Initiates complex N-linked carbohydrate formation. Essential for the conversion of high-mannose to hybrid and complex N-glycans. N(4)-(alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc)-L-asparaginyl-[protein] (N-glucan mannose isomer 5A1,2) + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP The cofactor is mostly bound to the substrate. Protein modification; protein glycosylation. Interacts with MGAT4D. Interacts with BRI3 (isoforms 1 and 2); the interaction with isoform 2 is weaker than with isoform 1 (PubMed:30983867). Co-localizes with BRI3 isoform 1 at the perinuclear region. Belongs to the glycosyltransferase 13 family. Alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase