O-linked N-acetylglucosamine is attached to proteins of the nuclear pore. Evidence for cytoplasmic and nucleoplasmic glycoproteins

Organization of the nuclear pore complex in Saccharomyces cerevisiae

Fractions enriched for nuclear pore complexes (NPCs) have been isolated from Saccharomyces cerevisiae. The sequential extraction of nuclei with detergent, nucleases, and salt reveals an organization of the yeast NPC similar to other eukaryotes. Yeast NPCs contain a 30-nm "ring" structure not previously described in other organisms. This structure appears to organize 10-nm filaments into an assembly which exhibits an eight-fold rotational symmetry. Some proteins in the NPC fraction are capable of forming intermediate-sized filaments. These studies suggest that some component of the nuclear pore complex organizes an interaction between nuclear and cytoplasmic networks of intermediate filaments.

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.

Evidence for the presence of a carbohydrate moiety in fluorescein isothiocyanate labeled fragments of rat gastric (H+-K+)-ATPase

Limited tryptic digestion of fluorescein isothiocyanate (FITC)-labeled (H+-K+)-ATPase from rat resting light gastric membranes produced a soluble 27-kDa polypeptide which retained the fluorescence of the parent enzyme. Its production was markedly enhanced in the presence of an amphiphilic detergent, Zwittergent 3-14, which potently inhibits the ATPase activity. This increase is probably due to protection of certain tryptic cleavage sites through conformational changes of the membrane enzyme by the detergent. The NH2-terminal sequence of the 27-kDa polypeptide corresponded exactly to that beginning at Asn-369 of the cDNA-deduced primary structure of the rat ATPase. The presence of the phosphorylation site, Asp-385, and FITC-labeled Lys-517, which is known to be a part of the ATP-binding site, indicates that the 27-kDa polypeptide contains a major cytoplasmic portion of (H+-K+)-ATPase. Interestingly, the polypeptide was stained with periodate-Schiff's base, indicating its glycoprotein nature. The carbohydrate group attached to the polypeptide seems to include at least an N-linked high-mannose moiety, since the polypeptide showed Con A binding activity as detected with a Con A-biotin/avidin-peroxidase assay on nitrocellulose transblots. Also, its Con A binding activity was inhibited by excess methyl alpha-D-mannopyranoside and disappeared upon treatment of the polypeptide with endoglycosidase H and N-glycanase. Further tryptic action converted the 27-kDa polypeptide to 2 smaller FITC-labeled polypeptides of 25 and 15 kDa, which lost 18 and 96 amino acid residues, respectively, from the NH2 terminus of the parent polypeptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the maturation-associated galactose oxidase-sensitive glycoproteins of rat caudal sperm plasma membrane and epididymal fluid

This paper explores the relationship between the galactose oxidase-sensitive glycoproteins from rat caudal epididymal sperm and fluid and, in addition, their relatedness to the 32,000-Da major acidic secretory glycoproteins of caudal epididymal fluid. The major acidic secretory glycoproteins were purified by a combination of high-resolution anion-exchange (Mono Q) and gel permeation (Bio-Sil TSK 125) chromatographic steps. Immunoprecipitation studies, peptide mapping, and the inability to label the purified glycoprotein by galactose oxidase/sodium boro[3H]hydride clearly established that the galactose oxidase-sensitive fluid and membrane glycoproteins were not related to these acidic secretory glycoproteins. Membrane and fluid tritium-labeled glycoproteins were shown to be closely related, but not identical, polypeptides. Sugar analysis indicated that both glycoproteins contain N- and O-linked saccharide chains and that the galactose oxidase-sensitive residue was present only on O-linked sugars. It was also found that efficient labeling of the 32,000-Da fluid glycoprotein was possible only if protease inhibitors were omitted from all buffers used in the isolation of caudal epididymal fluid and subsequent labeling procedures. This suggests that the fluid glycoprotein was acquired by the unintentional proteolysis of the membrane glycoprotein. Polyclonal antibodies raised against caput sperm plasma membranes immunoprecipitated tritium-labeled glycoproteins from both caudal epididymal fluid and sperm membrane, suggesting that a precursor form of the caudal galactose oxidase-sensitive glycoprotein may be present on caput sperm.

