In eukaryotic protein N-glycosylation, a series of glycosyltransferases catalyse the biosynthesis of a dolichylpyrophosphate-linked oligosaccharide before its transfer onto acceptor proteins1. The final seven steps occur in the lumen of the endoplasmic reticulum (ER) and require dolichylphosphate-activated mannose and glucose as donor substrates2. The responsible enzymesâALG3, ALG9, ALG12, ALG6, ALG8 and ALG10âare glycosyltransferases of the C-superfamily (GT-Cs), which are loosely defined as containing membrane-spanning helices and processing an isoprenoid-linked carbohydrate donor substrate3,4. Here we present the cryo-electron microscopy structure of yeast ALG6 at 3.0Â Ã
resolution, which reveals a previously undescribed transmembrane protein fold. Comparison with reported GT-C structures suggests that GT-C enzymes contain a modular architecture with a conserved module and a variable module, each with distinct functional roles. We used synthetic analogues of dolichylphosphate-linked and dolichylpyrophosphate-linked sugars and enzymatic glycan extension to generate donor and acceptor substrates using purified enzymes of the ALG pathway to recapitulate the activity of ALG6 in vitro. A second cryo-electron microscopy structure of ALG6 bound to an analogue of dolichylphosphate-glucose at 3.9Â Ã
resolution revealed the active site of the enzyme. Functional analysis of ALG6 variants identified a catalytic aspartate residue that probably acts as a general base. This residue is conserved in the GT-C superfamily. Our results define the architecture of ER-luminal GT-C enzymes and provide a structural basis for understanding their catalytic mechanisms. Analyses reveal a previously undescribed transmembrane protein fold in the endoplasmic reticulum-based glucosyltransferase ALG6 and provide a structural basis for understanding the glucose transfer mechanism.
在真核蛋白 N-糖基化中,一系列糖基转移酶在转移到受体蛋白上之前催化多
乙醇焦磷酸连接的
寡糖的
生物合成。最后七个步骤发生在内质网 (ER) 的腔中,需要多
乙醇磷酸酯激活的
甘露糖和
葡萄糖作为供体底物2。负责的酶——ALG3、ALG9、ALG12、ALG6、ALG8 和 ALG10——是 C 超家族 (GT-Cs) 的糖基转移酶,其被粗略地定义为含有跨膜螺旋并处理类
异戊二烯连接的
碳水化合物供体底物3 ,4.在这里,我们展示了酵母 ALG6 分辨率为 3.0 × 的冷冻电子显微镜结构,揭示了以前未描述的跨膜蛋白折叠。与已报道的 GT-C 结构进行比较表明,GT-C 酶包含具有保守模块和可变模块的模块化结构,每个模块都具有不同的功能作用。我们使用长效
磷酸酯连接糖和长效醇
焦磷酸酯连接糖的合成类似物以及酶促聚糖延伸来生成供体和受体底物,并使用 ALG 途径的纯化酶在体外重现 ALG6 的活性。 ALG6 与多醇
磷酸-葡萄
糖类似物结合的第二个冷冻电子显微镜结构以 3.9 × 分辨率显示了该酶的活性位点。 ALG6 变体的功能分析鉴定出可能充当通用碱基的催化
天冬氨酸残基。该残基在 GT-C 超家族中是保守的。我们的结果定义了 ER-luminal GT-C 酶的结构,并为理解其催化机制提供了结构基础。分析揭示了基于内质网的
葡萄糖基转移酶 ALG6 中先前未描述的跨膜蛋白折叠,并为理解
葡萄糖转移机制提供了结构基础。