A new beta-1,2-N-acetylglucosaminyltransferase that may play a role in thebiosynthesis of mammalian O-mannosyl glycans

Recent studies have shown that O-mannosyl glycans ape present in several ma mmalian glycoproteins, Although knowledge on the functional roles of these glycans is accumulating, their biosynthetic pathways are poorly understood. Here we report the identification and initial characterization of a novel enzyme capable of forming GlcNAc beta1-2Man linkage, namely UDP-N-acetylglu cosamine: O-linked mannose beta -1,2-N-acetylglucosaminyltransferase in the microsome fraction of newborn rat brains. The enzyme transfers GlcNAc to b eta -linked mannose residues, and the formed linkage was confirmed to be be ta1-2 on the basis of diplococcal beta -N-acetylhexosaminidase susceptibili ty and by high-pH anion-exchange chromatography. Its activity is linearly d ependent on time, protein concentration, and substrate concentration and is enhanced in the presence of manganese ion. Its activity is not due to UDP- N-acetylglucosamine: alpha -3-D-mannoside beta -1,2-N-acetylglucosaminyltra nsferase I(GnT-I) or UDP-N-acetylglucosamine: alpha -6-D-mannoside beta -1, 2-D-acetylglucosaminyltransferase II (GnT-II), which acts on the early step s of N-glycan biosynthesis, because GnT-I or GnT-II expressed in yeast cell s did not show any GlcNAc transfer activity against a synthetic mannosyl pe ptide. Taken together, the results suggest that the GlcNAc transferase acti vity described here is relevant to the O-mannosyl glycan pathway in mammals .