Reaction catalyzed by galactosylxylosylprotein 3-beta-galactosyltransferase (2.4.1.134)

		+			=			+
	UDP-alpha-D-galactose	+		[protein]-3-O-(beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-serine	=		UDP	+		[protein]-3-O-(beta-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-serine