Effects of N-glycosylation on the activity and localization of GlcNAc-6-sulfotransferase 1

N-Acetylglucosamine-6-sulfotransferase-1 (GlcNAc6ST-1) is a Golgi-resident glycoprotein that is responsible for sulfation of the l-selectin ligand on endothelial cells. Here, we report the sites at which GlcNAc6ST-1 is modified with N-linked glycans and the effects that each glycan has on enzyme activity, specificity, and localization. We determined that glycans are added at three of four potential N-linked glycosylation sites: N196, N410, and N428. The N428 glycan is required for the production of sulfated cell surface glycans: cells expressing a mutant enzyme lacking this glycan were unable to sulfate the sialyl Lewis X tetrasaccharide or a putative extended core 1 O-linked glycan. The N196 and N410 glycans differentially affect sulfation of two different substrates: cells that express an enzyme lacking the N410 glycan are able to sulfate the sialyl Lewis X substrate, but produce reduced levels of a sulfated peripheral lymph node addressin epitope and cells that express an enzyme lacking the N196 glycan are able to produce a sulfated peripheral lymph node addressin epitope, but are impaired in their ability to sulfate sialyl Lewis X. The glycans' effects on enzyme activity may be mediated, in part, by changes in enzyme localization. While most mutants that lacked glycans localized normally within the Golgi, the N428A mutant and a mutant lacking all glycans were also found to localize ectopically. Altered trafficking of mutants may be associated with the mechanisms by which misglycosylated enzyme is degraded.

Photocrosslinking of glycoconjugates using metabolically incorporated diazirine-containing sugars

Transient interactions among glycoconjugates underlie developmental, immunological and metastatic recognition. Glycan-mediated interactions have low binding affinities and rapid dissociation rates. As a result, these complexes dissociate when removed from their cellular context, complicating characterization. Photocrosslinkers introduce a covalent bond between glycoconjugates and their binding partners, allowing physiologically relevant complexes to be isolated. This protocol describes metabolic incorporation of a diazirine photocrosslinker into sialic acids in cellular glycoconjugates. Subsequent irradiation results in photocrosslinking of sialic acid to neighboring macromolecules, providing a photochemical 'snapshot' of binding events. As photocrosslinking sugars are light activated, these reagents have the potential to be used for temporally and/or spatially restricted crosslinking. We provide instructions for the synthesis of photocrosslinking sugar precursors, cell culture for metabolic incorporation, flow cytometry to evaluate metabolic incorporation, photoirradiation and analysis of the crosslinked complexes. Synthesis of photocrosslinking sugars requires 4-6 d, and photocrosslinking experiments can be completed in an additional 6 d.

Association of beta-1,3-N-acetylglucosaminyltransferase 1 and beta-1,4-galactosyltransferase 1, trans-Golgi enzymes involved in coupled poly-N-acetyllactosamine synthesis

Poly-N-acetyllactosamine (polyLacNAc) is a linear carbohydrate polymer composed of alternating N-acetylglucosamine and galactose residues involved in cellular functions ranging from differentiation to metastasis. PolyLacNAc also serves as a scaffold on which other oligosaccharides such as sialyl Lewis X are displayed. The polymerization of the alternating N-acetylglucosamine and galactose residues is catalyzed by the successive action of UDP-GlcNAc:betaGal beta-1,3-N-acetylglucosaminyltransferase 1 (B3GNT1) and UDP-Gal:betaGlcNAc beta-1,4-galactosyltransferase, polypeptide 1 (B4GALT1), respectively. The functional association between these two glycosyltransferases led us to investigate whether the enzymes also associate physically. We show that B3GNT1 and B4GALT1 colocalize by immunofluorescence microscopy, interact by coimmunoprecipitation, and affect each other's subcellular localization when one of the two proteins is artificially retained in the endoplasmic reticulum. These results demonstrate that B3GNT1 and B4GALT1 physically associate in vitro and in cultured cells, providing insight into possible mechanisms for regulation of polyLacNAc production.

