Reaction catalyzed by UDP-D-xylose:beta-D-glucoside alpha-1,3-D-xylosyltransferase (2.4.2.42)

		+		[protein with EGF-like domain]-3-O-(beta-D-glucosyl)-L-serine	=			+		[protein with EGF-like domain]-3-O-[alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-serine
	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