Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. Has activity toward non-glycosylated peptides such as Muc5AC, Muc1a and EA2, and no detectable activity with Muc2 and Muc7. Displays enzymatic activity toward the Gal-NAc-Muc5AC glycopeptide, but no detectable activity to mono-GalNAc-glycosylated Muc1a, Muc2, Muc7 and EA2. May play an important role in the initial step of mucin-type oligosaccharide biosynthesis in digestive organs.
Subcellular locations: Golgi apparatus membrane
Widely expressed at different levels of expression. Highly expressed in digestive organs such as small intestine, stomach, pancreas and colon. Expressed at intermediate level in testis, thyroid gland and spleen. Weakly expressed in whole brain, cerebral cortex, cerebellum, fetal brain, bone marrow, thymus, leukocytes, heart, skeletal muscle, liver, lung, esophagus, kidney, adrenal gland, mammary gland, uterus, placenta, ovary and prostate.