Alpha-1,3-xylosyltransferase, which elongates the O-linked xylose-glucose disaccharide attached to EGF-like repeats in the extracellular domain of target proteins by catalyzing the addition of the second xylose (PubMed:22117070, PubMed:8982869). Known targets include Notch proteins and coagulation factors, such as F9 (PubMed:22117070, PubMed:8982869). 3-O-[alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + UDP-alpha-D-xylose = 3-O-[alpha-D-xylosyl-(1->3)-alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + H(+) + UDP Has the highest in vitro activity with 20 mM Mn(2+), a concentration entirely out of the physiological range. Can also utilize Mg(2+), suggesting this may be the physiological cofactor. Optimum pH is 7.2. Homodimer (PubMed:22117070). Dimer formation may be essential for the retention in endoplasmic reticulum (Probable). Belongs to the glycosyltransferase 8 family. It is uncertain whether Met-1 or Met-14 is the initiator. Truncated N-terminus.