A bifunctional enzyme catalyzes the first two steps in N-acetylneuraminic acid biosynthesis of rat liver. Purification and characterization of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase

Biosynthesis of N-acetylneuraminic acid (Neu5Ac), a prominent component of glycoconjugates, is initiated by the action of UDP-N-acetylglucosamine 2-epimerase (UDP-GlcNAc 2-epimerase, EC 5.1. 3.14) and N-acetylmannosamine kinase (ManNAc kinase, EC 2.7.1.60). We demonstrate for the first time that the two activities are parts of one bifunctional enzyme in rat liver. The enzyme was purified to homogeneity from rat liver cytosol using salmine sulfate precipitation and chromatography on phenyl-Sepharose, ATP-agarose, and Mono Q. The purification resulted in one polypeptide with an apparent molecular mass of 75 kDa. Immunoprecipitation with a polyclonal antibody against the polypeptide reduced both enzyme activities in equal amounts. Gel filtration analysis of purified UDP-GlcNAc 2-epimerase/ManNAc kinase showed that the polypeptide self-associates as a dimer and as a hexamer with apparent molecular masses of 150 and 450 kDa, respectively. The hexamer was fully active for both enzyme activities, whereas the dimer catalyzed only the phosphorylation of N-acetylmannosamine (ManNAc). Incubation of the dimer with UDP-N-acetylglucosamine led to reassembly of the fully active hexamer; maximal quantities of the hexamer were produced after incubation for 3 h. Kinetic analysis of purified hexameric and dimeric enzyme revealed significantly lower Michaelis constants (93 +/- 3 to 121 +/- 15 microM for ManNAc and 1.18 +/- 0. 13 to 1.67 +/- 0.20 mM for ATP) and higher cooperativity (Hill coefficients of 1.42 +/- 0.16 to 1.17 +/- 0.06 for ManNAc and 1.30 +/- 0.09 to 1.05 +/- 0.14 for ATP) for the hexamer for both substrates of ManNAc kinase. The Michaelis constant of UDP-GlcNAc 2-epimerase for its substrate was 11 +/- 2 microM. The Hill coefficient of 0.45 +/- 0.07 represents strongly negative cooperativity in substrate binding. UDP-GlcNAc 2-epimerase was feedback inhibited by CMP-Neu5Ac. Complete inhibition was achieved with 60 microM CMP-Neu5Ac, and highly positive cooperativity (Hill coefficient of 4.1) was found for inhibitor binding.

Consequences of a subtle sialic acid modification on the murine polyomavirus receptor

Polyomaviruses are small, nonenveloped DNA tumor viruses with restricted host ranges. Virus binding to cell surface receptors is one determinant of viral tropism. Although murine polyomavirus is among the best characterized viruses, little is known about the sialic acid-containing receptor and its interaction with viral particles. By using nonradioactive virus binding assays as recently described for the B-lymphotropic papovavirus, murine polyomavirus particles were found to bind in a saturable and noncooperative manner to 25,000 receptors per 3T6 mouse fibroblast. The virus-receptor interaction at 4 degrees C was of high affinity (Kd = 1.8 x 10(-11) M), very fast (k1 = 1.7 x 10(7) M(-1) s(-1)), and stable (half-life = 38 min). Elongation of the N-acyl side chain of sialic acid by biosynthetic modulation with synthetic precursor analogs has been shown for other polyomaviruses to influence both sialic acid-dependent binding and infection (O. T. Keppler, P. Stehling, M. Herrmann, H. Kayser, D. Grunow, W. Reutter, and M. Pawlita, J. Biol. Chem. 270:1308-1314, 1995). In 3T6 cells in which about one-third of the sialic acids were modified, infection and binding of polyomavirus particles were significantly reduced. The number of receptors per cell was decreased to 18,000, with the remaining receptors displaying the same affinity as in untreated cells. Molecular modeling studies based on the three-dimensional structure of a mouse polyomavirus-sialyllactose complex recently solved by T. Stehle and coworkers (T. Stehle, Y. W. Yan, T. L. Benjamin, and S. C. Harrison, Nature 369:160-163, 1994) were performed. They suggest that the elongation of the N-acyl side chain by a single methylene group leads to steric hinderence, with the peptide backbone of a loop walling the tip of the shallow sialic acid binding groove. This collision appears to be incompatible with functional binding. The data are taken as a basis to discuss possible features of the organization and topology of the cellular receptor for mouse polyomavirus.

