O-glycosidically linked fucose in high molecular weight glycoproteins, in normal and virus-transformed rat cells

Extracellular and cellular proteins of rat cells with O-glycosidically linked fucose

Fucose-labelled proteins were examined for the release of low-Mr O-linked fucose substituents after mild alkaline-borohydride treatment. A component tentatively identified as glucosylfucitol (DS) and an apparently higher-Mr component (TS), which also contained fucitol, were observed to be released over a broad molecular-size range of proteins. Approx. 90% of the DS-releasing proteins were in the particulate fraction, whereas only approx. 66% of the TS-releasing proteins were in that fraction. In addition to cell-associated proteins, a substantial proportion of DS-containing proteins were shed into the medium. For example, after 96 h of labelling there was 6-fold more of these components in the growth medium than were cell-associated. Moreover, the incorporation of labelled fucose into both the DS and TS appeared to be cell-population-density-dependent. Despite the apparent wide distribution of these novel fucose substituents in cellular proteins, it seems reasonable to suggest that they have not been routinely observed largely because each represents less than 0.5% of the fucose bound to protein.

Different half-lives of the carbohydrate and protein moieties of a 110,000-dalton glycoprotein isolated from plasma membranes of rat liver

By using a four-step procedure (i, solubilization with Triton X-100; ii, affinity chromatography on concanavalin A-Sepharose; iii, affinity chromatography on wheat germ lectin-Sepharose; iv, preparative sodium dodecyl sulfate gel electrophoresis) a glycoprotein was isolated from rat liver plasma membrane. The molecular weight is 110,000 and the isoelectric point is 5.8. It contains L-fucose, N-acetylneuraminic acid, D-galactose, D-mannose, N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, and considerable quantities of aspartate, threonine, serine, and leucine. In pulse-chase experiments the half-lives of methionine and arginine, representing the half-life of the protein, were determined as 70 hr and 78 hr, respectively. The half-lives of the terminal carbohydrates L-fucose and N-acetylneuraminic acid were 12.5 and 33 hr, respectively. The galactose half-life was 20 hr. From this it is concluded that terminal sugars turn over several times in the life-span of this protein molecule. This process may be operative during membrane recycling mechanisms.

Effects of sodium butyrate on the membrane glycoconjugates of murine sarcoma virus-transformed rat cells

The temporal relationship between butyrate-induced cellular flattening of murine sarcoma virus-transformed rat cells (MSV-NRK) and alterations in certain surface-associated biochemical markers of transformation, e.g., surface glycopeptides, glycolipids, fibronectin, hexose uptake, and cell-substrate adhesion was examined. The induction of elevated levels of the ganglioside GM3 and of a GDla-like ganglioside were observed to precede or to parallel cellular flattening. Likewise, enhanced incorporation of radioisotopically labeled fucose into a novel fucose-containing component, i.e., glucopyranosyl (1 leads to 3) fucopyranosyl-threonine, was also observed to occur at an early stage of cellular flattening. In contrast, a shift in the molecular weight distribution of trypsin-sensitive, surface fucopeptides was observed to occur at a late stage of cellular flattening. Moreover, surface fibronectin was not detectable in the butyrate-flattened MSV-NRK cells despite the fact that the cells manifested significantly enhanced cell-substrate adhesion. Thus, butyrate appears to be a useful tool for understanding the sequential changes associated with expression of the transformed phenotype of MSV-NRK cells.