Beta-1,4-glucuronyltransferase involved in O-mannosylation of alpha-dystroglycan (DAG1) (PubMed:23359570). Transfers a glucuronic acid (GlcA) residue onto a xylose (Xyl) acceptor to produce the glucuronyl-beta-1,4-xylose-beta disaccharide primer, which is further elongated by LARGE, during synthesis of phosphorylated O-mannosyl glycan (By similarity). Phosphorylated O-mannosyl glycan is a carbohydrate structure present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity (By similarity). Required for axon guidance; via its function in O-mannosylation of alpha-dystroglycan (DAG1) (By similarity). 3-O-[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-glucuronate = 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] + H(+) + UDP Protein modification; protein glycosylation. Localizes near the trans-Golgi apparatus. Muscular defects and reduced alpha-dystroglycan (dag1) glycosylation. Belongs to the glycosyltransferase 49 family.