Carbohydrate-binding domain of the POMGnT1 stem region modulates O-mannosylation sites of α-dystroglycan

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 113; no. 33; pp. 9280 - 9285
• Main Authors: Kuwabara, Naoyuki, Manya, Hiroshi, Yamada, Takeyuki, Tateno, Hiroaki, Kanagawa, Motoi, Kobayashi, Kazuhiro, Akasaka-Manya, Keiko, Hirose, Yuriko, Mizuno, Mamoru, Ikeguchi, Mitsunori, Toda, Tatsushi, Hirabayashi, Jun, Senda, Toshiya, Endo, Tamao, Kato, Ryuichi
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 16.08.2016
• Subjects: Binding Sites; Biological Sciences; Crystallization; Dystroglycans - chemistry; Glycosylation; Humans; Mannose - chemistry; N-Acetylglucosaminyltransferases - chemistry; O-mannosyl glycan; muscular dystrophy; α-dystroglycanopathy; glycosyltransferase; lectin
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1525545113

The dystrophin glycoprotein complex, which connects the cell membrane to the basement membrane, is essential for a variety of biological events, including maintenance of muscle integrity. An O-mannose–type GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man structure of α-dystroglycan (α-DG), a subunit of the complex that is anchored to the cell membrane, interacts directly with laminin in the basement membrane. Reduced glycosylation of α-DG is linked to some types of inherited muscular dystrophy; consistent with this relationship, many disease-related mutations have been detected in genes involved in O-mannosyl glycan synthesis. Defects in protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), a glycosyltransferase that participates in the formation of GlcNAc-β1,2-Man glycan, are causally related to muscle-eye-brain disease (MEB), a congenital muscular dystrophy, although the role of POMGnT1 in postphosphoryl modification of GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan remains elusive. Our crystal structures of POMGnT1 agreed with our previous results showing that the catalytic domain recognizes substrate O-mannosylated proteins via hydrophobic interactions with little sequence specificity. Unexpectedly, we found that the stem domain recognizes the β-linked GlcNAc of O-mannosyl glycan, an enzymatic product of POMGnT1. This interaction may recruit POMGnT1 to a specific site of α-DG to promote GlcNAc-β1,2-Man clustering and also may recruit other enzymes that interact with POMGnT1, e.g., fukutin, which is required for further modification of the GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan. On the basis of our findings, we propose a mechanism for the deficiency in postphosphoryl modification of the glycan observed in POMGnT1-KO mice and MEB patients.