A new β-1,2-N-acetylglucosaminyltransferase that may play a role in the biosynthesis of mammalian O-mannosyl glycans

Abstract

Recent studies have shown that O-mannosyl glycans are present in several mammalian 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 GlcNAcβ1-2Man linkage, namely UDP-N-acetyl-glucosamine: O-linked mannose β-1,2-N-acetylglucosaminyl-transferase in the microsome fraction of newborn rat brains. The enzyme transfers GlcNAc to β-linked mannose residues, and the formed linkage was confirmed to be β1-2 on the basis of diplococcal β-N-acetylhexosaminidase susceptibility and by high-pH anion-exchange chromatography. Its activity is linearly dependent 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: α-3-D-mannoside β-1,2-N-acetylglucosaminyl- transferase I (GnT-I) or UDP-N-acetylglucosamine: α-6-D-mannoside β-1,2-D-acetylglucosaminyltransferase II (GnT-II), which acts on the early steps of N-glycan biosynthesis, because GnT-I or GnT-II expressed in yeast cells did not show any GlcNAc transfer activity against a synthetic mannosyl peptide. Taken together, the results suggest that the GlcNAc transferase activity described here is relevant to the O-mannosyl glycan pathway in mammals.