Recognition of diazirine-modified O-GlcNAc by human O-GlcNAcase

Growing and dividing: how O-GlcNAcylation leads the way

Cell cycle errors can lead to mutations, chromosomal instability, or death; thus, the precise control of cell cycle progression is essential for viability. The nutrient-sensing posttranslational modification, O-GlcNAc, regulates the cell cycle allowing one central control point directing progression of the cell cycle. O-GlcNAc is a single N-acetylglucosamine sugar modification to intracellular proteins that is dynamically added and removed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. These enzymes act as a rheostat to fine-tune protein function in response to a plethora of stimuli from nutrients to hormones. O-GlcNAc modulates mitogenic growth signaling, senses nutrient flux through the hexosamine biosynthetic pathway, and coordinates with other nutrient-sensing enzymes to progress cells through Gap phase 1 (G1). At the G1/S transition, O-GlcNAc modulates checkpoint control, while in S Phase, O-GlcNAcylation coordinates the replication fork. DNA replication errors activate O-GlcNAcylation to control the function of the tumor-suppressor p53 at Gap Phase 2 (G2). Finally, in mitosis (M phase), O-GlcNAc controls M phase progression and the organization of the mitotic spindle and midbody. Critical for M phase control is the interplay between OGT and OGA with mitotic kinases. Importantly, disruptions in OGT and OGA activity induce M phase defects and aneuploidy. These data point to an essential role for the O-GlcNAc rheostat in regulating cell division. In this review, we highlight O-GlcNAc nutrient sensing regulating G1, O-GlcNAc control of DNA replication and repair, and finally, O-GlcNAc organization of mitotic progression and spindle dynamics.

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

4-Deoxy-4-fluoro-GalNAz (4FGalNAz) Is a Metabolic Chemical Reporter of O-GlcNAc Modifications, Highlighting the Notable Substrate Flexibility of O-GlcNAc Transferase

Bio-orthogonal chemistries have revolutionized many fields. For example, metabolic chemical reporters (MCRs) of glycosylation are analogues of monosaccharides that contain a bio-orthogonal functionality, such as azides or alkynes. MCRs are metabolically incorporated into glycoproteins by living systems, and bio-orthogonal reactions can be subsequently employed to install visualization and enrichment tags. Unfortunately, most MCRs are not selective for one class of glycosylation (e.g., N-linked vs O-linked), complicating the types of information that can be gleaned. We and others have successfully created MCRs that are selective for intracellular O-GlcNAc modification by altering the structure of the MCR and thus biasing it to certain metabolic pathways and/or O-GlcNAc transferase (OGT). Here, we attempt to do the same for the core GalNAc residue of mucin O-linked glycosylation. The most widely applied MCR for mucin O-linked glycosylation, GalNAz, can be enzymatically epimerized at the 4-hydroxyl to give GlcNAz. This results in a mixture of cell-surface and O-GlcNAc labeling. We reasoned that replacing the 4-hydroxyl of GalNAz with a fluorine would lock the stereochemistry of this position in place, causing the MCR to be more selective. After synthesis, we found that 4FGalNAz labels a variety of proteins in mammalian cells and does not perturb endogenous glycosylation pathways unlike 4FGalNAc. However, through subsequent proteomic and biochemical characterization, we found that 4FGalNAz does not widely label cell-surface glycoproteins but instead is primarily a substrate for OGT. Although these results are somewhat unexpected, they once again highlight the large substrate flexibility of OGT, with interesting and important implications for intracellular protein modification by a potential range of abiotic and native monosaccharides.

Photocrosslinking O-GlcNAcylated Proteins to Neighboring Biomolecules

This protocol enables identification of the interaction partners of O-GlcNAcylated proteins. The method involves the introduction of the diazirine photocrosslinker onto the O-GlcNAc modification within living cells. The photocrosslinker is activated by UV light to yield covalent crosslinking between O-GlcNAcylated proteins and neighboring molecules. The binding partners can be further characterized by immunoblot or proteomics mass spectrometry methods. The benefits of using the photocrosslinker include the capacity to trap low-affinity binding interactions and the ability to selectively target the interaction partners of the O-GlcNAcylated form of the protein of interest. © 2021 Wiley Periodicals LLC. Basic Protocol 1: In-cell production and crosslinking of O-GlcNDAzylated proteins Basic Protocol 2: Immunoblot analysis to assess O-GlcNDAz crosslinking Support Protocol: Detection of UDP-GlcNDAz from cell lysates.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule-protein and macromolecule-protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Site-Directed Glycosylation of Peptide/Protein with Homogeneous O-Linked Eukaryotic N-Glycans

