidenced by using glycosylated proteins (neoglycoproteins) introduced into the cytosol of cells either by electroporation or on digitonin-permeabilization and was shown to be distinct from the peptide NLS-mediated pathway. In this work, we used a microinjection approach to compare the two nuclear import pathways in intact living cells. The intracellular localization of fluorescent NLS-BSA or Glc-BSA injected into the cytosol was analyzed by confocal microscopy. Novel differences between the two mechanisms were evidenced. First, Glc-BSA migrated less efficiently into the nucleus than NLS-BSA because of a cytosolic retention. Second, the import of neoglycoproteins was not affected by microinjection of antinuclear import factor importin/karyopherin beta antibodies, whereas the NLS-dependent transport was completely abolished. Third, the nuclear import activity of Glc-BSA was found to be cell cycle-dependent in thymidine and hydroxyurea-treated HeLa cells, with greatest efficiency during G1/S transition and S phases, whereas NLS-BSA was imported with the same efficiency during any stage of the cell cycle but the G2 phase. Fourth, we show that after mitosis, nonglycosylated BSA was excluded from the nucleus contrary to Glc-BSA. In both cases, the nuclear import signals (NLS or alpha-glucoside) were grafted onto BSA; such tools led to a clear-cut conclusion, which will reach a full physiological significance when they are confirmed in the case of endogenous (glyco)proteins.

Photoaffinity glycoprobes-a new tool for the identification of lectins

One of the proposed functions for the carbohydrate structures on glycoconjugates is the transfer of information through interaction with specific lectin receptors. However, the number of elucidated functional lectin-carbohydrate interactions is still relatively small, largely due to the lack of adequate methods to identify lectin activity in complex biological samples. Aiming to solve this problem, we have developed a method based on the novel group of compounds we named glycoprobes. The glycoprobe consists of three vital parts: (1) glycan, (2) digoxin tag, and (3) photoreactive crosslinker. When incubated in dark, oligosaccharide part of the glycoprobe forms a complex with lectin. After illumination, covalent link between the probe and the lectin is formed resulting in a digoxin-tagged lectin. Using antibodies against digoxin, this complex can easily be identified immuno/cytochemically, or by Western blots. To demonstrate the applicability of glycoprobes we have used Man(9)-glycoprobe (containing Man(9)oligosaccharide) and YEE(ahGalNAc)(3)-glycoprobe (containing a synthetic neoglycopeptide with three terminal N-acetyl-galactosamine residues; Lee and Lee, Glycoconjugate J., 1987,4, 317) to identify lectins in bovine serum and rat liver membranes. The simplicity of the method enables its application in routine monitoring of changes in lectin activity during various developmental or pathological processes. An example of GalNAc-binding analysis in human serum is shown.

Purification and MALDI-MS characterization of stressin, a stress-associated glycoprotein

Glycoconjugates have a whole spectrum of biological roles, from those that appear trivial to those that are crucial. Results accumulated in the past years indicate they might also play an important role in the response to stress, a complex physiological response of the human organism to various threats. We have recently identified stressin, a human serum glycoprotein, which was found to be increased under stress conditions. Here we report the purification of stressin from sera of professional soldiers and partial characterization of its protein and carbohydrate parts using lectin blotting and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Stressin was purified using a combination of ammonium sulfate precipitation, ion exchange chromatography, preparative gel electrophoresis and reverse-phase HPLC. It was found to be a highly glycosylated protein. Only 21.9 kDa (out of 36.7 kDa) was the protein part, whereas the remaining 40% of the mass originated from N-linked oligosaccharides. The carbohydrate part contained 12 sialic acids moieties, nearly 90% of which were lost due to post-source decay in the field-free tube. Tryptic fragments were produced from glycosylated and deglycosylated stressin, separated by reverse-phase HPLC and their exact molecular masses were determined using MALDI-MS. Comparison with tryptic maps of other proteins in computer databases indicated that stressin does not correspond to any already described protein.