Turnover studies of the neural cell adhesion molecule NCAM: degradation of NCAM in PC12 cells depends on the presence of NGF

The neural cell adhesion molecule NCAM is a member of the immunoglobulin superfamily and involved in path finding and outgrowth of neurites in vitro. PC12 cells express two major isoforms of NCAM (NCAM180 and NCAM140) and undergo neuronal differentiation, e.g., neurite formation, in response to NGF. Using this cell system, we determined the half-life time of both NCAM isoforms in the absence and presence of NGF. Half-life time of NCAM140 is similar in the presence and absence of NGF, whereas the half-life time of NCAM180 is increased in the presence of NGF. Furthermore, we quantified protein expression of both NCAM isoforms in the presence or absence of NGF and found a decrease of NCAM protein expression in course of neuronal differentiation.

Domain-specific N-glycosylation of the membrane glycoprotein dipeptidylpeptidase IV (CD26) influences its subcellular trafficking, biological stability, enzyme activity and protein folding

Dipeptidyl peptidase IV (DPPIV, CD26) is an N-glycosylated type II plasma membrane protein. The primary structure of rat wild-type DPPIV contains eight potential N-glycosylation sites. To investigate the role of N-glycosylation in the function of DPPIV, three of its asparagine residues were separately converted to glutamine by site-directed mutagenesis. The resulting N-glycosylation mutants of rat DPPIV were studied in stable transfected Chinese hamster ovary cells. All three N-glycosylation mutants of DPPIV showed a reduced half-life, as well as differing degrees of inhibition of the processing of their N-glycans. Mutation of the first (Asn83-->Gln) or eighth (Asn686-->Gln) N-glycosylation site had only a small effect on its enzymatic activity, cell-surface expression and dimer formation, whereas the mutation of the sixth N-glycosylation site (Asn319-->Gln) abolished the enzymatic activity, eliminated cell-surface expression and prevented the dimerization of the DPPIV protein. The mutant [Gln319]DPPIV is retained in the cytoplasm and its degradation was drastically increased. Our data suggest that the N-glycosylation at Asn319 is involved in protein trafficking and correct protein folding.

Analysis of site-specific N-glycosylation of recombinant Desmodus rotundus salivary plasminogen activator rDSPA alpha 1 expressed in Chinese hamster ovary cells

The recombinant plasminogen activator (rDSPA alpha 1) from the vampire bat Desmodus rotundus is a promising new thrombolytic agent that exhibits a superior pharmacological profile if compared to tissue-type plasminogen activator (t-PA) or streptokinase. In the present study the structures of the carbohydrate moieties at the two N-glycosylation sites (Asn-117, Asn-362) of rDSPA alpha 1 expressed in Chinese hamster ovary cells were determined. N-Linked glycans were enzymatically released from isolated tryptic glycopeptides by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F digestion and separated by two-dimensional HPLC. Oligosaccharide structures were characterized by analysis of carbohydrate composition and linkage, by mass spectrometry, and by sequence analysis in which the fluorescently labeled glycans were cleaved with an array of specific exoglycosidases. More than 30 different oligosaccharides were identified. The results revealed that Asn-117 carried a mixture of one high-mannose structure (17% of site-specific glycosylation), three hybrid glycans (26%) and predominantly biantennary complex N-glycans (54%). Glycosylation site Asn-362 was found to comprise complex glycans with biantennary (50%), 2,4- and 2,6-branched triantennary (21%, 11%), and tetraantennary structures (10%), which were fucosylated at the innermost residue of N-acetylglucosamine. Mainly neutral and monosialylated glycans, and smaller quantities of disialylated glycans, were detected at both glycosylation sites. Sialic acid was alpha 2-3 linked to galactose exclusively. As shown in this study the N-glycans attached to Asn-117 of rDSPA alpha 1 are more processed during biosynthesis than the high-mannose structures linked to Asn-117 of t-PA, to which the polypeptide backbone of rDSPA alpha 1 is structurally closely related.

