Fig. 1.

(A) Initiation of mucin-type O-linked glycan biosynthesis by the ppGalNAcTs. UDP–GalNAc is the nucleotide sugar donor substrate for the ppGalNAcT family. (B) Strategy for metabolic labeling of mucin-type O-linked glycoproteins with an azido GalNAc analog (GalNAz) for proteomic analysis with phosphine probes. This strategy capitalizes on the highly chemoselective Staudinger ligation reaction between azides and phosphines. R and R′ are oligosaccharide elaborations from the core α-GalNAc residue.