Conditional glycosylation in eukaryotic cells using a biocompatible chemical inducer of dimerization

Chemical inducers of dimerization (CIDs) are cell-permeable small molecules capable of dimerizing two protein targets. The most widely used CID, the natural product rapamycin and its relatives, is immunosuppressive due to interactions with endogenous targets and thus has limited utility in vivo. Here we report a new biocompatible CID, Tmp-SLF, which dimerizes E. coli DHFR and FKBP and has no endogenous mammalian targets that would lead to unwanted in vivo side effects. We employed Tmp-SLF to modulate gene expression in a yeast three-hybrid assay. Finally, we engineered the Golgi-resident glycosyltransferase FucT7 for tunable control by Tmp-SLF in mammalian cells.

Chemical methods for glycoprotein discovery

An important frontier in glycoproteomics is the discovery of proteins with post-translational glycan modifications. The first step in glycoprotein identification is the isolation of glycosylated proteins from the remainder of the proteome. New enzymatic and metabolic methods are being used to chemically tag proteins to enable their isolation. Once isolated, glycoproteins can be identified by mass spectrometry. Additional information can be obtained by using either enzymatic or chemoselective reactions to incorporate isotope labels at specific sites of glycosylation. Isotopic labeling facilitates mass spectrometry-based confirmation of glycoprotein identity, identification of glycosylation sites, and quantification of the extent of modification. By combining chemical tagging for isolation and isotope labeling for mass spectrometry analysis, researchers are developing highly effective strategies for glycoproteomics. These techniques are enabling cancer biologists to identify biomarkers whose glycosylation state correlates with disease states, and developmental biologists to characterize stage-specific changes in glycoprotein expression. Next-generation methods will make functional analyses of the glycoproteome possible, including the discovery of glycoprotein interaction partners and the identification of enzymes responsible for synthesis of particular glycan structures.

Regulating cell surface glycosylation with a small-molecule switch

Correct localization of Golgi-resident enzymes is essential for the formation of specific glycan epitopes. In this chapter, we describe a method to control the localization, and thus the activity, of an individual glycosyltransferase by administration of a small molecule. Our method takes advantage of the modularity of most Golgi-resident enzymes, which are composed of localization and catalytic domains. These domains can be physically separated and fused to the small molecule binding proteins FRB and FKBP, which dimerize in the presence of rapamycin. In this way, rapamycin serves as a "switch" for enzyme activity.

A century at the chemistry-biology interface

The field of chemical biology is now hitting its stride. Chemical biologists have developed essential tools that are being used to illuminate complex cellular events. The application of chemical principles to biological phenomena has revealed new opportunities for drug discovery. This report highlights recent progress and exciting new directions in chemical genetics and drug discovery.

A small-molecule switch for Golgi sulfotransferases

The study of glycan function is a major frontier in biology that could benefit from small molecules capable of perturbing carbohydrate structures on cells. The widespread role of sulfotransferases in modulating glycan function makes them prime targets for small-molecule modulators. Here, we report a system for conditional activation of Golgi-resident sulfotransferases using a chemical inducer of dimerization. Our approach capitalizes on two features shared by these enzymes: their requirement of Golgi localization for activity on cellular substrates and the modularity of their catalytic and localization domains. Fusion of these domains to the proteins FRB and FKBP enabled their induced assembly by the natural product rapamycin. We applied this strategy to the GlcNAc-6-sulfotransferases GlcNAc6ST-1 and GlcNAc6ST-2, which collaborate in the sulfation of L-selectin ligands. Both the activity and specificity of the inducible enzymes were indistinguishable from their WT counterparts. We further generated rapamycin-inducible chimeric enzymes comprising the localization domain of a sulfotransferase and the catalytic domain of a glycosyltransferase, demonstrating the generality of the system among other Golgi enzymes. The approach provides a means for studying sulfate-dependent processes in cellular systems and, potentially, in vivo.