Inhibition of L-fucokinase from rat liver by L-fucose analogues in vitro

By investigating the effects of more than 15 different L-fucose analogues on the activity of L-fucokinase (EC 2.7.1.52) from rat liver in vitro, certain structural requirements for potent inhibition of this enzyme were established. Of the novel compounds, 4,6-dideoxy-L-xylo-hexopyranose (4) and methyl 4,6-dideoxy-4-iodo-L-glucopyranose (9) were found to be competitive inhibitors with Ki-values of 0.5 mM and 5.0 mM respectively. Thus 4,6-dideoxy-L-xylo-hexopyranose is a better inhibitor of L-fucokinase than methyl-alpha-L-fucoside (1). Uptake of L-fucose into rat hepatoma cells is reduced by 52% in the presence of the deoxy derivative (4), leading to a decrease of 45% in the incorporation of L-fucose into total cellular glycoproteins.

Quantification of cell-matrix and cell-cell adhesion using horseradish peroxidase

A simple, universal, and rapid enzymatic method for the quantitative determination of cell adhesion in 96-well cell culture plates has been established. The assay is based on cellular steady-state endocytosis, which is used to label cells with horseradish peroxidase (HRP) prior to adhesion. Subsequently, attached cells can be detected by a simple enzymatic reaction, in which the accumulated HRP catalyzes dye formation from a colorless hydrogen donor, e.g., o-phenylenediamine, in the presence of hydrogen peroxide. As demonstrated with different cell lines and test systems, the method can be used to quantify cell-matrix as well as cell-cell interactions and allows a very sensitive quantification of adherent cells. The HRP label is nontoxic and does not affect the adhesion properties of tested cell lines; the quantity of dye formed is proportional to the number of adherent cells. Furthermore, the assay represents an alternative method to isotopic cell labeling, e.g., with 51Cr, which is usually used for quantifying cell-cell interactions.

Chemical cross-linking detects different conformational arrangements of platelet integrin alpha IIb beta III (gpIIb/IIIa)

Treatment of Triton X-100 solubilized platelet membrane with the homobifunctional cross-linker dithiobis(succinimidyl propionate) resulted in covalent cross-linking of the platelet integrin alpha IIb beta 3 (gpIIb/IIIa), the fibrinogen receptor, into three high-molecular-mass complexes with apparent M of 200, 220 and 240 k. Generation of these cross-linked alpha IIb beta 3 aggregates depended on the presence of a native receptor structure and was not influenced by Ca2+ ions and other experimental conditions. Immunoblotting analysis of purified 200/220/240 kDa aggregates revealed that they were made up exclusively by alpha IIb and beta 3 integrin chains in roughly stoichiometric amounts. We therefore conclude that alpha IIb beta 3 integrin occurs in different conformations in the platelet membrane that can be directly detected by chemical cross-linking.

Is the lectin binding pattern of human breast and colon cancer cells influenced by modulators of sialic acid metabolism?

Sialic acid residues are the most abundant terminal carbohydrate residues of mammalian cells. Modification of the sialic acid residues by exposure of cells in culture to sialic acid precursor analogues resulted in a modified susceptibility to polyoma viruses. In the present study, human breast and colon cancer cell lines were exposed for 65 h to these acid precursor analogues at 5 mM and their lectin binding pattern was analysed. Use of a panel of several different lectins indicated that the pretreatment of these cell lines with the sialic acid analogues did not change their lectin binding profile. The incorporation of these precursors into membrane glycoproteins was assessed by reversed phase high-performance liquid chromatography, which clearly demonstrated that the precursors were incorporated. The results therefore indicate that these analogues are highly specific for sialic acid and do not interfere with other biosynthetic pathways of membrane glycoconjugates.

Transforming growth factor-beta (TGF-beta) stimulates the expression of beta1 integrins and adhesion by rat mesangial cells

Transforming growth factor-beta (TGF-beta) has been implicated in the pathogenesis of mesangial matrix (MM) accumulation in human and experimental glomerulonephritis. To clarify molecular mechanisms responsible for pathological MM deposition, we examined the effect of TGF-beta on the production of beta1 integrins and on adhesion function of rat mesangial cells (MC). In immunoprecipitation experiments using [35S]methionine-labeled MC, stimulation of MC with TGF-beta for 48 h resulted in an increase in the synthesis of alpha1beta1 (collagen/laminin receptor) and alpha5beta1 (fibronectin receptor) integrins accompanied by increases in the synthesis of their ligands, collagen type I (collagen I), and fibronectin. A time-dependent increase in beta1, alpha1 integrin subunit mRNA peaking 48 h after exposure to TGF-beta was shown by Northern blot analysis. After 48 h of treatment with TGF-beta, MC displayed significant increases in adhesion to fibronectin, collagen I, and laminin as compared to untreated MC. Anti-beta1 antiserum significantly inhibits MC adhesion to fibronectin, collagen I, and laminin. Anti-alpha1 subunit antibody very strongly inhibited adhesion to collagen I and laminin, but not to fibronectin. Synthetic peptides containing RGD sequences specifically blocked adhesion to fibronectin. These data suggest that TGF-beta may promote MM deposition by increasing MC synthesis of both matrix proteins and beta1 integrins which facilitate binding of these proteins to the MC surface and thus enhance their incorporation into MM.

