Catalyzes the transfer of a ribitol-phosphate from CDP-ribitol to the distal N-acetylgalactosamine of the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan (DAG1) ( ). This constitutes the first step in the formation of the ribitol 5-phosphate tandem repeat which links the phosphorylated O-mannosyl trisaccharide to the ligand binding moiety composed of repeats of 3-xylosyl-alpha-1,3-glucuronic acid-beta-1 (  ). Required for normal location of POMGNT1 in Golgi membranes, and for normal POMGNT1 activity . May interact with and reinforce a large complex encompassing the outside and inside of muscle membranes . Could be involved in brain development (Probable).
Subcellular locations: Golgi apparatus membrane, Cytoplasm, Nucleus
In retinal tissue, does not localize with the Golgi apparatus.
Expressed in the retina (at protein level) . Widely expressed with highest expression in brain, heart, pancreas and skeletal muscle . Expressed at similar levels in control fetal and adult brain . Expressed in migrating neurons, including Cajar-Retzius cells and adult cortical neurons, as well as hippocampal pyramidal cells and cerebellar Purkinje cells . No expression observed in the glia limitans, the subpial astrocytes (which contribute to basement membrane formation) or other glial cells .