DIMERIZATION OF TRANSCOBALAMIN II RECEPTOR

转钴胺II受体(TC II-R)是以单体形式存在的 微粒体内分子量为62和124 kDa的二聚体 分别为质膜。体内纯TC II-R单体 二聚体插入卵磷脂/胆固醇(4：1)脂质体中。这个 目前已有研究明确TC II-R的作用机制 二聚化。酶脱糖(45-47 kDa)和 成熟的纯TC II-R(62 KDa)表现出最佳的结合和 在22℃形成了分子质量分别为95和124 kDa的二聚体 C与含有至少10摩尔%的 胆固醇。成熟的受体也会在插入到 二肉豆蔻基磷脂酰胆碱囊泡在5℃，无 胆固醇或在22℃下使用脂类制备脂泡 从血浆中提取，但不是微粒体膜。胆固醇 天然肠道质膜的耗尽或其浓缩 微粒体膜导致124 kDa的原位转化 二聚体为62 kDa单体或单体为二聚体形式， 分别进行了分析。磷脂酶A2对质膜的处理 导致受体的二聚体形式的原位转化 转变为单体形式。1-棕榈酰基的自旋标记研究 Dxsteroyl PC显示与TC II-R的相互作用增加 在探测器周围下达命令。基于这些结果，我们建议 TC II-R的二聚化是1)由其与A的相互作用所介导 刚性更有序的脂质双层膜；2)在质膜中， 受胆固醇水平调节；以及3)独立于 糖基化介导的折叠。

Transcobalamin II receptor (TC II-R) exists as a monomer and a dimer of molecular masses of 62 and 124 kDa in the microsomal and plasma membranes, respectively. In vivo, pure TC II-R monomer dimerizes upon insertion into egg PC/cholesterol (4:1) liposomes. The current studies were carried out to define the mechanism of TC II-R dimerization. Both enzymatically deglycosylated (45-47 kDa) and mature pure TC II-R (62 kDa) demonstrated optimal association and formed dimers of molecular mass of 95 and 124 kDa, respectively, at 22 C in association with egg PC vesicles containing at least 10 mole% of cholesterol. Mature receptor also dimerized upon insertion into dimyristoyl phosphatidylcholine vesicles at 5 C in the absence of cholesterol or at 22 C with lipid vesicles prepared using lipid extract from the plasma, but not microsomal membranes. Cholesterol depletion of native intestinal plasma membranes or its enrichment of microsomal membranes resulted in the in situ conversion of the 124 kDa dimer to the 62 kDa monomer or of the monomer into the dimer form, respectively. Treatment of the plasma membranes with phospholipase A2 resulted in the in situ conversion of the dimer form of the receptor to the monomer form. Spin-label studies using 1-palmitoyl, 12 doxylsteroyl PC revealed that interactions with TC II-R increased order around the probe. Based on these results, we suggest that dimerization of TC II-R is 1) mediated by its interactions with a rigidly more ordered lipid bilayer membrane; 2) in plasma membranes, is regulated by cholesterol levels; and 3) is independent of glycosylation-mediated folding.