The Widespread Effect of β1,4-Galactosyltransferase on N-Glycan Processing

We investigated β1,4-GalT (UDP-galactose: β-d-N-acetylglucosaminide β1,4-galactosyltransferase) in terms of intracellular competition with GnT-IV (UDP-N-acetylglucosamine: α1,3-d-mannoside β1,4-N-acetylglucosaminyltransferase) and GnT-V (UDP-N-acetylglucosamine: α1,6-d-mannoside β1,6-N-acetylglucosaminyltransferase). The β1,4-GalT-I gene was introduced into Chinese hamster ovary (CHO) cells producing human interferon (hIFN)-γ (IM4/V/IV cells) and five clones expressing various levels of β1,4-GalT were isolated. As we previously reported, parental IM4/V/IV cells express high levels of GnT-IVa and -V and produce hIFN-γ having primarily tetraantennary sugar chains. The branching of sugar chains on hIFN-γ was suppressed in the β1,4-GalT-enhanced clones to a level corresponding to the intracellular activity of β1,4-GalT relative to GnTs. Moreover, the contents of hybrid-type and high-mannose-type sugar chains increased in these clones. The results showed that β1,4-GalT widely affects N-glycan processing by competing with GnT-IV, GnT-V, and α-mannosidase II in cells and also by some other mechanisms that suppress the conversion of high-mannose-type sugar chains to the hybrid type.