Reilly B D, Skanes V M, Levine R P
Department of Microbiology, University of New Mexico, School of Medicine, Albuquerque 87131.
Mol Immunol. 1994 Jul;31(10):761-9. doi: 10.1016/0161-5890(94)90150-3.
Human C4A and C4B have different functions that may stem from their ability to bind hydroxyl or free amino groups on complement activating surfaces. Previous studies suggest that C4B binds to hydroxyl or amino groups whereas C4A binds to free amino groups on acceptor molecules. Comparison of the derived amino acid sequences of C4A and C4B has shown that differences exist between them at positions 1101, 1102, 1105 and 1106. These residues appear to be involved in the binding specificity of C4B. Less is known about the corresponding residues of C4A. It has been suggested that the aspartic acid of C4A at position 1106 is involved in amide bond formation by serving as a catalytic residue for the reaction or by promoting an increased interaction with amino nucleophilic groups. To examine the functional role of residues 1101-1106, we studied the effects of the C4A site-specific antipeptide mAb, AII-1 in assays dependent on the covalent binding properties of C4A; the C4 mediated inhibition of hemolysis and the C4 mediated inhibition of immune precipitation. This study shows that mAb AII-1 has no effect on C4-mediated hemolysis or its ability to inhibit the rate of immune precipitate formation. The lack of interference by AII-1 in these assays could not be explained by low affinity interaction between antibody and C4A showing that mAb AII-1 does not affect the covalent binding activity of C4A. Furthermore, results from epitope mapping studies show that AII-1 binds to Leu1105 and Asp1106 suggesting that these residues are not critical for amide bond formation by C4A.
人C4A和C4B具有不同的功能，这可能源于它们在补体激活表面结合羟基或游离氨基的能力。先前的研究表明，C4B结合羟基或氨基，而C4A结合受体分子上的游离氨基。C4A和C4B推导的氨基酸序列比较表明，它们在第1101、1102、1105和1106位存在差异。这些残基似乎与C4B的结合特异性有关。关于C4A的相应残基了解较少。有人提出，C4A第1106位的天冬氨酸通过作为反应的催化残基或促进与氨基亲核基团的相互作用增强而参与酰胺键形成。为了研究第1101 - 1106位残基的功能作用，我们在依赖C4A共价结合特性的试验中研究了C4A位点特异性抗肽单克隆抗体AII - 1的作用；C4介导的溶血抑制和C4介导的免疫沉淀抑制。本研究表明，单克隆抗体AII - 1对C4介导的溶血或其抑制免疫沉淀形成速率的能力没有影响。AII - 1在这些试验中缺乏干扰不能用抗体与C4A之间的低亲和力相互作用来解释，这表明单克隆抗体AII - 1不影响C4A的共价结合活性。此外，表位作图研究结果表明，AII - 1结合Leu1105和Asp1106，提示这些残基对C4A形成酰胺键并非关键。