Cell-matrix interactions of normal and transformed human keratinocytes in vitro are modulated by the synthetic phospholipid analogue hexadecylphosphocholine

We have investigated the effect of the synthetic phospholipid analogue, hexadecylphosphocholine (HePC), a member of a new class of membrane-affecting antiproliferative drugs, on adhesion to extracellular matrix components, reorganization of three-dimensional collagen lattices, and proliferative activity of human keratinocyte populations in vitro. Six transformed keratinocyte lines were compared with their normal counterparts from interfollicular epidermis. Adhesion to collagen types I and IV, laminin, and fibronectin was weakest in normal keratinocytes (binding of 3-10% of the cells), followed by HaCaT and HaCaT-II/3 (25-30% binding), and the squamous carcinoma lines and SV-40 transformed keratinocytes (35-50% binding). Treatment with non-toxic doses of 10-20 mumol/l HePC led to a clear inhibition of the adhesive interactions by 50-75% in all populations. The impaired adhesion capacity was paralleled by a clear decrease of the ability of all keratinocyte populations to contract three-dimensional collagen lattices, an experimental model system for the reorganization of extracellular matrices. Histological examination revealed that these effects were due to a reduced cell number in the collagen lattices, a finding underscored by significant inhibition of proliferative activity of all keratinocyte populations by HePC. Furthermore, HePC led to marked changes of cellular morphology in the contracted gels. FACS analysis revealed that the impaired interaction with components of the extracellular matrix was not due to a specific downregulation of beta 1-integrins, the major cell-surface receptors for the respective matrix proteins. In conclusion, our results provide new insights into the potential action of HePC in a variety of skin disorders. Decreased proliferative activity and changes of cellular morphology, combined with impaired interactions with extracellular matrix proteins and a consecutive loss of matrix organization capacity, may cause suppression of different hyperproliferative skin diseases.

In vivo modulated N-acyl side chain of N-acetylneuraminic acid modulates the cell contact-dependent inhibition of growth

Sialylation of plasma membrane glycoproteins is thought to be involved in the regulation of differentiation and in the process of tumorigenesis. Here we show that sialylation also affects cell-cell contact-dependent growth regulation. When cultured in the presence of non-physiological synthetic sialic acid precursors, human diploid fibroblasts no longer exhibited density-dependent inhibition of growth. Concomitantly, increased sialylation of contactinhibin, a glycoprotein involved in density-dependent inhibition of growth, was observed. These results indicate that sialidase-resistant sialic acid modifications lead to dysregulated growth control. The modifications have been induced by N-propanoyl and other N-acyl derivatives of D-mannosamine.

Rapid isolation of endosomes from BHK cells: identification of DPP IV (CD26) in endosomes

In plasma membrane glycoproteins the peripheral monosaccharides of the N-glycan side chains are degraded faster than the core oligosaccharides and the protein backbone. This intramolecular heterogeneous turnover is a typical characteristic of membrane glycoproteins and is termed remodeling or reprocessing. The mechanism of the reprocessing has been shown first for dipeptidyl peptidase IV (DPP IV, CD26). However, it is still a question in which subcellular compartment the enzyme machinery for the reprocessing is located. Since lysosomes could be excluded, it has been proposed that the responsible glycosidases are located at the plasma membrane, in endosomes, or in the trans-Golgi network. The present study is concerned with the possible role of endosomes in this process of reprocessing. We transfected nonpolarized BHK cells with rat DPP IV cDNA. By establishing a fast and efficient method to purify endosomes, we could identify for the first time significant amounts of DPP IV in endosomes and we suggest therefore that endosomes are closely related with the regulation of reprocessing of plasma membrane glycoproteins.

