Participates in O-mannosyl glycosylation by catalyzing the addition of N-acetylglucosamine to O-linked mannose on glycoproteins (PubMed:11709191, PubMed:27493216, PubMed:28512129). 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 (PubMed:11709191, PubMed:27493216). Is specific for alpha linked terminal mannose and does not have MGAT3, MGAT4, MGAT5, MGAT7 or MGAT8 activity. 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. Optimum pH is 6.0. Protein modification; protein glycosylation. Interacts with DAG1 (via O-linked mannose moiety) (PubMed:27493216). Interacts (via transmembrane domain) with FKTN; the interaction is direct and is required for normal location in Golgi membranes (PubMed:17034757). Constitutively expressed. An additional weaker band is also detected in spinal cord, lymph node, and trachea. Expressed especially in astrocytes. Also expressed in immature and mature neurons. Amino acid residues between 299-311 are important for both protein expression and enzymatic activity. The minimal catalytic domain is located between positions 299-651. Single amino acid substitutions in the stem domain from MEB patients abolished the activity of the membrane-bound form but not the soluble form. This suggests that the stem domain of the soluble form is unnecessary for activity, but that some amino acids play a crucial role in the membrane-bound form. 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. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase 13 family. Truncated N-terminus. Protein O-linked-mannose beta-1,2-N-acetylglucosaminyltransferase 1