Monoclonal antibodies to a Mr 68,000 pore complex glycoprotein interfere with nuclear protein uptake in Xenopus oocytes

Using a monoclonal antibody (PI1) raised against mouse lymphocyte nuclear matrix fractions we have identified a N-acetylglucosamine (GlcNAc)-containing glycoprotein of Mr 68,000 as a component of the nuclear pore complexes of Xenopus laevis oocytes. The antigenic determinant recognized by antibody PI1 comprises both the sugar moiety and protein sequences since, on the one hand, added GlcNAc competed effectively for antibody binding and, on the other hand, the antibody reacted in immunoblots with only one member of the GlcNAc-containing pore complex glycoprotein family. By using immunogold-electron microscopy we could demonstrate that the Mr 68,000 glycoprotein was located preferentially to the cytoplasmic side of the pore complex channel. When radiolabeled soluble nuclear proteins were injected into the cytoplasm of Xenopus oocytes, their reentry into the nucleus was almost completely inhibited in the presence of antibody PI1 as shown by two-dimensional gel electrophoresis. The results indicate that the evolutionarily conserved Mr 68,000 glycoprotein is involved in transport processes of karyophilic proteins from the cytoplasm into the nucleus.

Nuclear protein import: specificity for transport across the nuclear pore

Transport of proteins into the cell nucleus is thought to require specific localization sequences and may be mediated by nuclear pores. Following microinjection into fused cultured cells, nuclear protein import was directly monitored by fluorescence microscopy using B-phycoerythrin (PE; Mr 240,000) coupled to synthetic peptides corresponding to the simian virus 40 (SV-40) large T antigen nuclear localization signal. Peptides with a single amino acid replacement found in a cytoplasmic mutant of T antigen (cT) failed to promote uptake. Further studies with deletion peptides revealed the minimum sequence requirements for efficient nuclear import of PE conjugates to be similar to those previously defined genetically for large T antigen itself. No competitive inhibition of uptake was observed in cells expressing nuclear or cytoplasmic T antigen. Nuclear import was time- and temperature-dependent. The lectin wheat germ agglutinin (WGA) binds to glycoproteins bearing O-linked GlcNAc on the cytoplasmic face of the nuclear pore in vitro [J.A. Hanover et al. (1987) J. Biol. Chem. 262, 9887-9894] and in vivo. Microinjection of WGA into the cytoplasm of living cells did not alter the diffusion of dextran (Mr 10,000) into the nucleus, but blocked the uptake of PE conjugates. This inhibition was reversed when a competing saccharide was introduced into the cytoplasm.

Nuclear import can be separated into distinct steps in vitro: nuclear pore binding and translocation

Large nuclear proteins must possess a signal sequence to pass through the nuclear pores. Using an in vitro system, we have been able experimentally to dissect nuclear protein transport into two distinct steps: binding and translocation. In the absence of ATP, we observe a binding of nuclear proteins to the pore that is signal sequence-dependent. Translocation through the pore, on the other hand, strictly requires ATP. These steps, visualized in the fluorescence and electron microscopes, were observed both with a natural nuclear protein, nucleoplasmin, and a synthetic nuclear protein, composed of the signal sequence of SV40 T antigen coupled to HSA. When a mutant signal sequence was coupled to HSA, neither transport nor binding were observed, indicating that both result from the presence of a functional signal sequence. An inhibitor of transport, the lectin WGA, also arrested nuclear proteins in a bound state at the cytoplasmic face of the pore. Therefore, only the translocation step is sensitive to the inhibitor WGA, which is known to bind specifically to proteins of the nuclear pore.