pp-GalNAc-T initiates the formation of mucin-type O-linked glycans.
UDP-GalNAc: polypeptide alpha-N-acetylgalactosaminyltransferases (pp-GalNAc-T) initiate the formation of mucin-type, O-linked glycans by catalyzing the transfer of alpha-N-acetylgalactosamine (GalNAc) from UDP-GalNAc to hydroxyl groups of Ser or Thr residues of core proteins to form the Tn antigen (GalNAc-a-1-O-Ser/Thr). These enzymes are type II membrane proteins with a GT-A type catalytic domain and a lectin domain located on the lumen side of the Golgi apparatus. In human, there are 15 isozymes of pp-GalNAc-Ts, representing the largest of all glycosyltransferase families. Each isozyme has unique but partially redundant substrate specificity for glycosylation sites on acceptor proteins.