Here we report a facile and efficient method for site-directed glycosylation of peptide/protein. The method contains two sequential steps: generation of a GlcNAc-O-peptide/protein, and subsequent ligation of a eukaryotic N-glycan to the GlcNAc moiety. A pharmaceutical peptide, glucagon-like peptide-1 (GLP-1), and a model protein, bovine α-Crystallin, were successfully glycosylated using such an approach. It was shown that the GLP-1 with O-linked N-glycan maintained an unchanged secondary structure after glycosylation, suggesting the potential application of this approach for peptide/protein drug production. In summary, the coupled approach provides a general strategy to produce homogeneous glycopeptide/glycoprotein bearing eukaryotic N-glycans.

Enhanced transfer of a photocross-linking N-acetylglucosamine (GlcNAc) analog by an O-GlcNAc transferase mutant with converted substrate specificity

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification of proteins in multicellular organisms. O-GlcNAc modification is catalyzed by the O-GlcNAc transferase (OGT), which transfers N-acetylglucosamine (GlcNAc) from the nucleotide sugar donor UDP-GlcNAc to serine or threonine residues of protein substrates. Recently, we reported a novel metabolic labeling method to introduce the diazirine photocross-linking functional group onto O-GlcNAc residues in mammalian cells. In this method, cells are engineered to produce diazirine-modified UDP-GlcNAc (UDP-GlcNDAz), and the diazirine-modified GlcNAc analog (GlcNDAz) is transferred to substrate proteins by endogenous OGT, producing O-GlcNDAz. O-GlcNDAz-modified proteins can be covalently cross-linked to their binding partners, providing information about O-GlcNAc-dependent interactions. The utility of the method was demonstrated by cross-linking highly O-GlcNAc-modified nucleoporins to proteins involved in nuclear transport. For practical application of this method to a broader range of O-GlcNAc-modified proteins, efficient O-GlcNDAz production is critical. Here we examined the ability of OGT to transfer GlcNDAz and found that the wild-type enzyme (wtOGT) prefers the natural substrate, UDP-GlcNAc, over the unnatural UDP-GlcNDAz. This competition limits O-GlcNDAz production in cells and the extent of O-GlcNDAz-dependent cross-linking. Here we identified an OGT mutant, OGT(C917A), that efficiently transfers GlcNDAz and, surprisingly, has altered substrate specificity, preferring to transfer GlcNDAz rather than GlcNAc to protein substrates. We confirmed the reversed substrate preference by determining the Michaelis-Menten parameters describing the activity of wtOGT and OGT(C917A) with both UDP-GlcNAc and UDP-GlcNDAz. Use of OGT(C917A) enhances O-GlcNDAz production, yielding improved cross-linking of O-GlcNDAz-modified molecules both in vitro and in cells.

Cellular metabolism of unnatural sialic acid precursors

Carbohydrates, in addition to their metabolic functions, serve important roles as receptors, ligands, and structural molecules for diverse biological processes. Insight into carbohydrate biology and mechanisms has been aided by metabolic oligosaccharide engineering (MOE). In MOE, unnatural carbohydrate analogs with novel functional groups are incorporated into cellular glycoconjugates and used to probe biological systems. While MOE has expanded knowledge of carbohydrate biology, limited metabolism of unnatural carbohydrate analogs restricts its use. Here we assess metabolism of SiaDAz, a diazirine-modified analog of sialic acid, and its cell-permeable precursor, Ac4ManNDAz. We show that the efficiency of Ac4ManNDAz and SiaDAz metabolism depends on cell type. Our results indicate that different cell lines can have different metabolic roadblocks in the synthesis of cell surface SiaDAz. These findings point to roles for promiscuous intracellular esterases, kinases, and phosphatases during unnatural sugar metabolism and provide guidance for ways to improve MOE.