O-linked L-fucose is present in Desmodus rotundus salivary plasminogen activator

DSPAalpha1 (Desmodus rotundus salivary plasminogen activator), a plasminogen activator from the saliva of the vampire bat Desmodus rotundus, is an effective thrombolytic agent. An unusual type of posttranslational modification, in which L-fucose is O-glycosidically linked to threonine 61 in the epidermal growth factor domain was found for natural DSPAalpha1 and its recombinant form isolated from Chinese hamster ovary cells. In the present study a combination of carbohydrate and amino acid composition analysis, amino acid sequencing, and mass spectrometry revealed that the L-fucose is bound to residues 56-68 of DSPAalpha1. The amino acid sequence of this glycosylation site agreed with the suggested consensus sequence Cys-Xaa-Xaa-Gly-Gly-Ser/Thr-Cys described for other proteins. Anew strategy for the identification of the modified amino acid was established. Direct evidence for the occurrence of fucosyl-threonine was obtained by mass spectrometry after digestion of the glycopeptide with a mixture of peptidases. On the basis of these results, DSPAalpha1 is a suitable model for studying the influence of O-fucosylation on clearance rates, particularly in comparative studies with the identically fucosylated and structurally related tissue plasminogen activator.

Determination of cytidine 5'-monophospho-N-acetylneuraminic acid pool size in cell culture scale using high-performance anion-exchange chromatography with pulsed amperometric detection

A simultaneous determination of cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-Neu5Ac) and its metabolic products, cytidine, CMP and Neu5Ac by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) using a Carbo-Pac PA1 column is described. Preparation of the samples involved a single purification step of the crude cell extract on DEAE-Sepharose. The method is adequate to quantify the amount of CMP-Neu5Ac produced by one culture dish; equivalent to 6.10(6) cells. In addition, a method for desalting and recovery of the separated material was developed to determine the cellular concentration of CMP-Neu5Ac in Madin Darby canine kidney (MDCK) cells. The addition of 5 mM N-acetylmannosamine to the culture medium gave rise to a 6.4-fold elevation of this value.

The cysteine-rich region of dipeptidyl peptidase IV (CD 26) is the collagen-binding site

A remarkable property of the integral glycoprotein dipeptidyl peptidase IV (DPP IV, CD 26) is its affinity to proteins of the extracellular matrix (ECM). By in vitro binding assays we have shown that DPP IV binds to collagens; preferentially to the collagens I and III, which are both characterized by the formation of large triplehelical domains. No binding of DPP IV to laminin or fibronectin could be observed. Within collagen I, the alpha 1(I) chain was found to be the most prominent binding ligand of DPP IV. A monoclonal anti DPP IV antibody (13.4) specifically inhibited the interaction of DPP IV with collagen I. Peptide mapping and N-terminal sequencing revealed that the corresponding epitope of mAb 13.4 is located in the cysteine-rich domain of DPP IV. We therefore conclude that the putative collagen binding site of DPP IV is different from the region of the catalytic site containing the exopeptidase activity, which is located at the C-terminal portion of the molecule.

Influence of extracellular matrices on ganglioside pattern of two hepatoma cell lines with different adhesive properties

A cell culture model was developed to investigate the involvement of gangliosides in cell-matrix adhesion. Two cell lines with different adhesive properties derived from solid Morris hepatoma 7777 were established. Cultured in horse serum-containing medium, the adhesive cell line (MH 7777A) adheres and spreads on uncoated culture dishes, whereas the revertant cell line (MH 7777A > N) does not adhere and grows in suspension. The adhesiveness of both cell lines is dependent on the coating protein used (none, bovine serum albumin, fibronectin or collagen I) and the horse serum concentration in the culture medium. Both cell lines, although of the same origin, differed in their ganglioside composition. The most abundant ganglioside of both MH 7777A and MH 7777A > N cell lines was fucosyl-GM1, 0.78 and 0.72 microgram per mg cellular protein, respectively. The GM3 and GD1a content of MH 7777A > N cells was significantly higher than that of MH 7777A cells. Furthermore, a matrix-dependency of the ganglioside pattern of both cell lines was demonstrated.

