Glycosyltransferase that acts on alpha-linked mannose of N-glycans and O-mannosyl glycans. Catalyzes the transfer of N-acetylglucosamine (GlcNAc) to the beta 1-6 linkage of the mannose residue of GlcNAc-beta1,2-Man-alpha on both the alpha1,3- and alpha1,6-linked mannose arms in the core structure of N-glycan. Also acts on the GlcNAc-beta1,2-Man-alpha1-Ser/Thr moiety, forming a 2,6-branched structure in brain O-mannosyl glycan. Plays an active role in modulating integrin and laminin-dependent adhesion and migration of neuronal cells via its activity in the O-mannosyl glycan pathway. N(4)-{beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->4)]-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAcl-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-{beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->4)]-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->2)-[beta-D-GlcNAc-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAcl-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP 3-O-[N-acetyl-beta-D-glucosaminyl-(1->2)-alpha-D-mannosyl]-L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + O(3)-{N-acetyl-beta-D-glucosaminyl-(1->2)-[N-acetyl-beta-D-glucosaminyl-(1->6)]-alpha-D-mannosyl}-L-seryl-[protein] + UDP 3-O-[N-acetyl-beta-D-glucosaminyl-(1->2)-alpha-D-mannosyl]-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + O(3)-{N-acetyl-beta-D-glucosaminyl-(1->2)-[N-acetyl-beta-D-glucosaminyl-(1->6)]-alpha-D-mannosyl}-L-threonyl-[protein] + UDP Optimum pH is 8.0. Protein modification; protein glycosylation. Predominantly expressed in brain. Expressed in all areas of the adult and fetal brain. Also expressed at much lower levels in testis, spleen and thymus. Belongs to the glycosyltransferase 18 family. Probable cloning artifact. Probable cloning artifact. Chimeric sequence. Truncated N-terminus. Extended N-terminus.