The Golgi apparatus, the main glycosylation station of the cell, consists of a stack of discontinuous cisternae. Glycosylation enzymes are usually concentrated in one or two specific cisternae along the cis-trans axis of the organelle. How such compartmentalized localization of enzymes is achieved and how it contributes to glycosylation are not clear. Here, we show that the Golgi matrix protein GRASP55 directs the compartmentalized localization of key enzymes involved in glycosphingolipid (GSL) biosynthesis. GRASP55 binds to these enzymes and prevents their entry into COPI-based retrograde transport vesicles, thus concentrating them in the trans-Golgi. In genome-edited cells lacking GRASP55, or in cells expressing mutant enzymes without GRASP55 binding sites, these enzymes relocate to the cis-Golgi, which affects glycosphingolipid biosynthesis by changing flux across metabolic branch points. These findings reveal a mechanism by which a matrix protein regulates polarized localization of glycosylation enzymes in the Golgi and controls competition in glycan biosynthesis.

中文翻译：

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GRASP55 调节糖基化酶的高尔基体内定位以控制糖鞘脂生物合成

高尔基体是细胞的主要糖基化站，由一堆不连续的池组成。糖基化酶通常集中在沿细胞器顺反轴的一个或两个特定池中。酶的这种区室化定位是如何实现的以及它如何促进糖基化尚不清楚。在这里，我们表明高尔基体基质蛋白 GRASP55 指导参与糖鞘脂 (GSL) 生物合成的关键酶的区室化定位。GRASP55 与这些酶结合并阻止它们进入基于 COPI 的逆行转运囊泡，从而将它们集中在反式中-高尔基体。在缺乏 GRASP55 的基因组编辑细胞中，或在表达没有 GRASP55 结合位点的突变酶的细胞中，这些酶重新定位到顺式高尔基体，它通过改变代谢分支点之间的通量来影响糖鞘脂生物合成。这些发现揭示了一种机制，基质蛋白通过该机制调节高尔基体中糖基化酶的极化定位并控制聚糖生物合成中的竞争。