Bifunctional glycosyltransferase with both alpha-1,3-xylosyltransferase and beta-1,3-glucuronyltransferase activities involved in the maturation of alpha-dystroglycan (DAG1) by glycosylation leading to DAG1 binding to laminin G-like domain-containing extracellular proteins with high affinity and in a phosphorylated-O-mannosyl trisaccharide dependent manner (PubMed:15661757, PubMed:15752776, PubMed:25138275). Elongates the glucuronyl-beta-1,4-xylose-beta disaccharide primer structure by adding repeating units [-3-Xylose-alpha-1,3-GlcA-beta-1-] to produce a heteropolysaccharide (By similarity). Supports the maturation of DAG1 more effectively than LARGE1 (PubMed:15752776). In addition, can modify both heparan sulfate (HS)- and chondroitin/dermatan sulfate (CS/DS)-proteoglycans (PGs), namely GPC4, with a glycosaminoglycan (GAG)-like polysaccharide composed of xylose and glucuronic acid to confer laminin binding (By similarity).