Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
J Biol Chem. 2011 Apr 1;286(13):11529-42. doi: 10.1074/jbc.M110.173591. Epub 2011 Jan 24.
The molecular basis for retention of integral membrane proteins in the endoplasmic reticulum (ER) is not well understood. We recently discovered a novel ER molecular chaperone termed Cosmc, which is essential for folding and normal activity of the Golgi enzyme T-synthase. Cosmc, a type II single-pass transmembrane protein, lacks any known ER retrieval/retention motifs. To explore specific ER localization determinants in Cosmc we generated a series of Cosmc mutants along with chimeras of Cosmc with a non-ER resident type II protein, the human transferrin receptor. Here we show that the 18 amino acid transmembrane domain (TMD) of Cosmc is essential for ER localization and confers ER retention to select chimeras. Moreover, mutations of a single Cys residue within the TMD of Cosmc prevent formation of disulfide-bonded dimers of Cosmc and eliminate ER retention. These studies reveal that Cosmc has a unique ER-retention motif within its TMD and provide new insights into the molecular mechanisms by which TMDs of resident ER proteins contribute to ER localization.
内质网（ER）中完整膜蛋白保留的分子基础尚不清楚。我们最近发现了一种新型的 ER 分子伴侣，称为 Cosmc，它对于高尔基体酶 T-合成酶的折叠和正常活性是必需的。Cosmc 是一种 II 型单次跨膜蛋白，缺乏任何已知的 ER 回收/保留基序。为了探索 Cosmc 中特定的 ER 定位决定因素，我们生成了一系列 Cosmc 突变体以及 Cosmc 与非 ER 驻留的 II 型蛋白（人转铁蛋白受体）的嵌合体。在这里，我们表明 Cosmc 的 18 个氨基酸跨膜结构域（TMD）对于 ER 定位是必需的，并赋予选择嵌合体 ER 保留。此外，Cosmc 的 TMD 内单个半胱氨酸残基的突变阻止 Cosmc 二硫键二聚体的形成，并消除 ER 保留。这些研究揭示了 Cosmc 在其 TMD 中具有独特的 ER 保留基序，并为驻留 ER 蛋白的 TMD 如何有助于 ER 定位的分子机制提供了新的见解。