Regulating cell surface glycosylation by small molecule control of enzyme localization

Cell surface carbohydrates mediate interactions between the cell and its environment. Glycosyltransferases responsible for synthesis of cell surface oligosaccharides are therefore essential administrators of cellular communication. These enzymes often comprise large families. Redundancy of related family members and embryonic lethality both complicate genetic methods for deconvoluting functions of glycosyltransferases. We report a chemical method in which the activity of an individual glycosyltransferase is controlled by a small molecule. The approach exploits the requirement of Golgi localization, a common feature of glycosyltransferase superfamily members. In our approach, the glycosyltransferase is separated into two domains, one that determines localization and one responsible for catalysis. Control of enzyme activity is achieved using a small molecule to regulate association of the two domains. We used this method to regulate production of sialyl Lewis x by alpha1,3-fucosyltransferase VII in living cells.

Effects of nucleic acids and polyanions on dimer formation and DNA binding by bZIP and bHLHZip transcription factors

A large fraction of known transcription factors form 2:1 complexes with DNA. In our studies of the assembly of such ternary (protein-protein-DNA) complexes formed by bZIP and bHLHZip proteins, we found that the proteins recognize DNA as monomers. Here we show that protein monomer-DNA complexes are favored at high DNA concentrations. Further, we show that, due to fast rates of association with protein monomers, DNA and other polyanions accelerate the rate of protein dimer formation. Finally, we find that DNA-assisted formation of protein dimers provides a mechanism by which dimeric transcription factors can rapidly discriminate between specific and nonspecific sites.

Kinetic studies of Fos.Jun.DNA complex formation: DNA binding prior to dimerization

The bZIP proteins Fos and Jun bind DNA rapidly and with high affinity, forming a heteromeric complex that mediates activated transcription. Here we use stopped-flow fluorescence resonance energy transfer (FRET) to study the kinetic pathway by which Fos.Jun. DNA complexes assemble. Though dimerization of Fos and Jun occurs rapidly in the absence of DNA, the rate of dimerization is enhanced in the presence of DNA. Global analysis of the kinetic data shows that the favored DNA binding pathway is one is which the two protein monomers bind DNA sequentially and assemble their dimerization interface while bound to DNA.

Long-term immunological memory induced by recombinant oral Salmonella vaccine vectors

We have previously shown that Salmonella enterica serovar Typhimurium expressing the hagB hemagglutinin gene from Porphyromonas gingivalis can induce primary and recall immune responses in serum and secretions in mice; however, the longevity of memory induced by oral Salmonella carriers has not been adequately demonstrated. In this study, we examined the capacity of mice to mount a recall response 52 weeks after primary immunization. Recall responses were seen in serum immunoglobulin G (IgG) and IgA following boosting at week 52, and in most cases, they were equal to or greater than the primary responses. Significant mucosal IgA recall responses in saliva and vaginal wash were also detected following boosting at week 52. In addition, there was a considerable residual response in secretions at week 51, prior to boosting. These results indicate that oral Salmonella vectors can induce long-term memory to recombinant HagB and are particularly effective at inducing long-lasting mucosal responses as well as at inducing the capacity for mucosal recall responses.

Effect of preexisting immunity to Salmonella on the immune response to recombinant Salmonella enterica serovar typhimurium expressing a Porphyromonas gingivalis hemagglutinin

