This entry represents a domain found in N-acetylgalactosaminyltransferases (also known as pp-GalNAc-T). They initiate the formation of mucin-type, O-linked glycans by catalysing 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 [].Interestingly, some members, such as human GALNTL5, lack the C-terminal ricin B-type lectin domain, which contributes to the glycopeptide specificity. No glycosyltransferase activity has been detected for human GALNTL5 in an in vitro assay [].This entry also includes Putative inactive polypeptide N-acetylgalactosaminyltransferase 11/12 from Drosophila, which, although they are strongly related to polypeptide N-acetylgalactosaminyltransferase proteins, they lack the conserved His at position 211 which is part of the Asp-X-His motif that binds the cofactor Mn2, suggesting that they may have lost its activity.
