Transfers N-acetylgalactosamine (GalNAc) from UDP-GalNAc to N-acetylglucosamine-beta-benzyl with a beta-1,4-linkage to form N,N'-diacetyllactosediamine, GalNAc-beta-1,4-GlcNAc structures in N-linked glycans and probably O-linked glycans. Mediates the N,N'-diacetyllactosediamine formation on gastric mucosa.
Subcellular locations: Golgi apparatus, Golgi stack membrane
Localizes to apical Golgi.
Highly expressed in testis, colon and stomach. Weakly expressed in other tissues.