Takeda Koh, Qin Sheng-Ying, Matsumoto Naoki, Yamamoto Kazuo
Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, 277-8562 Chiba, Japan.
Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, 277-8562 Chiba, Japan.
Biochem Biophys Res Commun. 2014 Nov 21;454(3):436-40. doi: 10.1016/j.bbrc.2014.10.102. Epub 2014 Oct 27.
We previously demonstrated that malectin associates with ribophorin I, which is a subunit of oligosaccharyltransferase in the endoplasmic reticulum (ER). When malectin and ribophorin I are overexpressed in the ER, secretion of an α1-antitrypsin (AT) variant whose folding is defective, termed null Hong Kong (AT(NHK)), is decreased. To confirm whether the interaction between malectin and ribophorin I is involved in the decreased secretion of misfolded glycoproteins, we constructed an expression vector encoding truncated malectin, which could not associate with ribophorin I and had an Lys-Asp-Glu-Leu ER-retention sequence at its C-terminus. Expression of wild-type malectin abrogated AT(NHK) secretion, whereas expression of truncated malectin did not affect AT(NHK) secretion. Both wild-type and truncated malectin bound to AT(NHK), and the level of AT(NHK) was similar in cells expressing wild-type malectin and those expressing truncated malectin. Furthermore, we previously showed that decreased secretion of misfolded AT(NHK) by malectin overexpression is restored by treatment with a proteasome inhibitor. These results clearly demonstrate that the association of malectin with ribophorin I is required to capture misfolded glycoproteins and direct them to the degradation pathway.
我们之前证明，malectin与核糖体结合蛋白I相关联，核糖体结合蛋白I是内质网（ER）中寡糖基转移酶的一个亚基。当malectin和核糖体结合蛋白I在内质网中过表达时，一种折叠有缺陷的α1-抗胰蛋白酶（AT）变体（称为零香港型（AT(NHK)））的分泌会减少。为了确认malectin与核糖体结合蛋白I之间的相互作用是否参与了错误折叠糖蛋白分泌的减少，我们构建了一个编码截短型malectin的表达载体，该截短型malectin不能与核糖体结合蛋白I结合，并且在其C末端有一个赖氨酸-天冬氨酸-谷氨酸-亮氨酸内质网滞留序列。野生型malectin的表达消除了AT(NHK)的分泌，而截短型malectin的表达不影响AT(NHK)的分泌。野生型和截短型malectin都与AT(NHK)结合，并且在表达野生型malectin的细胞和表达截短型malectin的细胞中，AT(NHK)的水平相似。此外，我们之前表明，用蛋白酶体抑制剂处理可恢复因malectin过表达而导致的错误折叠的AT(NHK)分泌减少。这些结果清楚地表明，malectin与核糖体结合蛋白I的结合是捕获错误折叠的糖蛋白并将它们导向降解途径所必需的。
