| Research Abstract |
Mannan-binding protein (MBP) is a C-type serum lectin specific for mannose, fucose and N-acetylglucosamine, and is associated with the removal of pathogenic microorganisms from the host animals. While the pig has been proposed to be an attractive source of xeno transplantable tissues and organs, little is known about porcine MBP. In our previous studies, phosphomannan, but not mannan, was found to be an effective inhibitor of the C1q-independent bactericidal activity of newborn piglet serum against some rough strains of gram-negative bacteria. Here we show that a novel phosphomannan-binding lectin (PMBL) of 33 kDa under reducing conditions was isolated from both newborn and adult porcine serum and characterized. Porcine PMBL activated the complement system via the lectin pathway triggered by binding with both phosphomannan and mannan, which, unlike MBP, was effectively inhibited by mannose 6-phosphate-containing oligosaccharides. Our observations suggest that porcine PMBL plays a criti
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cal role in the innate immune defense from the newborn stage to adulthood. 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. On the other hand, the subcellular localization of I-MBP and its functional implication have not been clarified yet. Here, we demonstrated that I-MBP shows distinct accumulation in cytoplasmic granules, and is predominantly localized in the 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. These results strongly indicate that I-MBP may function as a cargo transport lectin facilitating ER-to-Golgi traffic in glycoprotein quality control. In addition, we are preparing monoclonal antibodies recognizing MBP-ligand oligosaccharides expressed on the surfaces of a human colorectal carcinoma SW1116 cells. Less
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