A short isoform of carcinoembryonic-antigen-related rat liver cell-cell adhesion molecule (C-CAM/gp110) mediates intercellular adhesion. Sequencing and recombinant functional analysis

Rat liver cell-cell adhesion molecule (C-CAM) is a type I transmembrane glycoprotein belonging to the immunoglobulin (Ig)-superfamily. Within this family it is related to the carcinoembryonic antigen (CEA) proteins. C-CAM, previously known as gp110, cell-CAM 105, HA4/pp120 or ecto-ATPase, is a highly glycosylated protein with an apparent M(r) or 100,000-115,000 and an isoelectric point of 3-3.5. It was analysed as a molecule that stimulates reaggregation of isolated hepatocytes. So far three different isoforms have been cloned. Only the isoform with a long intracellular tail (71 amino acids), C-CAM1, was shown to be involved in intercellular adhesion. C-CAM2, an isoform with only 10 cytoplasmic amino acids and a slightly different N-terminal Ig-like loop did not function as an adhesion molecule. In this study we show the existence of another short C-CAM isoform (C-CAM2a), which is an alternatively spliced product of the C-CAM1 gene. Like C-CAM2, it has a short cytoplasmic tail, but in the extracellular region it is identical to C-CAM1. To investigate whether C-CAM2a can function as an adhesion molecule, we stably expressed the corresponding cDNA in Chinese hamster ovary (CHO) cells. In these cells, we detected a specific increase of intercellular adhesion, indicating that, in contrast to the other short isoform, C-CAM2a can induce adhesion. This adhesion is homophilic and Ca2+ independent.

Chemical cross-linking leads to two high molecular mass aggregates of rat alpha 1 beta 1 integrin differing in their conformation but not in their composition

In order to detect protein interactions of the collagen/laminin receptor alpha 1 beta 1 integrin, covalent chemical cross-linking was performed with the homo-bifunctional, amine reactive reagents DSS (disuccinimidylsuberate) and DSP (dithiobis(succinimidylpropionate)). After cross-linking of the 190 kDa rat alpha 1 integrin subunit, immunoblotting revealed two additional, immunoreactive, high molecular mass complexes (M(r) 240/290 k). Generation of the 240/290 kDa aggregates depended on the presence of the intact tertiary protein structure. As shown with immunoaffinity purified proteins, the 240/290 kDa aggregates consist exclusively of alpha 1 and beta 1 integrin subunits. No other cross-linked proteins associated with the alpha 1 or beta 1 subunit were detected. In contrast to the non-cross-linkable alpha 1 beta 1 integrin, the 240/290 kDa aggregates presumably represent active forms of the adhesion receptor, because both bound in vitro to collagen I and IV. This ability of alpha 1 beta 1 integrin to cross-link and produce two additional high molecular mass forms is shared by rat alpha 9 beta 1 integrin. Thus, the cross-linking approach directly indicates that beta 1 integrins occur in different conformations caused by variations in the folding and/or spatial arrangement of their subunits.

Characterization of molecular aggregates of alpha 1 beta 1-integrin and other rat liver membrane proteins by combination of size-exclusion chromatography and chemical cross-linking

Many membrane proteins display their biological activity in molecular aggregates of interacting counterparts. The analysis of these aggregates remains difficult; especially intermolecular complexes of membrane proteins tend to dissociate or artificially aggregate during detergent extraction out of membranes. Thus, the existence of protein aggregates was investigated by two approaches. First, after modest detergent extraction, the presence of three well characterized rat liver membrane proteins, alpha 1 beta 1-integrin, dipeptidyl aminopeptidase IV (DPP IV) and cell-CAM 105 (CAM = cell adhesion molecule), in aggregates could be demonstrated when investigated by size-exclusion chromatography (SEC) under non-denaturating conditions. However, the applied detergents partially influenced the resolution of the separation reducing the ability to discriminate between native and artificial protein aggregates. To circumvent these problems, a second approach based on covalent cross-linking of native protein complexes by dithiobis(succinimidylpropionate) was combined with the performance of denaturating SEC. Under such optimized some high-molecular-mass complexes of all model proteins consisting of unknown components could also be detected. Taken together, non-denaturating SEC and chemical cross-linking in combination with denaturating SEC represent methodological approaches for the characterization of protein aggregates.

cDNA cloning and tissue-specific regulation of expression of rat calcium-binding protein 65/67. Identification as a homologue of annexin VI

