Use of novel mutant galactosyltransferase for the bioconjugation of terminal N-acetylglucosamine (GlcNAc) residues on live cell surface

A Time-Resolved FRET Assay Identifies a Small Molecule that Inhibits the Essential Bacterial Cell Wall Polymerase FtsW

The peptidoglycan cell wall is essential for bacterial survival. To form the cell wall, peptidoglycan glycosyltransferases (PGTs) polymerize Lipid II to make glycan strands and then those strands are crosslinked by transpeptidases (TPs). Recently, the SEDS (for shape, elongation, division, and sporulation) proteins were identified as a new class of PGTs. The SEDS protein FtsW, which produces septal peptidoglycan during cell division, is an attractive target for novel antibiotics because it is essential in virtually all bacteria. Here, we developed a time-resolved Förster resonance energy transfer (TR-FRET) assay to monitor PGT activity and screened a Staphylococcus aureus lethal compound library for FtsW inhibitors. We identified a compound that inhibits S. aureus FtsW in vitro. Using a non-polymerizable Lipid II derivative, we showed that this compound competes with Lipid II for binding to FtsW. The assays described here will be useful for discovering and characterizing other PGT inhibitors.

[Comparison of the performance of secretome analysis based on metabolic labeling by three unnatural sugars]

分泌蛋白质是调控细胞间信号转导的重要生物大分子。由于分泌蛋白的丰度相比于胞内蛋白以及培养基添加剂更低,因此分泌蛋白的高通量鉴定是目前蛋白质组学界研究的热点和难点。目前,基于生物质谱的分泌蛋白质组学分析一般均需要从无血清的条件培养基中获得分泌蛋白质,再对其进行富集和分析。该流程操作步骤繁琐,易造成分泌蛋白质的损失和降解。本工作采用基于生物正交化学生物学技术实现对分泌蛋白质的高选择性标记和高效富集。通过结合点击化学技术,综合评估了分泌蛋白质分析中用于代谢标记的不同糖类似物。采用3种最常用的商品化糖类似物,N-叠氮乙酰甘露糖胺(ManNAz)、N-叠氮乙酰半乳糖胺(GalNAz)和N-叠氮乙酰葡萄糖胺(GlcNAz)分别对HeLa细胞进行代谢标记,之后通过炔基生物素探针对条件培养基中的分泌蛋白进行富集,结合质谱分析来对比3种糖类似物对分泌蛋白的标记效率。最后通过无标定量蛋白质组学分析,系统评估了3种糖类似物用于分泌蛋白质组分析的性能。结果表明,基于ManNAz的分泌蛋白标记方法鉴定到了282个分泌蛋白、224个细胞质膜蛋白以及846个N-糖基化位点;对分泌蛋白的富集效率分别较GalNAz和GlcNAz提高了130%和67.2%;对细胞质膜蛋白的富集效率较GalNAz和GlcNAz分别提高了273.3%和148.7%,体现出了明显的优势。本研究的实验结果为分泌蛋白高选择性富集和系统分析提供了有益的对比分析和新技术策略。

Aberration of Serum and Tissue N-Glycans in Mouse β1,4-GalT1 Y286L Mutant Variants

β1,4-GalT1 is a type II membrane glycosyltransferase. It catalyzes the production of lactose in the lactating mammary gland and is supposedly also involved in the galactosylation of terminal GlcNAc of complex-type N-glycans. In-vitro studies of the bovine β4Gal-T1 homolog showed that replacing a single residue of tyrosine with leucine at position 289 alters the donor substrate specificity from UDP-Gal to UDP-N-acetyl-galactosamine (UDP-GalNAc). The effect of this peculiar change in β1,4GalT1 specificity was investigated in-vivo, by generating biallelic Tyr286Leu β1,4GalT1 mice using CRISPR/Cas9 and crossbreeding. Mice bearing this mutation showed no appreciable defects when compared to wild-type mice, with the exception of biallelic female B4GALT1 mutant mice, which were unable to produce milk. The detailed comparison of wild-type and mutant mice derived from liver, kidney, spleen, and intestinal tissues showed only small differences in their N-glycan pattern. Comparable N-glycosylation was also observed in HEK 293 wild-type and knock-out B4GALT1 cells. Remarkably and in contrast to the other analyzed tissue samples, sialylation and galactosylation of serum N-glycans of biallelic Tyr286Leu GalT1 mice almost disappeared completely. These results suggest that β1,4GalT1 plays a special role in the synthesis of serum N-glycans. The herein described Tyr286Leu β1,4GalT1 mutant mouse model may, therefore, prove useful in the investigation of the mechanism which regulates tissue-dependent galactosylation.

