A new β1,4-N-acetylglucosaminyltransferase (GnT) responsible for the formation of branched N-linked complex-type sugar chains has been purified 64,000-fold in 16% yield from a homogenate of hen oviduct by column chromatography procedures using Q-Sepharose FF, Ni2+-chelating Sepharose FF, and UDP-hexanolamineagarose. This enzyme catalyzes the transfer of GlcNAc from UDP-GlcNAc to tetraantennary oligosaccharide and produces pentaantennary oligosaccharide with the β1-4-linked GlcNAc residue on the Manα1-6 arm. It requires a divalent cation such as Mn2+ and has an apparent molecular weight of 72,000 under nonreducing conditions. The enzyme does not act on biantennary oligosaccharide (GnT I and II product), and β1,6-N-acetylglucosaminylation of the Manα1-6 arm (GnT V product) is essential for its activity. This clearly distinguishes it from GnT IV, which is known to generate a β1-4-linked GlcNAc residue only on the Manα1-3 arm. Based on these findings, we conclude that this enzyme is UDP-GlcNAc:GlcNAcβ1-6(GlcNAcβ1-2)Manα1-R [Glc-NAc to Man]-β1,4-N-acetylglucosaminyltransferase VI. This is the only known enzyme that has not been previously purified among GnTs responsible for antenna formation on the cores of N-linked complex-type sugar chains.
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