Self-regulation of MGAT4A and MGAT4B activity toward glycoproteins through interaction of lectin domain with their own N-glycans

<i>N</i>-Acetylglucosaminyltransferases-IVa (GnT-IVa or MGAT4A) and -IVb (MGAT4B) are glycosyltransferase isozymes synthesizing the β1,4-GlcNAc branch in <i>N</i>-glycans, a glycan structure involved in diabetes. These enzymes uniquely have a non-catalytic lectin domain, which selectively recognizes the GnT-IV product <i>N</i>-glycan branch, but the role of this lectin domain has remained unclear. Here, using UDP-Glo enzyme assays, we discovered that this domain is required for activity toward glycoprotein substrates but not toward free glycans. Furthermore, we found that the lectin domain itself is decorated with an <i>N</i>-glycan, which can serve as a self-ligand and interact with the ligand binding site of the lectin domain in a glycan structure-dependent manner. Enzyme assays using glycan-remodeled GnT-IVa demonstrated that the interaction of the self-ligand with the lectin domain suppresses GnT-IVa activity toward glycoprotein substrates. These findings unveiled a lectin-assisted self-regulatory mechanism of glycosyltransferases, which deepens our understanding of the complex pathway of <i>N</i>-glycan biosynthesis.