We isolated a cDNA encoding the rat membrane-associated 65/67-kDa calcium-binding protein, CBP 65/67, from a lambda ZAP II cDNA-expression library of rat liver by immunoscreening using monospecific polyclonal anti-(CBP 65/67) antibodies and monoclonal anti-(CBP 65/67) IgG. The product of this cDNA expressed in Escherichia coli was confirmed as CBP 65/67 both by immunostaining and by comparison of the molecular mass with the CBP 65/67 isolated from rat liver by SDS/PAGE. The cDNA sequence and the deduced amino acid sequence of CBP 65/67 both show a high degree of identity to human p68 and human calelectrin, which belong to a family of calcium-dependent, membrane-associated, phospholipid-binding proteins, called annexins. This means that CBP 65/67 is a homolog of the two human proteins just mentioned above. We are not aware that a rat annexin VI has previously been isolated and sequenced. The mRNA expression of CBP 65/67 in different rat organs during development was investigated by Northern blot analysis. In adult tissues, high mRNA levels of CBP 65/67 were found in lung, heart, muscle, spleen and especially in thymus and pancreas, whereas in liver, kidney, intestine, stomach and brain only low levels of CBP 65/67 mRNA could be detected. The amount of mRNA during tissue development in kidney, stomach and muscle showed only slight changes. In contrast, a significant increase of CBP 65/67 expression was observed in liver, lung, heart and brain. In most of the organs investigated, the level of mRNA correlated closely with the level of protein expression, indicating that the expression of CBP 65/67 in most organs is controlled primarily at the transcriptional level.

Anti-peptide sera against cell-CAM 105 determine high molecular-mass variants of the long isoform in rat hepatocytes

The glycoprotein cell-CAM 105 is a member of the carcinoembryonic-antigen-(CEA)-gene family, involved in cell-cell adhesion of rat hepatocytes and expressed on the cell surface as a long (L) and a short (S) isoform with slightly differing molecular masses and isoelectric points. The cDNA of the L-isoform has been isolated and sequenced, as confirmed by the preparation of specific anti-peptide sera [Lin, S.-H., Culic, O., Flanagan, D. & Hixson, D. C. (1991) Biochem. J. 278, 155-161]. Recently, two additional cDNAs have been sequenced, which possess identical deduced primary structures, including short intracellular domains 10 amino acids in length, which differ from the cytoplasmic domain of the L-isoform specifically in the last four C-terminal amino acids. Here, we report on the production of the polyclonal antiserum [anti-(peptide 2)] by immunization with a synthetic hexapeptide (GGSGSF) corresponding to the unique intracellular C-terminal domain of these short cell-CAM 105 cDNA isoforms. This antiserum was specific in ELISA, immunoblot and immunoprecipitation assays for a protein with the same biochemical properties as the S-isoform of cell-CAM 105 expressed in rat liver. In addition, CNBr peptide maps of the S-isoform and the protein immunoprecipitated with anti-(peptide 2) serum were identical. Together, these results provide strong evidence that anti-(peptide 2) serum is specific for the S-isoform of rat liver cell-CAM 105. In immunoblot analysis on liver plasma membrane extracts prepared without collagenase perfusion, at least seven high molecular-mass proteins were observed which showed strong reactivity with mAbs against extracellular epitopes and L-isoform-specific antibodies but no reactivity with anti-(peptide 2) serum. Like the L-isoform, these proteins are expressed on the cell surface and might represent structural variants of cell-CAM 105.

Distribution and quantification of alpha 1-integrin subunit in rat organs

The alpha 1 beta 1-integrin is known to be a receptor for collagen and laminin mediating cell-matrix interactions. A monoclonal antibody, 33.4, which specifically inhibits the alpha 1-integrin-mediated in vitro cell-collagen binding of rat hepatocytes and hepatoma-derived A-cells (Löster et al., 1994), was used to purify by immunoaffinity chromatography the alpha 1-integrin subunit from rat liver in large quantities for inducing a polyclonal antiserum. In immunoblot analysis on membrane extracts of several rat organs this polyclonal antiserum recognized only a 190 kDa-band, suggesting that it is highly specific for the alpha 1-integrin subunit. A sandwich-ELISA with monoclonal antibody 33.4 and the polyclonal antiserum against the alpha 1-integrin subunit, respectively, enabled the quantitative expression pattern of the alpha 1-integrin subunit to be studied in different rat organs. With the exceptions of brain (not detectable) and muscle (low concentration), the alpha 1-integrin subunit was detectable in almost all organs of the digestive, respiratory and urogenital system as well as in lymphatic organs. The highest relative concentrations of alpha 1-integrin subunit were found in uterus, lung and spleen, whereas in seminal vesicle, stomach, parotid gland, epididymis, kidney and liver only modest concentrations were evident. The organ distribution and localization of alpha 1-integrin subunit were studied by immunohistochemistry with monoclonal and polyclonal antibodies. Immunoreactivity was present in the plasma membranes of all smooth muscle cells, vascular endothelial cells of many organs and fibrocyte-fibroblast sheaths in the heart and kidney. Since these cells are in close contact with collagen-containing basal membranes as well as reticular fibrils, strong evidence exists that in rat tissues the alpha 1-integrin subunit is expressed at sites where collagen is present and might be involved in vivo in cell-collagen binding.

