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

内质网蛋白质的N-糖基化在蛋白质质量控制中起着关键作用。在这里，我们报道了N-葡聚糖作为降解信号，通过胞浆中新的SCF^&lt；Fbx2&gt；泛素连接酶复合体。Fbx2是一种F-box蛋白，它与N-连接的高甘露糖型低聚糖结合，进而促进N-糖基化蛋白的泛素化。我们确定前整合素β1是Fbx2的靶标之一。当蛋白酶体被抑制时，这种相互作用在胞浆中被检测到。此外，缺乏形成Δ复合体所必需的F-盒结构域的Fbx2SCF的表达明显阻止了典型的ER相关降解底物的降解，例如囊性纤维化跨膜电导调节因子突变体ΔF508和T细胞受体α亚单位，以及Fbx2抑制了ΔF508的聚集。这些结果表明，SCF^&lt；Fbx2&gt；泛素化的N-糖基化蛋白通过质量控制机制从内质网转移到胞浆。

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.