Subcellular localization and physiological significance of intracellular mannan-binding protein
Subcellular localization and physiological significance of intracellular mannan-binding protein
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DOI：
10.1074/jbc.m700992200
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发表时间：
2007-06-15
影响因子：
4.8
通讯作者：
Kawasaki, Toshisuke
中科院分区：
文献类型：
--
作者：
Nonaka, Motohiro;Ma, Bruce Yong;Kawasaki, Toshisuke
甘露聚糖结合蛋白(MBP)是一种C型哺乳动物凝集素，专为甘露糖和N-乙酰氨基葡萄糖所特有。MBP主要在肝脏合成，天然存在两种形式：血清MBP(S-MBP)和细胞内MBP(I-MBP)。S-丝氨酸蛋白通过凝集素途径激活补体，并与丝氨酸蛋白酶结合。尽管我们以前的研究(Mori，K.，K.Kawasaki，T.和Yamashina，I.(1984)Arch.生物化学。生物群落。232,223-233)，I-MBP的亚细胞定位及其功能尚未阐明。在这里，作为我们先前研究的延伸，我们已经证明了人MBP基因的表达在人肝癌细胞中复制了S-MBP和I-MBP的天然MBP分化。I-MBP在细胞质颗粒中有明显聚集，主要定位于内质网，参与COPII囊泡介导的内质网到高尔基体的转运。然而，无论是缺乏糖结合活性的突变体(C236S/C244S)I-MBP还是野生型I-MBP在衣霉素处理的细胞中的亚细胞定位都显示出同样弥散的细胞质分布，这表明I-MBP在ER和COPII囊泡中的独特积累是由N-糖-凝集素相互作用介导的。此外，I-MBP与糖蛋白中间体的结合发生在内质网中，内质网是碳水化合物和pH依赖的，并受到葡萄糖修剪的高甘露糖型寡糖的影响。这些结果有力地表明，I-MBP在糖蛋白质量控制中可能作为一种货物运输凝集素，促进内质网到高尔基体的运输。
Mannan-binding protein (MBP) is a C-type mammalian lectin specific for mannose and N-acetylglucosamine. MBP is mainly synthesized in the liver and occurs naturally in two forms, serum MBP (S-MBP) and intracellular MBP (I-MBP). S-MBP activates complement in association with MBP-associated serine proteases via the lectin pathway. Despite our previous study (Mori, K., Kawasaki, T., and Yamashina, I. (1984) Arch. Biochem. Biophys. 232, 223 -233), the subcellular localization of I-MBP and its functional implication have not been clarified yet. Here, as an extension of our previous studies, we have demonstrated that the expression of human MBP cDNA reproduces native MBP differentiation of S-MBP and I-MBP in human hepatoma cells. I-MBP shows distinct accumulation in cytoplasmic granules, and is predominantly localized in the endoplasmic reticulum (ER) and involved in COPII vesicle-mediated ER-to-Golgi transport. However, the subcellular localization of either a mutant (C236S/ C244S) I-MBP, which lacks carbohydrate-binding activity, or the wild-type I-MBP in tunicamycin-treated cells shows an equally diffuse cytoplasmic distribution, suggesting that the unique accumulation of I-MBP in the ER and COPII vesicles is mediated by an N-glycan-lectin interaction. Furthermore, the binding of I-MBP with glycoprotein intermediates occurs in the ER, which is carbohydrate-and pH-dependent, and is affected by glucose-trimmed high-mannose-type oligosaccharides. These results strongly indicate that I-MBP may function as a cargo transport lectin facilitating ER-to-Golgi traffic in glycoprotein quality control.