Phage display uncovers a sequence motif that drives polypeptide binding to a conserved regulatory exosite of O-GlcNAc transferase

The modification of nucleocytoplasmic proteins by O-linked N-acetylglucosamine (O-GlcNAc) is an important regulator of cell physiology. O-GlcNAc is installed on over a thousand proteins by just one enzyme, O-GlcNAc transferase (OGT). How OGT is regulated is therefore a topic of interest. To gain insight into these questions, we used OGT to perform phage display selection from an unbiased library of ~109 peptides of 15 amino acids in length. Following rounds of selection and deep mutational panning, we identified a high-fidelity peptide consensus sequence, [Y/F]-x-P-x-Y-x-[I/M/F], that drives peptide binding to OGT. Peptides containing this sequence bind to OGT in the high nanomolar to low micromolar range and inhibit OGT in a noncompetitive manner with low micromolar potencies. X-ray structural analyses of OGT in complex with a peptide containing this motif surprisingly revealed binding to an exosite proximal to the active site of OGT. This structure defines the detailed molecular basis driving peptide binding and explains the need for specific residues within the sequence motif. Analysis of the human proteome revealed this motif within 52 nuclear and cytoplasmic proteins. Collectively, these data suggest a mode of regulation of OGT by which polypeptides can bind to this exosite to cause allosteric inhibition of OGT through steric occlusion of its active site. We expect that these insights will drive improved understanding of the regulation of OGT within cells and enable the development of new chemical tools to exert fine control over OGT activity.

Potent De Novo Macrocyclic Peptides That Inhibit O-GlcNAc Transferase through an Allosteric Mechanism

Glycosyltransferases are a superfamily of enzymes that are notoriously difficult to inhibit. Here we apply an mRNA display technology integrated with genetic code reprogramming, referred to as the RaPID (random non-standard peptides integrated discovery) system, to identify macrocyclic peptides with high binding affinities for O-GlcNAc transferase (OGT). These macrocycles inhibit OGT activity through an allosteric mechanism that is driven by their binding to the tetratricopeptide repeats of OGT. Saturation mutagenesis in a maturation screen using 39 amino acids, including 22 non-canonical residues, led to an improved unnatural macrocycle that is ≈40 times more potent than the parent compound (K_i ^app =1.5 nM). Subsequent derivatization delivered a biotinylated derivative that enabled one-step affinity purification of OGT from complex samples. The high potency and novel mechanism of action of these OGT ligands should enable new approaches to elucidate the specificity and regulation of OGT.

Discovery of a New Drug-Like Series of OGT Inhibitors by Virtual Screening

O-GlcNAcylation is an essential post-translational modification installed by the enzyme O-β-N-acetyl-d-glucosaminyl transferase (OGT). Modulating this enzyme would be extremely valuable to better understand its role in the development of serious human pathologies, such as diabetes and cancer. However, the limited availability of potent and selective inhibitors hinders the validation of this potential therapeutic target. To explore new chemotypes that target the active site of OGT, we performed virtual screening of a large library of commercially available compounds with drug-like properties. We purchased samples of the most promising virtual hits and used enzyme assays to identify authentic leads. Structure-activity relationships of the best identified OGT inhibitor were explored by generating a small library of derivatives. Our best hit displays a novel uridine mimetic scaffold and inhibited the recombinant enzyme with an IC50 value of 7 µM. The current hit represents an excellent starting point for designing and developing a new set of OGT inhibitors that may prove useful for exploring the biology of OGT.

Overview of the Assays to Probe O-Linked β-N-Acetylglucosamine Transferase Binding and Activity

O-GlcNAcylation is a posttranslational modification that occurs at serine and threonine residues of protein substrates by the addition of O-linked β-d-N-acetylglucosamine (GlcNAc) moiety. Two enzymes are involved in this modification: O-GlcNac transferase (OGT), which attaches the GlcNAc residue to the protein substrate, and O-GlcNAcase (OGA), which removes it. This biological balance is important for many biological processes, such as protein expression, cell apoptosis, and regulation of enzyme activity. The extent of this modification has sparked interest in the medical community to explore OGA and OGT as therapeutic targets, particularly in degenerative diseases. While some OGA inhibitors are already in phase 1 clinical trials for the treatment of Alzheimer's disease, OGT inhibitors still have a long way to go. Due to complex expression and instability, the discovery of potent OGT inhibitors is challenging. Over the years, the field has grappled with this problem, and scientists have developed a number of techniques and assays. In this review, we aim to highlight assays and techniques for OGT inhibitor discovery, evaluate their strength for the field, and give us direction for future bioassay methods.

Monitoring and modulating O-GlcNAcylation: assays and inhibitors of O-GlcNAc processing enzymes

O-linked N-acetylglucosamine (O-GlcNAc) is protein modification that is emerging as a regulator of diverse aspects of cellular physiology. Aberrant O-GlcNAcylation has been linked to several diseases, spurring the creation of methods to detect and perturb the activity of the two enzymes that govern this modification - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Here we summarize assays used for these two enzymes. We also detail the latest structure-guided development of inhibitors of these two enzymes and touch on selected reports that underscore the utility of inhibitors as tools for uncovering the diverse roles of O-GlcNAc in cell function. Finally, we summarize recent reports on the potential therapeutic benefits of antagonizing these enzymes and comment on outstanding challenges within the field.

Intracellular Hydrolysis of Small-Molecule O-Linked N-Acetylglucosamine Transferase Inhibitors Differs among Cells and Is Not Required for Its Inhibition

O-GlcNAcylation is an essential post-translational modification that occurs on nuclear and cytoplasmic proteins, regulating their function in response to cellular stress and altered nutrient availability. O-GlcNAc transferase (OGT) is the enzyme that catalyzes this reaction and represents a potential therapeutic target, whose biological role is still not fully understood. To support this research field, a series of cell-permeable, low-nanomolar OGT inhibitors were recently reported. In this study, we resynthesized the most potent OGT inhibitor of the library, OSMI-4, and we used it to investigate OGT inhibition in different human cell lines. The compound features an ethyl ester moiety that is supposed to be cleaved by carboxylesterases to generate its active metabolite. Our LC-HRMS analysis of the cell lysates shows that this is not always the case and that, even in the cell lines where hydrolysis does not occur, OGT activity is inhibited.