Fig. 1.

Initial steps of the biosynthesis of O-glycans in the mammary gland. Enzymes responsible for the biosynthesis of various O-glycan structures are indicated next to the arrows. Polypeptide GalNAc-transferase (ppGalNAcTs) initiates O-glycosylation by transferring GalNAc to serine or threonine residues (R). α6-Sialyltransferase (ST6GalNAc I) synthesizes the sialyl-Tn antigen by adding a sialylα6 residue to GalNAc. Alternatively, core 1 β3-Gal-transferase (C1GalT) synthesizes core 1, the T antigen. The sialyl-T structure is synthesized by α3-sialyltransferase ST3Gal-I. Further sialylation of sialyl-T can include α6-sialylation of GalNAc to form the desialylated T antigen and/or α2-8 sialylation of the α3-linked sialic acid residue. In the normal mammary gland, core 2 β6-GlcNAc-transferase (C2GnT) can introduce a branch, both arms of which can be further elongated and sialylated or fucosylated