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. Does not glycosylate apomucin or SDC3.
Subcellular locations: Golgi apparatus membrane
Specifically expressed in brain. Not expressed in heart, placenta, lung, liver, skeletal muscle, kidney, pancreas, spleen, thymus, prostate, testis, ovary, small intestine, colon and leukocyte. In brain, it is expressed in cerebellum, frontal lobe, temporal lobe, putamen and spinal cord, weakly expressed in cerebral cortex. Not expressed in medulla and occipital pole.