Catalyzes the reaction: UDP-alpha-D-xylose + -3-O-(alpha-D-xylosyl-(1->3)-beta-D-glucosyl)-L-serine = UDP + -3-O-(alpha-D-xylosyl-(1->3)-alpha-D-xylosyl-(1->3)-beta-D-glucosyl)-L-serine. The enzyme, found in animals and insects, is involved in the biosynthesis of the alpha-D-xylosyl-(1->3)-alpha-D-xylosyl-(1->3)- beta-D-glucosyl trisaccharide on epidermal growth factor-like (EGF- like) domains.
xylosyl alpha-1,3-xylosyltransferase activity