Glycosyltransferase involved in the O-galactosylation of several proteins including extensins. Catalyzes the transfer of alpha-galactosyl to Ser residues. Hydroxylation of proline residues adjacent to the serine acceptor is required for activity. Utilizes selectively UDP-galactose as a donor nucleotide sugar. Weak activity with Fe(2+), Co(2+) and Zn(2+), but no activity with Ca(2+), Mg(2+) or Cu(2+). Optimum pH is 6.0. Optimum temperature is 30 degrees Celsius.