It is involved in the regulation of the biosynthesis and biological function of glycoprotein oligosaccharides. Catalyzes the addition of N-acetylglucosamine in beta 1-4 linkage to the beta-linked mannose of the trimannosyl core of N-linked sugar chains, called bisecting N-acetylglucosamine (GlcNAc). It is one of the most important enzymes involved in the regulation of the biosynthesis of glycoprotein oligosaccharides (PubMed:25592972, PubMed:11986323). The addition of this bisecting GlcNAc residue alters not only the composition, but also the conformation of the N-glycan. The introduction of the bisecting GlcNAc residue results in the suppression of further processing and elongation of N-glycans, precluding the formation of beta-1,6 GlcNAc branching, catalyzed by MGAT5 since it is unable to use the bisected oligosaccharide as a substrate (By similarity). Addition of bisecting N-acetylglucosamine to CDH1/E-cadherin modulates CDH1 cell membrane location. Inhibits NeuAc-alpha-2,3-Gal-beta-1,4-GlcNAc- formation which modulates sialylation levels and plays a role in cell migration regulation (By similarity). In brain, addition of bisecting N-acetylglucosamine to BACE1 blocks its lysosomal targeting in response to oxidative stress and further degradation which increases its location to early endosome and the APP cleavage (PubMed:25592972, PubMed:26467158). N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[beta-D-GlcNAc-(1->4)]-[beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] + UDP Protein modification; protein glycosylation. Interacts with MGAT4D. Highly expressed in brain and kidney and to a much lesser extent in stomach, heart, intestine, uterus, testis, ovary and lung. Not present in spleen, liver and muscle. In brain, expressed in neurons of hippocampus (PubMed:11986323). Expressed in brain at 10.5 dpc and thereafter. Expression is up-regulated by CDH1/E-cadherin-mediated cell-cell interaction. Knockout mice are generally healthy, fertile, and behaviorally normal (PubMed:20805325). They have altered leg clasp reflex (PubMed:11986323). They show a lack of N-glycan species with an additional bisecting N-acetylglucosamine (PubMed:11986323). Knockout mice crossed with Alzheimer disease model mice have significantly lower levels of the beta-C-terminal fragment of APP (betaCTF) and soluble APP cleaved at the beta-site (sAPPbeta) in their brains than Alzheimer disease model mice. They have markedly decreased the number of amyloid-beta plaques and ameliorated cognitive function (PubMed:20805325). Belongs to the glycosyltransferase 17 family. Truncated N-terminus. beta-1,4-mannosyl-glycoprotein beta 1,4-GlcNAc T III (Mgat3)