Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
Department for Translational Neuroscience, Brain Center Rudolf Magnus, University Medical Center Utrecht, Universiteitsweg 100, 3584 CG Utrecht, The Netherlands.
Nat Commun. 2016 Dec 6;7:13584. doi: 10.1038/ncomms13584.
Myelin-associated glycoprotein (MAG) is a myelin-expressed cell-adhesion and bi-directional signalling molecule. MAG maintains the myelin-axon spacing by interacting with specific neuronal glycolipids (gangliosides), inhibits axon regeneration and controls myelin formation. The mechanisms underlying MAG adhesion and signalling are unresolved. We present crystal structures of the MAG full ectodomain, which reveal an extended conformation of five Ig domains and a homodimeric arrangement involving membrane-proximal domains Ig4 and Ig5. MAG-oligosaccharide complex structures and biophysical assays show how MAG engages axonal gangliosides at domain Ig1. Two post-translational modifications were identified-N-linked glycosylation at the dimerization interface and tryptophan C-mannosylation proximal to the ganglioside binding site-that appear to have regulatory functions. Structure-guided mutations and neurite outgrowth assays demonstrate MAG dimerization and carbohydrate recognition are essential for its regeneration-inhibiting properties. The combination of trans ganglioside binding and cis homodimerization explains how MAG maintains the myelin-axon spacing and provides a mechanism for MAG-mediated bi-directional signalling.
髓鞘相关糖蛋白（MAG）是一种在髓鞘上表达的细胞黏附分子和双向信号分子。MAG 通过与特定的神经元糖脂（神经节苷脂）相互作用，保持髓鞘-轴突的间距，抑制轴突再生并控制髓鞘形成。MAG 黏附和信号转导的机制尚未解决。我们呈现了 MAG 全长外显子的晶体结构，揭示了五个 Ig 结构域的延伸构象和涉及膜近端结构域 Ig4 和 Ig5 的同源二聚体排列。MAG-寡糖复合物结构和生物物理测定表明了 MAG 如何在 Ig1 结构域与轴突神经节苷脂结合。鉴定出两种翻译后修饰 - 二聚化界面上的 N 连接糖基化和靠近神经节苷脂结合位点的色氨酸 C-甘露糖基化 - 它们似乎具有调节功能。结构导向突变和神经突生长测定表明 MAG 二聚化和碳水化合物识别对于其抑制再生的特性至关重要。跨神经节苷脂结合和顺式同源二聚化的组合解释了 MAG 如何维持髓鞘-轴突的间距，并为 MAG 介导的双向信号转导提供了一种机制。
