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) (PubMed:
17034757, PubMed:
25279699, PubMed:
26923585, PubMed:
29477842). 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 (PubMed:
17034757, PubMed:
25279699, PubMed:
26923585, PubMed:
29477842). Required for normal location of POMGNT1 in Golgi membranes, and for normal POMGNT1 activity (PubMed:
17034757). May interact with and reinforce a large complex encompassing the outside and inside of muscle membranes (PubMed:
25279699). Could be involved in brain development (Probable).