UDP-D-Xylose:beta-D-Glucoside Alpha-1,3-D-Xylosyltransferase Activity

Catalyzes the reaction: UDP-alpha-D-xylose + [protein with EGF-like domain]-3-O-(beta-D-glucosyl)-L-serine <=> UDP + [protein with EGF-like domain]-3-O-(alpha-D-xylosyl-(1->3)-beta-D-glucosyl)-L-serine.