Glycosyltransferase that generates the core 1 O-glycan Gal-beta1-3GalNAc-alpha1-Ser/Thr (T antigen), which is a precursor for many extended O-glycans in glycoproteins. Plays a central role in many processes, such as angiogenesis, thrombopoiesis and kidney homeostasis development (By similarity). an N-acetyl-alpha-D-galactosaminyl derivative + UDP-alpha-D-galactose = a beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl derivative + H(+) + UDP Protein modification; protein glycosylation. Homodimer; disulfide-linked. Interacts with the C1GALT1C1 chaperone; required for galactosyltransferase activity (By similarity). Belongs to the glycosyltransferase 31 family. Beta3-Gal-T subfamily.