acetylglucosaminyltransferase activity acetylgalactosaminyl-O-glycosyl-glycoprotein beta-1,3-N-acetylglucosaminyltransferase activity https://github.com/geneontology/go-ontology/issues/27859 https://github.com/geneontology/go-ontology/issues/30193 mucin core 3 beta3-GlcNAc-transferase activity O-glycosyl-oligosaccharide-glycoprotein N-acetylglucosaminyltransferase III activity Catalysis of the reaction: 3-O-N-acetyl-D-galactosalaminyl-[protein] + UDP-N-acetyl-D-glucosamine = 3-O-N-acetyl-beta-D-glucosaminyl-1,3-N-acetyl-D-galactosaminyl-[protein] + UDP. Linkage of the glycan to the protein occurs via the oxygen atom in the side chain of an L-serine or L-threonine residue. Reactome:R-HSA-914010 GO:0047224 RHEA:46880 EC:2.4.1.147 RHEA:46884 MetaCyc:2.4.1.147-RXN UDP-N-acetyl-D-glucosamine:O-glycosyl-glycoprotein (N-acetyl-D-glucosamine to N-acetyl-D-galactosaminyl-R) beta-1,3-N-acetyl-D-glucosaminyltransferase activity core 3beta-GlcNAc-transferase activity molecular_function uridine diphosphoacetylglucosamine-mucin beta(1->3)-acetylglucosaminyltransferase activity core 3-beta-GlcNAc-transferase activity catalytic activity, acting on a glycoprotein