title: C1 binding by murine IgM. The effect of a Pro-to-Ser exchange at residue 436 of the mu-chain.
abstract: 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.
mesh_terms: D003166:Complement Activating Enzymes / Q000378:metabolism; D003167:Complement Activation; D003172:Complement C1 / Q000378:metabolism*; D015922:Complement C1q; D003181:Complement C4 / Q000378:metabolism; D015935:Complement C4b; D007075:Immunoglobulin M / Q000378:metabolism*; D007148:Immunoglobulin mu-Chains / Q000378:metabolism*; D009994:Osmolar Concentration; D011392:Proline / Q000378:metabolism*; D012694:Serine / Q000378:metabolism*
chemical_list: D003172:Complement C1; D003181:Complement C4; D007075:Immunoglobulin M; D007148:Immunoglobulin mu-Chains; D012694:Serine; D015922:Complement C1q; D015935:Complement C4b; D011392:Proline; D003166:Complement Activating Enzymes