Fibronectin glycosylation modulates fibroblast adhesion and spreading.

The role of the carbohydrate residues of fibronectin concerning the specificities of that glycoprotein to interact with fibroblastic cell surfaces, gelatin, and heparin was examined. Tunicamycin was used to produce carbohydrate-depleted fibronectin; it was synthesized by cultured fibroblasts. Unglycosylated and glycosylated fibronectins were analyzed for their ability to bind gelatin and heparin, using affinity columns. Fibronectin-coated surfaces were used to quantitatively measure cell adhesion and spreading. The results showed that the lack of carbohydrates significantly increased the interaction of the protein with gelatin and markedly enhanced its ability to promote adhesion and spreading of fibroblasts. In contrast, the binding of fibronectin to heparin was not influenced by glycosylation. The composite data indicate that the Asn-linked oligosaccharides of fibronectin act as modulators of biological functions of the glycoprotein.

Structures of the asparagine-linked sugar chains of laminin.

This investigation describes the isolation and characterization of oligosaccharides of the basement membrane glycoprotein, laminin. Pronase-released glycopeptides of isolated laminin, from a mouse Engelbreth-Holm-Swarm tumor, were fractionated using a combination of gel permeation chromatography and Con A-Sepharose affinity chromatography. The glycopeptides were analyzed for sugar linkage patterns by methylation analysis. Glycopeptides and hydrazine-released oligosaccharides were further analyzed using endo-beta-galactosidase, endo-beta-N-acetylglucosaminidase H and specific exoglycosidases in conjunction with calibrated gel permeation chromatography. Based on these experiments, murine tumor laminin was shown to contain asparagine-linked oligosaccharides with the following structures: bi-, tri- and tetraantennary complex-type oligosaccharides; polylactosaminyl side chains containing Gal(beta 1----4)GlcNAc(beta 1----3) repeating units attached to the trimannose core portion of the bi-, tri- and tetraantennary complex-type oligosaccharides; unusual complex-type oligosaccharides terminated at the nonreducing end with sialic acid, alpha-galactose, beta-galactose and beta-N-acetylglucosamine; alpha-galactosyl residues linked to N-acetyllactosamine sequences; high-mannose-type oligosaccharides. These results, in conjunction with analytical data, indicate that most of the carbohydrate of this laminin is N-linked to asparagine and that there are about 43 such N-linked oligosaccharides per laminin molecule.

Abnormal glycosylation of human cellular fibronectin in the presence of swainsonine.

The relationship between post-translational modifications of macromolecules and their intracellular routing is of fundamental importance. The availability of the indolizidine alkaloid, swainsonine, which interferes with glycoprotein processing, provides a new probe for studying relationships between glycosylation of proteins and their cellular routing. Using fibronectin as a model glycoprotein, we have explored the effect of swainsonine upon oligosaccharide structure, glycoprotein synthesis, and secretion. Confluent human fibroblasts were labeled with radioactive mannose or glucosamine in the presence or absence of swainsonine. Fibronectin was secreted into the medium of swainsonine-treated cultures and found to contain endo-beta-N-acetylglucosaminidase H-sensitive oligosaccharides, instead of the normal complex endo-beta-N-acetylglucosaminidase H-resistant oligosaccharides. Pronase-released glycopeptides and hydrazine-released oligosaccharides of isolated fibronectin from the culture media were analyzed using endo-beta-N-acetylglucosaminidase H and specific exoglycosidase digestions in conjunction with calibrated gel filtration chromatography. The structure of the swainsonine-modified endo-beta-N-acetylglucosaminidase H-sensitive oligosaccharide was found to be a hybrid type, Gal beta leads to GlcNAc beta leads to Man alpha leads to [Man alpha leads to (Man alpha leads to) Man alpha leads to]Man beta leads to GlcNAc beta leads to (+/- Fuc alpha leads to)-GlcNAc. Other experiments showed that the synthesis and secretion of fibronectin were not affected by its change in glycosylation. The results also infer that swainsonine inhibits Golgi mannosidase II in intact cells, and that removal of two mannose residues by that mannosidase is not a prerequisite for addition of galactose to the existing peripheral nonreducing GlcNAc or for the addition of fucose to the innermost reducing GlcNAc. The composite results indicate that significant changes in oligosaccharide structure had little effect upon the routing and cellular release of a typical N-asparagine-linked glycoprotein.

Swainsonine inhibits macrophage receptor-mediated uptake and degradation of a mannosyl-oligosaccharide.