Paramagnetic Tag for Glycosylation Sites in Glycoproteins: Structural Constraints on Heparan Sulfate Binding to Robo1

An enzyme- and click chemistry-mediated methodology for the site-specific nitroxide spin labeling of glycoproteins has been developed and applied. The procedure relies on the presence of single N-glycosylation sites that are present natively in proteins or that can be engineered into glycoproteins by mutational elimination of all but one glycosylation site. Recombinantly expressing glycoproteins in HEK293S (GnT1-) cells results in N-glycans with high-mannose structures that can be processed to leave a single GlcNAc residue. This can in turn be modified by enzymatic addition of a GalNAz residue that is subject to reaction with an alkyne-carrying TEMPO moiety using copper(I)-catalyzed click chemistry. To illustrate the procedure, we have made an application to a two-domain construct of Robo1, a protein that carries a single N-glycosylation site in its N-terminal domains. The construct has also been labeled with 15N at amide nitrogens of lysine residues to provide a set of sites that are used to derive an effective location of the paramagnetic nitroxide moiety of the TEMPO group. This, in turn, allowed measurements of paramagnetic perturbations to the spectra of a new high affinity heparan sulfate ligand. Calculation of distance constraints from these data facilitated determination of an atomic level model for the docked complex.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Glycosyltransferase engineering for carbohydrate synthesis

Glycosyltransferases (GTs) are powerful tools for the synthesis of complex and biologically-important carbohydrates. Wild-type GTs may not have all the properties and functions that are desired for large-scale production of carbohydrates that exist in nature and those with non-natural modifications. With the increasing availability of crystal structures of GTs, especially those in the presence of donor and acceptor analogues, crystal structure-guided rational design has been quite successful in obtaining mutants with desired functionalities. With current limited understanding of the structure-activity relationship of GTs, directed evolution continues to be a useful approach for generating additional mutants with functionality that can be screened for in a high-throughput format. Mutating the amino acid residues constituting or close to the substrate-binding sites of GTs by structure-guided directed evolution (SGDE) further explores the biotechnological potential of GTs that can only be realized through enzyme engineering. This mini-review discusses the progress made towards GT engineering and the lessons learned for future engineering efforts and assay development.

One-Step Selective Exoenzymatic Labeling (SEEL) Strategy for the Biotinylation and Identification of Glycoproteins of Living Cells

Technologies that can visualize, capture, and identify subsets of biomolecules that are not encoded by the genome in the context of healthy and diseased cells will offer unique opportunities to uncover the molecular mechanism of a multitude of physiological and disease processes. We describe here a chemical reporter strategy for labeling of cell surface glycoconjugates that takes advantage of recombinant glycosyltransferases and a corresponding sugar nucleotide functionalized by biotin. The exceptional efficiency of this method, termed one-step selective exoenzymatic labeling, or SEEL, greatly improved the ability to enrich and identify large numbers of tagged glycoproteins by LC-MS/MS. We further demonstrated that this labeling method resulted in far superior enrichment and detection of glycoproteins at the plasma membrane compared to a sulfo-NHS-activated biotinylation or two-step SEEL. This new methodology will make it possible to profile cell surface glycoproteomes with unprecedented sensitivity in the context of physiological and disease states.

