Beta-1,4-glucuronyltransferase involved in O-mannosylation of alpha-dystroglycan (DAG1) (PubMed:19587235, PubMed:23359570, PubMed:25279699, PubMed:25279697). 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 LARGE1, during synthesis of phosphorylated O-mannosyl glycan (PubMed:25279699, PubMed:25279697). 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 (PubMed:25279699, PubMed:25279697). 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 Optimum pH is 7.0. Protein modification; protein glycosylation. Interacts with LARGE1 and LARGE2. Localizes near the trans-Golgi apparatus. In the adult, highly expressed in heart, brain, skeletal muscle and kidney and to a lesser extent in placenta, pancreas, spleen, prostate, testis, ovary, small intestine and colon. Very weak expression in lung, liver, thymus and peripheral blood leukocytes. In fetal highly expressed in brain and kidney and to a lesser extent in lung and liver. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase 49 family. Was initially characterized as a beta-1,3-N-acetylglucosaminyltransferase involved in the synthesis of poly-N-acetyllactosamine, able to initiate the synthesis or the elongation of the linear poly-N-acetyllactosaminoglycans (PubMed:9405606). However, it was later shown that it acts as a beta-1,4-glucuronyltransferase (PubMed:25279699, PubMed:25279697). N-acetyllactosaminide beta-1,3-N-acetylglucosaminyltransferase