Stressin and natural killer cell activity in professional soldiers

Chronic stress causes multiple biochemical and physiological changes in the human organism. Recently we have identified stressin, a human serum glycoprotein that was significantly increased in sera of prisoners released from Serbian concentration camps. To eliminate malnutrition and maltreatment as possible causes for the increased stressin concentration, we have analyzed stressin in sera of 40 professional soldiers after involvement in major military activity and compared it to stressin in sera of 20 control individuals. As expected, the sera of professional soldiers contained more than 2.2 times higher concentrations of stressin than control sera. It is interesting that, contrary to expectations, the natural killer cell activity of professional soldiers was normal or even increased. We hypothesize that this might be an effect of winning the war that could have, at least temporarily, erased the immunosuppressive effects of stress.

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.

Nuclear and cytoplasmic expressions of the carbohydrate-binding protein CBP70 in tumoral or healthy cells of the macrophagic lineage

The expression of the carbohydrate-binding protein CBP70 was analyzed in undifferentiated HL60 cells, HL60 cells differentiated into monocytes/macrophages or granulocytes, healthy monocytes and in vitro monocyte-derived macrophages (MDM) using an anti-CBP70 serum. This study was performed by immunoblotting analysis of nuclear and cytoplasmic extracts before and after N-acetylglucosamine affinity chromatography and by indirect immunofluorescence microscopy. The results of this study show, for the first time, that CBP70 is expressed in the nucleus and the cytoplasm of healthy or leukemic cells of the macrophagic lineage. However, striking differences were observed depending upon the leukemic or normal state of cells and cell differentiation. Indeed, the level of expression and the intracellular distribution of CBP70 were found to be different in undifferentiated HL60 cells and monocytes/macrophages differentiated from these cells. Major differences were also observed according to whether macrophages differentiated from leukemic HL60 cells or healthy monocytes. Thus, the total cellular expression of CBP70 was markedly lower in MDM than in HL60-derived macrophages and the intracellular distribution of the protein was different. Nevertheless, in both cases, the total cellular expression of CBP70 was enhanced during cell differentiation. Another important result is the finding that CBP70 behaviour was totally different when HL60 cells were induced to differentiate into macrophages or granulocytes. These data could therefore suggest that CBP70 is involved in phagocytic cell differentiation. Moreover, we show that an additional 60 kDa polypeptide (p60), recognized by the anti-CBP70 serum, is expressed in HL60 cells differentiated into macrophages or granulocytes as well as in healthy monocytes or MDM but not expressed in undifferentiated HL60 cells. Although CBP70 and p60 appeared to be closely related polypeptides, their relationship remains to be precised. These findings are discussed with regard to data available on galectin-3.

Sialoadhesin binds preferentially to cells of the granulocytic lineage

Sialoadhesin is a macrophage-restricted, sialic acid-dependent receptor of 185 kD that binds to the oligosaccharide sequence NeuAc alpha 2,3Gal on cell surface glycoconjugates. Recent cDNA cloning has shown that sialoadhesin is a new member of the immunoglobulin superfamily with sequence similarity to CD22, a sialic acid-dependent receptor of B lymphocytes. Sialoadhesin has been implicated in cellular interactions of stromal macrophages with developing myeloid cells. In this study, direct evidence for this interaction was obtained in cell-cell binding assays using both native and recombinant forms of the protein. In all assays, sialoadhesin exhibited specific, differential binding to various murine cell populations of hemopoietic origin. In rank order, sialoadhesin bound neutrophils > bone marrow cells = blood leukocytes > lymphocytes > thymocytes. Single-cell analyses confirmed that sialoadhesin selectively bound myeloid cells in complex cell mixtures obtained from the bone marrow and blood. In comparison, a recombinant Fc-chimeric form of murine CD22 showed high binding to B and T lymphocytes, but very low binding to immature and mature myeloid cells. These results are consistent with the notion that sialoadhesin in involved in interactions with granulocytes at different stages of their life histories.