Metabolic chemical reporters (MCRs) of glycosylation are analogues of monosaccharides that contain bioorthogonal functionalities and enable the direct visualization and identification of glycoproteins from living cells. Each MCR was initially thought to report on specific types of glycosylation. We and others have demonstrated that several MCRs are metabolically transformed and enter multiple glycosylation pathways. Therefore, the development of selective MCRs remains a key unmet goal. We demonstrate here that 6-azido-6-deoxy-N-acetyl-glucosamine (6AzGlcNAc) is a specific MCR for O-GlcNAcylated proteins. Biochemical analysis and comparative proteomics with 6AzGlcNAc, N-azidoacetyl-glucosamine (GlcNAz), and N-azidoacetyl-galactosamine (GalNAz) revealed that 6AzGlcNAc exclusively labels intracellular proteins, while GlcNAz and GalNAz are incorporated into a combination of intracellular and extracellular/lumenal glycoproteins. Notably, 6AzGlcNAc cannot be biosynthetically transformed into the corresponding UDP sugar-donor by the canonical salvage-pathway that requires phosphorylation at the 6-hydroxyl. In vitro experiments showed that 6AzGlcNAc can bypass this roadblock through direct phosphorylation of its 1-hydroxyl by the enzyme phosphoacetylglucosamine mutase (AGM1). Taken together, 6AzGlcNAc enables the specific analysis of O-GlcNAcylated proteins, and these results suggest that specific MCRs for other types of glycosylation can be developed. Additionally, our data demonstrate that cells are equipped with a somewhat unappreciated metabolic flexibility with important implications for the biosynthesis of natural and unnatural carbohydrates.

Seven plant lectins, Dolichos bijlorus agglutinin (DBA), Grijjfonia simplicifolia agglutinin (GSA, isolectin A4), Helix pomatia agglutinin (HPA), soybean (Glycine max) agglutinin (SBA), Salvia sclarea agglutinin (SSA), Vicia villosa agglutinin (VVA, isolectin B4) and Wistariafloribunda agglutinin (WFA), known to be specific for N-acetyl-D-galactosamine-(GalNAc) bearing glycoconjugates, have been compared by the binding of their radiolabelled derivatives, to eight well-characterized synthetic oligosaccharides immobilized via a spacer on an inert silica matrix (Synsorb). The eight oligosaccharides included the Forssman, the blood group A and the T antigens, as well as ctGalNAc coupled directly to the support (Tn antigen) and also structures with GalNAc linked a or /3 to positions 3 or 4 of an unsubstituted Gal. The binding studies clearly distinguished the lectins into aGalNAc-specific agglutinins like DBA, GSA and SSA, and lectins which recognize a-as well as P-linked GalNAc residues like HPA, VVA, WFA and SBA. HPA was the only lectin which bound to the P G a l l -+ 3aGalNAc-Synsorb adsorbent (T antigen) indicating that it also recognizes internal GalNAc residues. Among the aGalNAc-specific lectins, DBA strongly recognized blood group A structures while GSA displayed weaker recognition, and SSA bound only slightly to this affinity matrix. In addition, DBA and SSA were able to distinguish between GalNAc linked a1 -+ 3 and GalNAc linked a1 -+ 4, to the support, the latter being a much weaker ligand.These results were corroborated by the binding of the lectins to biological substrates as determined by their hemagglutination titers with native and enzyme-treated red blood cells carrying known GalNAc determinants, e.g. blood group A, and the Cad and Tn antigens. For SSA, the binding to the ctGalNAc matrix was inhibited by a number of glycopeptides and glycoproteins confirming the strong preference of this lectin for aGalNAc-Ser/ Thr-bearing glycoproteins.N-Acetyl-D-galactosamine (GalNAc) is an important constituent of the carbohydrate moieties of glycoproteins and glycolipids. Carbohydrate-binding proteins (lectins) which recognize this sugar residue were among the first to be studied in detail since these lectins were shown to agglutinate specifically blood group A erythrocytes ([l] binding of oligosaccharides having a terminal ct or PGalNAc linked in different positions to a or BGal, or a or PGalNAc. Some of these lectins have been used in immunocytochemistry since they are thought to bind specifically to defined carbohydrate structures (reviewed in [12]). Therefore it is of interest to compare, in the same analytical system, the binding specificities of lectins with similar recognition patterns in order to be able to choose the most appropriate lectin for a given purpose.Recently, two lectins have been purified from the seeds of Vicia villosa (VVA, isolectin B4) [13] and Salvia sclarea (SSA) [14] which were described as aGalNAc-specific and most interestingly both appeared to have a strong preference for the a...