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

• Published in: The Journal of biological chemistry Vol. 298; no. 9; p. 102400
• Main Authors: Osada, Naoko, Nagae, Masamichi, Nakano, Miyako, Hirata, Tetsuya, Kizuka, Yasuhiko
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.09.2022; American Society for Biochemistry and Molecular Biology
• Subjects: glycobiology / glycoprotein biosynthesis; glycosylation; glycosyltransferase; GnT-IV; substrate specificity; GnT-IV; glycobiology / glycoprotein biosynthesis; glycosylation; glycosyltransferase; substrate specificity
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/j.jbc.2022.102400

The N-glycans attached to proteins contain various N-acetylglucosamine (GlcNAc) branches, the aberrant formation of which correlates with various diseases. N-Acetylglucosaminyltransferase-IVa (GnT-IVa or MGAT4A) and -IVb (GnT-IVb or MGAT4B) are isoenzymes that catalyze the formation of the β1,4-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 GnT-IVb acted efficiently on glycoproteins bearing an N-glycan pre-modified by GnT-IV. To further understand the mechanism of this reaction, we focused on the non-catalytic C-terminal lectin domain, which selectively recognizes the product glycans. Replacement of a non-conserved amino acid in the GnT-IVb lectin domain with the corresponding residue in GnT-IVa altered the glycoprotein preference of GnT-IVb to resemble that of GnT-IVa. Our findings demonstrate that the C-terminal lectin domain regulates differential substrate selectivity of GnT-IVa and -IVb, highlighting a new mechanism by which N-glycan branches are formed on glycoproteins.