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. 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.