Structure and function of the ubiquitin ligase that recognizes N-linked oligosaccharides

内质网中蛋白质的N-糖基化在蛋白质质量控制中起着关键作用。在这里，我们报告，N-聚糖作为一个降解信号的新SCF β-<Fbx2>泛素连接酶复合物在胞质溶胶中。Fbx 2是一种F-box蛋白，与N-连接的高甘露糖型寡糖连接的蛋白质特异性结合，随后有助于N-糖基化蛋白的泛素化。我们将前整合素β1鉴定为Fbx 2靶点之一。当蛋白酶体被抑制时，在胞质溶胶中检测到这种相互作用。此外，缺乏形成SCF复合物所必需的F-box结构域的Fbx 2 ΔF的表达可明显阻断ER相关降解的典型底物的降解，例如囊性纤维化跨膜传导调节因子突变体CFTRΔF508和T细胞受体a亚基，并且Fbx 2抑制CFTRΔF508的聚集。这些结果表明，SCF<Fbx2>-2使N-糖基化蛋白泛素化，所述N-糖基化蛋白通过质量控制机制从ER易位到胞质溶胶。

N-glycosylation of proteins in the endoplasmic reticulum (ER) plays a key role in the protein quality control. Here we report that N-glycan serves as a degradation signal by the novel SCF^<Fbx2> ubiquitin-ligase complex in the cytosol. Fbx2,a F-box protein, binds specifically to proteins attached with N-linked high mannose type oligosaccharides, and subsequently contributes to ubiquitylation of N-glycosylated proteins. We identified pre-integrin β1 as one of the Fbx2 targets. This interaction was detected in the cytosol when the proteasome was inhibited. Furthermore, expression of Fbx2ΔF lacking the F-box domain essential for the formation of the SCF complex appreciably blocked degradation of typical substrates of ER-associated degradation, such as cystic fibrosis transmembrane conductance regulator mutant CFTRΔF508 and T-cell receptor a subunit, and Fbx2 suppressed aggregation of CFTRΔF508. These results indicate that SCF^<Fbx2> ubiquitylates N-glycosylated proteins, which are translocated from the ER to the cytosol by the quality control mechanism.