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 (PubMed:15661757, PubMed:15752776, PubMed:21987822, PubMed:22223806, PubMed:23125099, PubMed:25279697, PubMed:25279699). Elongates the glucuronyl-beta-1,4- xylose-beta disaccharide primer structure initiated by B4GAT1 by adding repeating units [-3-Xylose-alpha-1,3-GlcA-beta-1-] to produce a heteropolysaccharide (PubMed:22223806, PubMed:23125099, PubMed:25138275, PubMed:25279697, PubMed:25279699, PubMed:32975514). Requires the phosphorylation of core M3 (O-mannosyl trisaccharide) by POMK to elongate the glucuronyl-beta-1,4-xylose-beta disaccharide primer (PubMed:21987822). Plays a key role in skeletal muscle function and regeneration (By similarity). {ECO:0000250|UniProtKB:Q9Z1M7, ECO:0000269|PubMed:15661757, ECO:0000269|PubMed:15752776, ECO:0000269|PubMed:21987822, ECO:0000269|PubMed:22223806, ECO:0000269|PubMed:23125099, ECO:0000269|PubMed:25138275, ECO:0000269|PubMed:25279697, ECO:0000269|PubMed:25279699, ECO:0000269|PubMed:32975514}. This is the function of LARGE1 (LARGE xylosyl- and glucuronyltransferase 1, ENSG00000133424).