Biochemical properties of SV40 large tumor antigen as a glycosylated protein

SV40 large T antigen is modified with O-linked N-acetylglucosamine but not with other forms of glycosylation

SV40 large T antigen has been reported to be modified with several different sugars including N-acetylglucosamine, galactose, and mannose. In this report we have reexamined the glycosylation of T antigen and found that while we could detect modification with N-acetylglucosamine, we could not detect any other sugars on the protein. Surprisingly, even though [3H]galactose could be metabolically incorporated into the protein, analysis showed that all of the radioactivity in T antigen had been converted to other species. The N-acetylglucosamine was demonstrated to be linked to the protein in the form of O-linked N-acetylglucosamine, the best characterized form of nuclear and cytoplasmic glycosylation in mammalian systems. We have localized the major site of glycosylation to the amino terminal portion of the molecule. Analysis of mutated T antigen where serines 111/112 were substituted with alanine suggest that these residues constitute a glycosylation site on the protein. These two serines fall within a typical O-linked N-acetylglucosamine glycosylation site (PSS) and are also known to be phosphorylated. Thus, it is likely that competition between phosphorylation and glycosylation occurs at this site.

Role of glycosylation in the transport of recombinant glycoproteins through the secretory pathway of lepidopteran insect cells

Cell lines established from the Lepidopteran insect Spodoptera frugiperda (e.g., Sf9) are used routinely as hosts for the expression of foreign proteins by baculovirus vectors. Previously, we showed that human tissue plasminogen activator (t-PA) was expressed, N-glycosylated, and secreted by Sf9 cells infected with a recombinant baculovirus (Jarvis DL, Summers MD: Mol Cell Biol 9:214-223, 1989). We also showed that t-PA secretion was blocked by tunicamycin (TM), an inhibitor of N-glycosylation, but not by castanospermine (CS) or N-methyldeoxynojirimycin, inhibitors of the initial steps in N-linked oligosaccharide processing. This suggested that the addition, but not the processing, of N-linked oligosaccharides is required for the secretion of recombinant t-PA from baculovirus-infected Sf9 cells. In this study, we present a more generalized evaluation of the role of N-glycosylation in the transport of recombinant glycoproteins through the Sf9 cell secretory pathway. Several different secretory or membrane-bound glycoproteins were expressed in control, TM-treated, or CS-treated Sf9 cells, and their appearance in the medium or on the cell surface was measured. The results showed that TM blocked the transport of some, but not all, of these proteins, whereas CS did not block the transport of any. This suggests that N-glycosylation is sometimes required for the transport of recombinant glycoproteins through the Sf9 secretory pathway, while processing of the oligosaccharides is not. At least two other proteins, p80 and p31, consistently coimmunoprecipitated with the nonglycosylated precursors of recombinant glycoproteins expressed in TM-treated Sf9 cells. Neither was antigenically related to any of the recombinant proteins. Relatively larger amounts of p80 and p31 were coprecipitated when transport was completely blocked by TM compared to when transport was only reduced or was unaffected. These results suggest that p80 and p31 block the transport of some nonglycosylated glycoprotein precursors in TM-treated Sf9 cells by binding to them and producing transport-incompetent heterooligomeric complexes. If this speculation is correct, then p80 and p31 are functionally analogous to the mammalian immunoglobulin heavy chain binding/glucose-regulated 78 kilodalton protein (BiP/GRP78).

Glycosylation of chromosomal proteins: localization of O-linked N-acetylglucosamine in Drosophila chromatin

Drosophila polytene chromosomes contain a surprisingly large amount of terminal N-acetylglucosamine (GlcNAc) residues along their lengths, as determined by staining with a fluorescently tagged lectin, wheat germ agglutinin (FITC-WGA) and by specific radio-labeling with bovine galactosyltransferase and UDP-[3H]galactose. FITC-WGA intensely stains polytene chromosomes in a distinctive banding pattern in which condensed chromatin is brightly labeled and transcriptionally active "puff" regions are less intensely stained. Biochemical analyses of galactosyltransferase-radiolabeled chromatin indicates that nearly all of the chromatin-associated GlcNAc moieties exist as single monosaccharide residues attached to protein by an O-linkage (O-GlcNAc). Chromatin is enriched in O-GlcNAc (over 400 pmol/micrograms of chromatin protein) as compared with total nuclei and other cellular compartments. O-GlcNAc moieties are found on a myriad of chromatin proteins that have diverse types of intermolecular associations with other nuclear components.