Regulation of human GnT-IV family activity by the lectin domain.

N-Glycan branching critically regulates glycoprotein functions and is involved in various diseases. Among the glycosyltransferases involved in N-glycan branching is the human N-acetylglucosaminyltransferase-IV (GnT-IV) family, which has four members: GnT-IVa, GnT-IVb, GnT-IVc, and GnT-IVd. GnT-IVa and GnT-IVb have glycosyltransferase activity that generates the type-2 diabetes-related β1,4-GlcNAc branch on the α1,3-Man arm of N-glycans, whereas GnT-IVc and GnT-IVd do not. Recently, this enzyme family was found to have a unique lectin domain in the C-terminal region, which is essential for enzyme activity toward glycoprotein substrates but not toward free N-glycans. Furthermore, interaction between the lectin domain of GnT-IV and N-glycan attached to GnT-IV enables self-regulation of GnT-IV activity, indicating that the lectin domain plays a unique and pivotal role in the regulation of GnT-IV activity. In this review, we summarize the GnT-IV family's biological functions, selectivity for glycoprotein substrates, and regulation of enzymatic activity, with a focus on its unique C-terminal lectin domain.