Rat pulmonary macrophages were incubated in the presence of a radiolabeled mannosyl-oligosaccharide obtained from ovalbumin. Receptor-mediated endocytosis and degradation of this ligand by the cells was followed in the presence or absence of swainsonine, an inhibitor of alpha-mannosidases. The results indicated that at higher concentrations (greater than 1 microgram/ml) of swainsonine, both the internalization and degradation of the radiolabeled ligand were inhibited. At a concentration of 0.1 microgram/ml of swainsonine, only the degradation was inhibited while the uptake was unaltered. The degradation of the oligosaccharide was blocked due to the inhibition of lysosomal alpha-mannosidase. However, the inhibition of lysosomal alpha-mannosidase was reversible upon withdrawal of swainsonine.

Interprotein disulfide bonding between F and G glycoproteins of human respiratory syncytial virus.

The envelope glycoprotein G, of human respiratory virus was purified by immunoaffinity chromatography using a monoclonal antibody reacting with G glycoprotein. The purified material was analyzed for its protein patterns and by western blot for its reactivity with specific monoclonal antibodies. In addition to the G specific proteins at 90 and 55 kilodalton (kDa) range, high molecular weight species were coeluted with G protein. Three high molecular weight species were noticed: one (140 kDa) reacting with fusion protein (F) monoclonal antibody and two other species (230 and 195 kDa) reacting with both fusion protein and G protein monoclonal antibodies. The protein reacting only with F monoclonal antibody consists of fusion protein dimer. Western blot and two dimensional gel electrophoretic analysis revealed that each of the other two complexes is composed of two moles of F protein and one mole of G protein. These two complexes differ in their molecular sizes depending on whether G is in the form of 90 or 55 kDa. Upon heat denaturation, fusion protein monomer (70 kDa) is released from the complex, leaving the two complexes, consisting of one mole of F protein and one mole of G protein (160 and 125 kDa species respectively). Disulfide-reducing agents are required to break the monomers of F and G complexes. These results provide a direct evidence for the presence of envelope glycoprotein complexes linked by interprotein disulfide bonding. This may have implications on the structural and functional properties of envelope glycoproteins.

Evidence that the fusion protein of respiratory syncytial virus exists as a dimer in its native form. Brief report.

The quaternary structure of respiratory syncytial virus (RSV) fusion protein has been studied. Crosslinking studies were done to stabilize the noncovalently associated proteins. These stable, heat-resistant, covalently linked complexes were analyzed by sodium dodecyl sulfate-polyacrylamide electrophoresis. In situ crosslinking studies demonstrated that the fusion protein of RSV exists as a dimer in its native form on the surface of infected cells. The purified protein was also found to be present predominantly as a dimer. In addition, the results suggest that F1 subunits may play a role in the dimerization of the fusion protein.

Separation and characterization of the subunits of the laminin of EHS sarcoma.

A rapid and sensitive method was developed for the preparative separation of laminin subunits. Laminin was extracted and purified from mouse EHS sarcoma. On SDS-PAGE, the reduced and carboxymethylated molecule separated into two components corresponding to molecular weights of about 400 KDa (subunit A) and 200 KDa (subunit B). These two subunits were preparatively separated using heparin-agarose affinity chromatography. The larger subunit quantitatively adhered to the affinity column while the smaller one did not adhere. Amino acid analyses of the separated subunits showed distinct differences. Subunit B was further resolved into two distinct polypeptides of 200 KDa, B1 and B2, by means of reverse-phase HPLC. Although the amino acid compositions of B1 and B2 were very similar, the peptide maps generated by digestion of the B1 and B2 chains with Staphylococcus aureus V8 protease or by cyanogen bromide showed B1 and B2 to differ from each other. Thus, at least three different polypeptide subunits are present in this laminin and probably arise from separate gene origins. These studies provide a basis for the subsequent localization and analysis of the specialized structural and functional domains of laminin.

Fatty acid acylation of the fusion glycoprotein of human respiratory syncytial virus.

We describe the covalent attachment of palmitate to the fusion glycoprotein of respiratory syncytial virus and the identification of the attachment site. Labeling of respiratory syncytial virus-infected Vero cells with [3H]palmitate, followed by the purification and subsequent analysis of the fusion glycoprotein in conjunction with polyacrylamide gel electrophoresis, demonstrated that the fatty acid is covalently attached to the F1 subunit of the fusion glycoprotein. The bound palmitate was sensitive to 1 M hydroxylamine at neutral pH. In addition, the release of palmitate label by reduction with sodium borohydride showed that the palmitate is linked to the protein through a thioester bond. Isolation of a radiolabeled peptide from a tryptic digest of the protein and subsequent amino-terminal sequence analysis revealed that the cysteine residue (amino acid residue 550) within the anchor sequence, located at the carboxyl terminus of the F1 subunit, is the covalent attachment site for palmitate.