Subcellular Localization and Physiological Significance of Intracellular Mannan-binding Protein

Motohiro Nonaka,Bruce Yong Ma,Misato Ohtani,Akitsugu Yamamoto,Masayuki Murata,Kiichiro Totani,Yukishige Ito,Keiko Miwa,Wataru Nogami,Nobuko Kawasaki,Toshisuke Kawasaki

doi:10.1074/jbc.m700992200

Abstract

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.

Highlights

Exhibiting primary specificity for mannose, fucose, and N-acetylglucosamine [1]
We have demonstrated that the expression of human Mannan-binding protein (MBP) cDNA reproduces the native MBP differentiation of serum MBP (S-MBP) and intracellular MBP (I-MBP) depending on the different posttranslational modifications in human hepatoma cell lines, and have determined that I-MBP molecules accumulate predominantly as punctate structures in the endoplasmic reticulum (ER) and ER-to-Golgi COPII vesicles
Double staining of I-MBP and ER- or Golgi-associated protein markers revealed a high degree of co-localization of these two proteins in the ER and a low degree in the Golgi apparatus (Fig. 1B, right panels)

Summary

Introduction

Exhibiting primary specificity for mannose, fucose, and N-acetylglucosamine [1]. Because its discovery as a vital serum component associated with innate immunity, this lectin has been regarded as a multifunctional protein. Synthesized LAMP-1 Interacts with I-MBP—With respect to the subcellular localization of I-MBP described above, it is possible to speculate that I-MBP may serve as a cargo transport lectin for the trafficking of biosynthetic intermediates of some glycoproteins with high-mannose-type oligosaccharides from the ER-to-Golgi apparatus.

Results

Conclusion