Wright J F, Shulman M J, Isenman D E, Painter R H
Department of Biochemistry, University of Toronto, Ontario, Canada.
J Biol Chem. 1988 Aug 15;263(23):11221-6.
We have examined a defect in complement activation in a mutant trinitrophenyl-binding pentameric murine monoclonal IgM which has serine replacing the proline normally found at position 436 in the protein. The mutant protein showed equivalent hapten binding but a 100-fold decreased ability to initiate complement-dependent lysis of trinitrophenyl-coupled erythrocytes at physiological ionic strength (mu = 0.15). C4b deposition mediated by the mutant protein was impaired to a similar degree. C1 bound by the mutant protein showed C1s to C1-s conversion, suggesting normal activation. When measured at reduced ionic strength (mu = 0.06), the C1 and C1q binding affinity of the mutant protein was approximately one-half that of the wild type. However, the C1 binding affinity of the mutant protein showed a greater dependence upon ionic strength such that at physiological ionic strength we estimate a 50-fold lower C1 binding affinity for the mutant molecule. Kinetic studies suggested that this difference in affinity was largely attributable to differences in association rates. In addition, a fixed proportion of the mutant molecules showed no C1 binding. We conclude that the defect in complement activation occurs at the level of C1 binding. Our data support a role for the C mu 3 domain (residues 340-440) in C1 binding by IgM.
我们研究了一种突变型三硝基苯基结合五聚体小鼠单克隆IgM的补体激活缺陷，该突变体中丝氨酸取代了该蛋白正常位于436位的脯氨酸。突变蛋白表现出同等的半抗原结合能力，但在生理离子强度（μ = 0.15）下引发三硝基苯基偶联红细胞补体依赖性裂解的能力降低了100倍。由突变蛋白介导的C4b沉积也受到类似程度的损害。与突变蛋白结合的C1表现出C1s向C1-s的转化，表明激活正常。在降低的离子强度（μ = 0.06）下测量时，突变蛋白的C1和C1q结合亲和力约为野生型的二分之一。然而，突变蛋白的C1结合亲和力对离子强度的依赖性更大，以至于在生理离子强度下，我们估计突变分子的C1结合亲和力低50倍。动力学研究表明，这种亲和力差异很大程度上归因于结合速率的差异。此外，固定比例的突变分子不表现出C1结合。我们得出结论，补体激活缺陷发生在C1结合水平。我们的数据支持Cμ3结构域（残基340 - 440）在IgM与C1结合中的作用。