Site-Specific Quantification of Surface N-Glycoproteins in Statin-Treated Liver Cells

The frequent modification of cell-surface proteins by N-linked glycans is known to be correlated with many biological processes. Aberrant glycosylation on surface proteins is associated with different cellular statuses and disease progression. However, it is extraordinarily challenging to comprehensively and site-specifically analyze glycoproteins located only on the cell surface. Currently mass spectrometry (MS)-based proteomics provides the possibility to analyze the N-glycoproteome, but effective separation and enrichment methods are required for the analysis of surface glycoproteins prior to MS measurement. The introduction of bio-orthogonal groups into proteins accelerates research in the robust visualization, identification, and quantification of proteins. Here we have comprehensively evaluated different sugar analogs in the analysis of cell-surface N-glycoproteins by combining copper-free click chemistry and MS-based proteomics. Comparison of three sugar analogs, N-azidoacetylgalactosamine (GalNAz), N-azidoacetylglucosamine (GlcNAz), and N-azidoacetylmannosamine (ManNAz), showed that metabolic labeling with GalNAz resulted in the greatest number of glycoproteins and glycosylation sites in biological duplicate experiments. GalNAz was then employed for the quantification experiment in statin-treated HepG2 liver cells, and 280 unique N-glycosylated sites were quantified from 168 surface proteins. The quantification results demonstrated that many glycosylation sites on surface proteins were down-regulated in statin-treated cells compared to untreated cells because statin prevents the synthesis of dolichol, which is essential for the formation of dolichol-linked precursor oligosaccharides. Several glycosylation sites in proteins that participate in the Alzheimer's disease pathway were down-regulated. This method can be extensively applied for the global analysis of the cell-surface N-glycoproteome.

Current methods for the synthesis of homogeneous antibody-drug conjugates

Development of efficient and safe cancer therapy is one of the major challenges of the modern medicine. Over the last few years antibody-drug conjugates (ADCs) have become a powerful tool in cancer treatment with two of them, Adcetris® (brentuximab vedotin) and Kadcyla® (ado-trastuzumab emtansine), having recently been approved by the Food and Drug Administration (FDA). Essentially, an ADC is a bioconjugate that comprises a monoclonal antibody that specifically binds tumor surface antigen and a highly potent drug, which is attached to the antibody via either cleavable or stable linker. This approach ensures specificity and efficacy in fighting cancer cells, while healthy tissues remain largely unaffected. Conventional ADCs, that employ cysteine or lysine residues as conjugation sites, are highly heterogeneous. This means that the species contain various populations of the ADCs with different drug-to-antibody ratios (DARs) and different drug load distributions. DAR and drug-load distribution are essential parameters of ADCs as they determine their stability and efficacy. Therefore, various drug-loaded forms of ADCs (usually from zero to eight conjugated molecules per antibody) may have distinct pharmacokinetics (PK) in vivo and may differ in clinical performance. Recently, a significant progress has been made in the field of site-specific conjugation which resulted in a number of strategies for synthesis of the homogeneous ADCs. This review describes newly-developed methods that ensure homogeneity of the ADCs including use of engineered reactive cysteine residues (THIOMAB), unnatural amino acids, aldehyde tags, enzymatic transglutaminase- and glycotransferase-based approaches and novel chemical methods. Furthermore, we briefly discuss the limitation of these methods emphasizing the need for further improvement in the ADC design and development.

The challenge and promise of glycomics

Glycomics is a broad and emerging scientific discipline focused on defining the structures and functional roles of glycans in biological systems. The staggering complexity of the glycome, minimally defined as the repertoire of glycans expressed in a cell or organism, has resulted in many challenges that must be overcome; these are being addressed by new advances in mass spectrometry as well as by the expansion of genetic and cell biology studies. Conversely, identifying the specific glycan recognition determinants of glycan-binding proteins by employing the new technology of glycan microarrays is providing insights into how glycans function in recognition and signaling within an organism and with microbes and pathogens. The promises of a more complete knowledge of glycomes are immense in that glycan modifications of intracellular and extracellular proteins have critical functions in almost all biological pathways.

Selective exo-enzymatic labeling of N-glycans on the surface of living cells by recombinant ST6Gal I

A game of tag: N-Glycans on the surface of living cells were selectively tagged by exogenously administering recombinant ST6Gal I sialyltransferase and azide-modified CMP-Neu5Ac. This modification was followed by a strain-promoted cycloaddition using a biotin-modified dibenzylcyclooctynol (red star=biotin). The methodology will make it possible to dissect the mechanisms that underlie altered glycoconjugate recycling and storage in disease.