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:23125099, PubMed:23135544, PubMed:25138275, PubMed:32975514). 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 (By similarity). Requires the phosphorylation of core M3 (O-mannosyl trisaccharide) by POMK to elongate the glucuronyl-beta-1,4-xylose-beta disaccharide primer (PubMed:32975514). Plays a key role in skeletal muscle function and regeneration (PubMed:15184894, PubMed:24132234). 3-O-[beta-D-GlcA-(1->3)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-(O-6-P-alpha-D-Man)]-Thr-[protein] + UDP-alpha-D-xylose = 3-O-[alpha-D-Xyl-(1->3)-beta-D-GlcA-(1->4)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-(O-6-P-alpha-D-Man)]-Thr-[protein] + H(+) + UDP 3-O-{(1->[3)-alpha-D-Xyl-(1->3)-beta-D-GlcA-(1->](n)-4)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-O-6-P-alpha-D-Man}-L-Thr-[protein] + UDP-alpha-D-glucuronate = 3-O-{beta-D-GlcA-(1->[3)-alpha-D-Xyl-(1->3)-beta-D-GlcA-(1->](n)-4)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-O-6-P-alpha-D-Man}-L-Thr-[protein] + H(+) + UDP 3-O-{beta-D-GlcA-(1->[3)-alpha-D-Xyl-(1->3)-beta-D-GlcA-(1->](n)-4)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-O-6-P-alpha-D-Man}-L-Thr-[protein] + UDP-alpha-D-xylose = 3-O-{(1->[3)-alpha-D-Xyl-(1->3)-beta-D-GlcA-(1->](n+1)-4)-beta-D-Xyl-(1->4)-Rib-ol-P-Rib-ol-P-3-beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-O-6-P-alpha-D-Man}-L-Thr-[protein] + H(+) + UDP Binds 2 Mn(2+) ions per subunit. The xylosyltransferase part binds one Mn(2+) and the beta-1,3-glucuronyltransferase part binds one Mn(2+). Protein modification; protein glycosylation. Interacts with DAG1 (via the N-terminal domain of alpha-DAG1); the interaction increases binding of DAG1 to laminin (PubMed:15210115). Interacts with B4GAT1. Ubiquitous. Highest expression in heart, diaphragm and brain, where it is especially found in cerebral cortex, hippocampus, and trigeminal ganglion. Ubiquitously found at 14.5 dpc with strong expression in heart, central nervous system structures such as cerebral cortex, hippocampus, olfactory lobe, trigeminal ganglion and spinal cord. Also expressed in diaphragm and duodenum. Defects in Large are the cause of myodystrophy (myd), an autosomal recessive neuromuscular phenotype, probably due to abnormal post-translational modification of alpha-dystroglycan. In the C-terminal section; belongs to the glycosyltransferase 49 family. In the N-terminal section; belongs to the glycosyltransferase 8 family. Truncated N-terminus. Large like-glycosyltransferase