Recombinant Salmonella strains expressing foreign heterologous genes have been extensively studied as live oral vaccine delivery vectors. We have investigated the mucosal and systemic immune responses following oral immunization with a recombinant Salmonella enterica serovar Typhimurium expressing the hemagglutinin HagB from Porphyromonas gingivalis, a suspected etiological agent of adult periodontal disease. We have previously shown a primary mucosal and systemic response following oral immunization with chi4072/pDMD1 and recall responses following boosting at 14 weeks after primary immunization. In this study, we examined the effects of earlier boosting as well as the effects of deliberately induced immunity to the Salmonella carrier strain on subsequent immune responses. Mice boosted at week 7 following immunization, a point which corresponded to the peak of the primary response, generally showed lower responses than those boosted at week 14. When mice were preimmunized with the Salmonella carrier alone and then immunized with the recombinant strain 7 or 14 weeks later, significant reductions were seen for serum immunoglobulin G (IgG) antibodies at week 14 and for salivary IgA at week 7. No reductions were seen in serum IgA or vaginal wash IgA antibodies. Mice appear to be refractory to boosting with orally administered salmonellae at 7 weeks. Deliberate immunization with the carrier strain did not appreciably affect recall responses at 14 weeks, with the exception of the serum IgG responses, nor did it affect colonization of the Peyer's patches.

Gene regulation: protein escorts to the transcription ball

A new way by which the potency of a eukaryotic transcription factor can be regulated has been discovered, in which nuclear factors increase the concentration of the transcription factor's active form by modulating an otherwise unfavorable equilibrium between monomeric and dimeric forms of the protein.

DNA specificity enhanced by sequential binding of protein monomers

Transcriptional activation often requires the rapid assembly of complexes between dimeric transcription factors and specific DNA sites. Here we show that members of the basic region leucine zipper and basic region helix-loop-helix zipper transcription factor families follow an assembly pathway in which two protein monomers bind DNA sequentially and form their dimerization interface while bound to DNA. Nonspecific protein or DNA competitors have little effect on the rate of assembly along this pathway, but slow a competing pathway in which preformed dimers bind DNA. The sequential monomer-binding pathway allows the protein to search for and locate a specific DNA site more quickly, resulting in greater specificity prior to equilibrium.

Oral immunization with recombinant Salmonella typhimurium expressing a cloned Porphyromonas gingivalis hemagglutinin: effect of boosting on mucosal, systemic and immunoglobulin G subclass response

Live avirulent Salmonella typhimurium are convenient vaccine vectors for the delivery of recombinant antigens for the induction of mucosal and systemic immunity. The hagB gene encodes a hemagglutinin of Porphyromonas gingivalis, a suspected causal agent in human adult periodontal disease. In previous studies, we have shown that hagB can be expressed in avirulent S. typhimurium and is immunogenic when given orally to mice. In this study, we evaluated recall responses in both serum and mucosal secretions after boosting. In addition, we have examined the immunoglobulin G (IgG) subclass response in serum to both HagB and the Salmonella carrier. Mice were orally immunized with S. typhimurium expressing the hagB gene and then boosted 14 weeks later. Responses were measured through 27 weeks. Both primary and recall IgG and IgA responses were seen in serum to the purified HagB as well as to the Salmonella carrier. Likewise, mucosal primary and recall responses were seen in saliva, fecal extracts and vaginal washes although the kinetics of the responses differed. The anti-HagB response in serum was dominated by IgG2a during the peak of primary response, prior to boosting and during the peak of the recall response. The anti-S. typhimurium response shifted from predominantly IgG3 following primary immunization to IgG2a after boosting. The IgG1 response was minimal against each antigen. This pattern of IgG subclass distribution is consistent with a Th1-type response. These data indicate that avirulent S. typhimurium is capable of delivering a putative virulence factor from P. gingivalis and inducing a primary and recall response in both serum and secretions and provides a means of studying P. gingivalis virulence factors and for the development of a potential vaccine.

[Villous tumors of the rectum. Apropos of 30 cases]

Although high localisations of villous tumours may be treated by wide removal, which alone guarantees cure without risk of relapse, the use of such an operation in low villous tumours is less obvious owing to the mutilation which may result. Local removal of small volume tumours by the anal route, by the trans-sacral route when they are larger or more broadly implanted, is preferred by us in rectal localisations, i.e. each villous tumour should be treated taking into considération all its characteristics.