Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8530, Japan.
Glycoscience Group, National University of Ireland, Galway, Ireland; Structural Glycobiology Team, RIKEN-Max Planck Joint Research Center, Global Research Cluster, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Mol Cell Proteomics. 2019 Oct;18(10):2044-2057. doi: 10.1074/mcp.RA119.001534. Epub 2019 Aug 2.
Glycoproteins are decorated with complex glycans for protein functions. However, regulation mechanisms of complex glycan biosynthesis are largely unclear. Here we found that bisecting GlcNAc, a branching sugar residue in -glycan, suppresses the biosynthesis of various types of terminal epitopes in -glycans, including fucose, sialic acid and human natural killer-1. Expression of these epitopes in -glycan was elevated in mice lacking the biosynthetic enzyme of bisecting GlcNAc, GnT-III, and was conversely suppressed by GnT-III overexpression in cells. Many glycosyltransferases for -glycan terminals were revealed to prefer a nonbisected -glycan as a substrate to its bisected counterpart, whereas no up-regulation of their mRNAs was found. This indicates that the elevated expression of the terminal -glycan epitopes in GnT-III-deficient mice is attributed to the substrate specificity of the biosynthetic enzymes. Molecular dynamics simulations further confirmed that nonbisected glycans were preferentially accepted by those glycosyltransferases. These findings unveil a new regulation mechanism of protein -glycosylation.
糖蛋白通过复杂的聚糖进行修饰，以发挥其蛋白质功能。然而，复杂聚糖生物合成的调控机制在很大程度上尚不清楚。在这里，我们发现双分支 GlcNAc（一种 - 聚糖中的分支糖残基）抑制了 - 聚糖中各种末端表位（包括岩藻糖、唾液酸和人自然杀伤细胞-1 配体）的生物合成。缺乏双分支 GlcNAc 生物合成酶 GnT-III 的小鼠中 - 聚糖上这些表位的表达升高，而细胞中 GnT-III 的过表达则相反地抑制了它们的表达。许多 - 聚糖末端的糖基转移酶被揭示优先以非双分支 - 聚糖作为其双分支对应物的底物，而其 mRNA 并未上调。这表明 GnT-III 缺陷小鼠中末端 - 聚糖表位的高表达归因于生物合成酶的底物特异性。分子动力学模拟进一步证实，非双分支聚糖更优先被这些糖基转移酶接受。这些发现揭示了一种新的蛋白质糖基化调控机制。
