Examination of differential glycoprotein preferences of N-acetylglucosaminyltransferase-IV isozymes a and b

The N-glycans attached to proteins contain various GlcNAc branches, the aberrant formation of which correlates with various diseases. N-Acetylglucosaminyltransferase-IVa (GnT-IVa or MGAT4A) and Gnt-IVb (or MGAT4B) are isoenzymes that catalyze the formation of the beta 1,4-GlcNAc branch in N-glycans. However, the functional differences between these isozymes remain unresolved. Here, using cellular and UDP-Glo enzyme assays, we discovered that GnT-IVa and GnT-IVb have distinct glycoprotein preferences both in cells and in vitro. Notably, we show that GnT-IVb acted efficiently on glycopro-teins bearing an N-glycan premodified by GnT-IV. To further understand the mechanism of this reaction, we focused on the noncatalytic C-terminal lectin domain, which selectively rec-ognizes the product glycans. Replacement of a nonconserved amino acid in the GnT-IVb lectin domain with the corre-sponding residue in GnT-IVa altered the glycoprotein prefer-ence of GnT-IVb to resemble that of GnT-IVa. Our findings demonstrate that the C-terminal lectin domain regulates dif-ferential substrate selectivity of GnT-IVa and GnT-IVb, high-lighting a new mechanism by which N-glycan branches are formed on glycoproteins.

Automated summary

In this study, the authors found that GnT-IVa and GnT-IVb have different preferences for glycoprotein substrates in both cells and in vitro. They discovered that the noncatalytic C-terminal lectin domain regulates the substrate selectivity of these enzymes, providing a new mechanism for the formation of N-glycan branches on glycoproteins.