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. Although it displays a much weaker activity toward all substrates tested compared to GALNT2, it is able to transfer up to seven GalNAc residues to the Muc5AC peptide, suggesting that it can fill vicinal Thr/Ser residues in cooperation with other GALNT proteins. Prefers Muc1a as substrate.
Subcellular locations: Golgi apparatus membrane
Widely expressed. Highly expressed in small intestine, placenta, spleen, cerebral cortex and ovary. Expressed at intermediate level in uterus, mammary gland, stomach, cerebellum and whole brain. Weakly expressed in fetal brain, bone marrow, thyroid gland, thymus, heart, skeletal muscle, lung, liver, colon, pancreas, kidney and testis. Not expressed in leukocyte. Expressed in both normal and osteoarthritic cartilage. Expressed at low level in chondrocytes in all zones of both normal and osteoarthritic cartilage.