Yu H, Kaung G, Kobayashi S, Kopito R R
Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA.
J Biol Chem. 1997 Aug 15;272(33):20800-4. doi: 10.1074/jbc.272.33.20800.
The T-cell antigen receptor (TCR) is an hetero-oligomeric membrane complex composed of at least seven transmembrane polypeptide chains that has served as a model for the assembly and degradation of integral membrane proteins in the endoplasmic reticulum (ER). Unassembled TCRalpha chains fail to mature to the Golgi apparatus and are rapidly degraded by a non-lysosomal "ER degradation" pathway that has been proposed to be autonomous to the ER. In these studies we show that the degradation of core-glycosylated TCRalpha is blocked by N-acetyl-L-leucyl-L-leucyl-L-norleucinal (ALLN) and lactacystin, implicating the proteasome in ER degradation. Either acute or chronic treatment of TCRalpha-transfected cells with proteasome inhibitors cause the core-glycosylated TCRalpha chains to progressively shift to an approximately 28-kDa form that lacks N-linked oligosaccharides and the N-terminal signal peptide. The susceptibility of this 28-kDa species to extravesicular protease indicates that it is not protected by the ER membrane and, hence, cytoplasmic. These data suggest a model in which TCRalpha chains that are translocated across the membrane, core-glycosylated, but fail to assemble are dislocated back to the cytoplasm for degradation by cytoplasmic proteasomes. Our data also suggest that covalent modification of TCRalpha with ubiquitin is not required for its degradation.
T细胞抗原受体（TCR）是一种异源寡聚体膜复合物，由至少七条跨膜多肽链组成，它已成为内质网（ER）中整合膜蛋白组装和降解的模型。未组装的TCRα链无法成熟转运至高尔基体，并通过一种非溶酶体的“ER降解”途径迅速降解，该途径被认为是内质网自主的。在这些研究中，我们表明核心糖基化的TCRα的降解被N-乙酰-L-亮氨酰-L-亮氨酰-L-正亮氨酸（ALLN）和乳胞素阻断，这表明蛋白酶体参与了内质网降解。用蛋白酶体抑制剂对转染TCRα的细胞进行急性或慢性处理，都会使核心糖基化的TCRα链逐渐转变为一种缺乏N-连接寡糖和N端信号肽的约28 kDa形式。这种28 kDa物质对胞外蛋白酶的敏感性表明它不受内质网膜保护，因此位于细胞质中。这些数据提出了一个模型，即跨膜转运、核心糖基化但未组装的TCRα链会重新定位回到细胞质中，由细胞质蛋白酶体进行降解。我们的数据还表明，TCRα的泛素化共价修饰对于其降解并非必需。
