Participates in O-mannosyl glycosylation by catalyzing the addition of N-acetylglucosamine to O-linked mannose on glycoproteins. Catalyzes the synthesis of the GlcNAc(beta1-2)Man(alpha1-)O-Ser/Thr moiety on alpha-dystroglycan and other O-mannosylated proteins, providing the necessary basis for the addition of further carbohydrate moieties. Is specific for alpha linked terminal mannose. 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl-(1->2)-alpha-D-mannosyl)-L-threonyl-[protein] + H(+) + UDP The manganese ion interacts primarily with the substrate UDP-N-acetylglucosamine. Protein modification; protein glycosylation. Interacts with DAG1 (via O-linked mannose moiety). Interacts (via transmembrane domain) with FKTN; the interaction is direct and is required for normal location in Golgi membranes. Expressed at basal body and daughter centriole of photoreceptor cells (at protein level). Broadly expressed in late embryonic and early postnatal cerebellar neurons, including premigratory granule neurons of the external granule cell layer. Expression is maintained in neurons of the internal granule cell layer after migration is complete. Expressed in Purkinje cells throughout development. Expressed in Bergmann glial scaffolds used by granule cells during early posnatal radial migration. The stem domain mediates specific interaction with beta-linked N-acetylglucosamine moieties of O-glycosylated proteins. It also interacts with its product, N-acetyl-beta-D-glucosaminyl-(1->2)-O-alpha-D-mannosylprotein. Belongs to the glycosyltransferase 13 family. POMGnT1