Biosynthetic modulation of sialic acid-dependent virus-receptor interactions of two primate polyoma viruses

Sialic acids are essential components of the cell surface receptors of many microorganisms including viruses. A synthetic, N-substituted D-mannosamine derivative has been shown to act as precursor for structurally altered sialic acid incorporated into glycoconjugates in vivo (Kayser, H., Zeitler, R., Kannicht, C., Grunow, D., Nuck, R., and Reutter, W. (1992) J. Biol. Chem. 267, 16934-16938). In this study we have analyzed the potential of three different sialic acid precursor analogues to modulate sialic acid-dependent virus receptor function on different cells. We show that treatment with these D-mannosamine derivatives can result in the structural modification of about 50% of total cellular sialic acid content. Treatment interfered drastically and specifically with sialic acid-dependent infection of two distinct primate polyoma viruses. Both inhibition (over 95%) and enhancement (up to 7-fold) of virus binding and infection were observed depending on the N-acyl substitution at the C-5 position of sialic acid. These effects were attributed to the synthesis of metabolically modified, sialylated virus receptors, carrying elongated N-acyl groups, with altered binding affinities for virus particles. Thus, the principle of biosynthetic modification of sialic acid by application of appropriate sialic acid precursors to tissue culture or in vivo offers new means to specifically influence sialic acid-dependent ligand-receptor interactions and could be a potent tool to further clarify the biological functions of sialic acid, in particular its N-acyl side chain.

A new type of Ca(2+)-dependent, Mg(2+)-stimulated ATPase of rat liver plasma membrane

Incubation of a glycoprotein fraction obtained from rat liver plasma membrane which has been previously well characterized using [gamma-32P]ATP results in the phosphorylation of a 230-kDa glycoprotein (pgp230). It is composed of a 120-kDa subunit (pgp120) and a 110-kDa subunit (pgp110) linked by interchain disulfide bonds. Peptide maps of pgp120 and pgp110 suggest extensive similarity in their polypeptide chains. Glycan analysis reveals between four and six hybrid-type oligosaccharide chains for both phosphoproteins. Immunoblotting using monoclonal antibodies and endoglycosidase digestion exclude an identity of pgp120 or pgp110 with the hepatocyte plasma membrane glycoproteins dipeptidylpeptidase IV or the taurocholate transport protein, which co-purify and co-migrate in SDS/PAGE. Protein phosphorylation is Ca(2+)-dependent (K0.5(Ca2+) = 0.35 microM, in the absence of Mg2+). In the presence of Mg2+, the glycoprotein undergoes rapid cycles of phosphorylation and dephosphorylation, resulting in ATPase activity. Analysis of phosphorylated amino acids identifies phosphothreonine as the major one. Photoaffinity labeling with 8-azido-[alpha-32P]ATP demonstrates the presence of one or more ATP binding site(s). Preincubation of pgp230 with various purine or pyrimidine nucleotides (ATP, UTP, TTP, ADP, GDP, AMP, CMP) or known P2-purinoceptor agonists or antagonists (adenosine 5'-[alpha,beta-methylene]triphosphate, 2-methyl-thio-adenosine 5'-triphosphate, suramin) inhibits its phosphorylation by [gamma-32P]ATP. The biological function of pgp230 is unknown at present. Several findings of the present study are compatible with the idea that pgp230 may be involved in a P2-purinoceptor function of the hepatocyte. Following this concept, a mechanism is discussed where a cytosolically exposed high-affinity Ca(2+)-binding site of pgp230 would allow for receptor feedback control, via phosphorylation and dephosphorylation, by sensing changes in cytosolic Ca2+ concentration.