Seya T, Turner J R, Atkinson J P
J Exp Med. 1986 Apr 1;163(4):837-55. doi: 10.1084/jem.163.4.837.
Based on preliminary evidence indicating that a cell-associated protein of U937 (a human monocyte-like cell line) possessed cofactor activity and was not the C3b/C4b receptor, we sought to further characterize this protein. A sequential four-column purification procedure was devised that includes C3(H2O) affinity chromatography to isolate in reasonable yields and purity a cell-associated protein of U937 and several other human cell lines. Based on its pattern and Mr on SDS-PAGE, acidic pI, and ligand specificity, it is identical to a recently described C3(H2O) or C3b-binding membrane glycoprotein of human PBL and cell lines; having no presently identified function, it was termed gp45-70. After purifying this protein, we determined its functional capabilities and compared them to those of the other complement proteins with regulatory activity directed at components comprising the C3 convertases. This protein was the most efficient (50 times that of H) yet-described cofactor for the I-mediated first cleavage of C3b. It also was a cofactor for the first cleavage of C4b, but was not as efficient as C4bp. The second cleavage of C3b and C4b was not efficiently mediated. It had no ability to accelerate decay in the classical or alternative pathway C3 convertases. Based on this unique activity profile and ability to be surface labeled, we have renamed this molecule membrane cofactor protein (MCP). We suggest that this protein plays a major role in preventing autologous complement activation.
基于初步证据表明U937（一种人单核细胞样细胞系）的一种细胞相关蛋白具有辅助因子活性且不是C3b/C4b受体，我们试图进一步表征这种蛋白。设计了一种连续四柱纯化程序，该程序包括C3(H2O)亲和层析，以合理的产量和纯度分离U937及其他几种人细胞系的一种细胞相关蛋白。基于其在SDS-PAGE上的图谱和相对分子质量、酸性pI以及配体特异性，它与最近描述的人外周血淋巴细胞（PBL）和细胞系的C3(H2O)或C3b结合膜糖蛋白相同；由于目前尚未确定其功能，故将其命名为gp45-70。纯化该蛋白后，我们确定了其功能能力，并将其与其他对构成C3转化酶的成分具有调节活性的补体蛋白的功能能力进行了比较。这种蛋白是I介导的C3b首次裂解中迄今描述的最有效的辅助因子（是H的50倍）。它也是C4b首次裂解的辅助因子，但不如C4bp有效。C3b和C4b的第二次裂解未得到有效介导。它没有加速经典或替代途径C3转化酶衰变的能力。基于这种独特的活性谱和表面标记能力，我们将该分子重新命名为膜辅助因子蛋白（MCP）。我们认为这种蛋白在防止自身补体激活中起主要作用。
