Catalyzes the transfer of 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) (By similarity). 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 (By similarity). May interact with and reinforce a large complex encompassing the outside and inside of muscle membranes. Could be involved in brain development (By similarity).
Subcellular locations: Golgi apparatus membrane, Cytoplasm, Nucleus
In retinal tissue, does not localize with the Golgi apparatus.
Expressed in the retina (at protein level).