Kurata H, Sannoh T, Kozutsumi Y, Yokota Y, Kawasaki T
Department of Biological Chemistry, Faculty of Pharmaceutical Sciences, Kyoto University.
J Biochem. 1994 Jun;115(6):1148-54. doi: 10.1093/oxfordjournals.jbchem.a124471.
Mannan-binding proteins (MBPs) occur in two forms, both of which are synthesized in the liver. Although two different MBP cDNAs have been cloned and characterized for rat and mouse, only one form of human MBP cDNA has been isolated. In this study, two forms of human MBP, liver MBP (L-MBP) and serum MBP (S-MBP), were purified from liver and serum and characterized, respectively. The amino acid sequences of these two human MBPs were identical and consistent with those deduced from the cDNA sequence. The most significant difference between L-MBP and S-MBP was the number of subunits, which was about 9 in L-MBP and 18 in S-MBP. Furthermore, S-MBP but not L-MBP had the ability to activate the complement. These results suggested that a newly synthesized protein is processed post-translationally into two forms, S-MBP and L-MBP, in human liver. Recombinant MBP synthesized in COS-1 cells, after transfection with human MBP cDNA, was secreted into the medium, suggesting that COS-1 cells lack a mechanism for differentiating S-MBP and L-MBP. A mutant MBP synthesized in COS-1 cells which lacked a sequence comprising 9 amino acid residues at the beginning of the collagen-like domain had no ability to activate the complement, suggesting that this sequence plays an important role in the activation of the complement by human MBP.
甘露聚糖结合蛋白（MBP）以两种形式存在，二者均在肝脏中合成。尽管已经克隆并鉴定了大鼠和小鼠的两种不同的MBP cDNA，但仅分离出了一种形式的人MBP cDNA。在本研究中，分别从肝脏和血清中纯化并鉴定了两种形式的人MBP，即肝脏MBP（L-MBP）和血清MBP（S-MBP）。这两种人MBP的氨基酸序列相同，且与从cDNA序列推导的序列一致。L-MBP和S-MBP之间最显著的差异在于亚基数量，L-MBP约为9个，S-MBP约为18个。此外，S-MBP具有激活补体的能力，而L-MBP则没有。这些结果表明，新合成的蛋白质在人肝脏中经翻译后加工成两种形式，即S-MBP和L-MBP。用人类MBP cDNA转染COS-1细胞后，在其中合成的重组MBP分泌到培养基中，这表明COS-1细胞缺乏区分S-MBP和L-MBP的机制。在COS-1细胞中合成的一种突变MBP，其胶原样结构域起始处缺少一个包含9个氨基酸残基的序列，该突变MBP没有激活补体的能力，这表明该序列在人MBP激活补体过程中起重要作用。
