The protein folding and lipid moiety status of glycosylphosphatidylinositol-anchored proteins (GPI-APs) are monitored in the endoplasmic reticulum (ER), with calnexin playing dual roles in the maturation of GPI-APs. In the present study, we investigated the functions of calnexin in the quality control and lipid remodeling of GPI-APs in the ER. By directly binding the N-glycan on proteins, calnexin was observed to efficiently retain GPI-APs in the ER until they were correctly folded. In addition, sufficient ER retention time was crucial for GPI-inositol deacylation, which is mediated by post-GPI attachment protein 1 (PGAP1). Once the calnexin/calreticulin cycle was disrupted, misfolded and inositol-acylated GPI-APs could not be retained in the ER and were exposed on the plasma membrane. In calnexin/calreticulin-deficient cells, endogenous GPI-anchored alkaline phosphatase was expressed on the cell surface, but its activity was significantly decreased. ER stress induced surface expression of misfolded GPI-APs, but proper GPI-inositol deacylation occurred due to the extended time that they were retained in the ER. Our results indicate that calnexin-mediated ER quality control systems for GPI-APs are necessary for both protein folding and GPI-inositol deacylation.

中文翻译：

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Calnexin 通过 ER 保留介导 GPI 锚的成熟。

在内质网 (ER) 中监测糖基磷脂酰肌醇锚定蛋白 (GPI-APs) 的蛋白质折叠和脂质部分状态，钙连接蛋白在 GPI-APs 的成熟中起双重作用。在本研究中，我们研究了钙连接蛋白在 ER 中 GPI-AP 的质量控制和脂质重塑中的功能。通过直接结合蛋白质上的 N-聚糖，观察到钙连接蛋白可以有效地将 GPI-AP 保留在 ER 中，直到它们正确折叠。此外，足够的 ER 保留时间对于 GPI-肌醇脱酰化至关重要，这是由 GPI 后附着蛋白 1 (PGAP1) 介导的。一旦钙连接蛋白/钙网蛋白循环被破坏，错误折叠和肌醇酰化的 GPI-AP 就不能保留在 ER 中并暴露在质膜上。在钙联蛋白/钙网蛋白缺陷细胞中，内源性 GPI 锚定碱性磷酸酶在细胞表面表达，但其活性显着降低。ER 应力诱导错误折叠的 GPI-APs 的表面表达，但由于它们在 ER 中保留的时间延长，因此发生了适当的 GPI-肌醇脱酰。我们的结果表明，钙连接蛋白介导的 GPI-AP 的 ER 质量控制系统对于蛋白质折叠和 GPI-肌醇脱酰都是必要的。