Fujita T, Gigli I, Nussenzweig V
J Exp Med. 1978 Oct 1;148(4):1044-51. doi: 10.1084/jem.148.4.1044.
We recently described the isolation from human serum of a high molecular weight protein with specific binding affinity for fluid-phase activated C4. We show here that the C4-binding protein (C4-Bp) functions as an essential cofactor in the proteolysis of C4b in the presence of C3b-inactivator (C3bINA). C4-bp, together with C3bINA, cleave the alpha'-chain of C4b into three fragments called alpha2, alpha3, and alpha4, with mol wt of 47,000, 25,000, and 17,000 daltons, respectively. The alpha2 fragment was dissociated from C4b without reduction, whereas the alpha3 and alpha4 fragments were disulfide bonded the other chains of C4b. The reaction did not occur when either C4-bp or C3bINA were omitted, nor in the presence of either protein in combination with beta1H. Native C4 was not affected by C3bINA aand C4-bp. C4b was not cleaved when incubated in serum of a patient with genetic deficiency of C3bINA. However, when purified C3bINA was added, the alpha'-chain of C4b was cleaved and fragments with the same molecular weight as alpha2, alpha3, and alpha4 were generated.
我们最近描述了从人血清中分离出一种对液相活化C4具有特异性结合亲和力的高分子量蛋白质。我们在此表明，C4结合蛋白（C4-Bp）在存在C3b灭活剂（C3bINA）的情况下，作为C4b蛋白水解的必需辅因子发挥作用。C4-bp与C3bINA一起，将C4b的α'-链切割成三个片段，分别称为α2、α3和α4，分子量分别为47,000、25,000和17,000道尔顿。α2片段在未还原的情况下从C4b上解离，而α3和α4片段通过二硫键与C4b的其他链相连。当省略C4-bp或C3bINA时，或者在任何一种蛋白质与β1H结合存在的情况下，反应都不会发生。天然C4不受C3bINA和C4-bp的影响。在C3bINA基因缺陷患者的血清中孵育时，C4b不会被切割。然而，当加入纯化的C3bINA时，C4b的α'-链被切割，并产生了与α2、α3和α4分子量相同的片段。
