Enables polypeptide N-acetylgalactosaminyltransferase activity. Acts upstream of or within protein O-linked glycosylation. Predicted to be located in perinuclear region of cytoplasm. Predicted to be active in Golgi apparatus. Is expressed in several structures, including alimentary system; brain; cranium; respiratory system; and sensory organ. Used to study heart valve disease. Orthologous to human GALNT1 (polypeptide N-acetylgalactosaminyltransferase 1). [provided by Alliance of Genome Resources, Apr 2022]