title: Mutual inhibition of the binding of Clq and protein A to rabbit IgG immune complexes.
abstract: A complex of rabbit IgG antibody with horseradish peroxidase covalently linked to Sepharose 4B was used as an insoluble immune complex for studying the binding of complement factor C1q protein A from Staphylococcus aureus, and its IgG-binding fragments AB and B, to rabbit IgG. It was shown that protein A (mol. wt approx. 42,000) and fragments AB and B (mol. wts approx. 14,000 and 7000, respectively) inhibited the binding of C1q to insoluble immune complex at 4 degrees C. However, at 37 degrees C fragment B did not inhibit this binding. On the other hand, C1q, when bound to an insoluble immune complex, almost completely blocked the binding of protein A and fragment B at both temps. The higher affinity of C1q for its CH2-binding site than of fragment B for its CH2-binding site may explain the displacement of the latter from the CH2 domain. The mutual inhibition of the binding of C1q and protein A (and its smaller fragments) indicates that the binding sites for C1q and protein A are closely located in the CH2 domain.
mesh_terms: D000818:Animals; D000936:Antigen-Antibody Complex / Q000378:metabolism*; D001667:Binding, Competitive; D003166:Complement Activating Enzymes / Q000378:metabolism*; D015922:Complement C1q; D004306:Dose-Response Relationship, Immunologic; D007074:Immunoglobulin G / Q000378:metabolism*; D011817:Rabbits; D013205:Staphylococcal Protein A / Q000378:metabolism*; D013696:Temperature
chemical_list: D000936:Antigen-Antibody Complex; D007074:Immunoglobulin G; D013205:Staphylococcal Protein A; D015922:Complement C1q; D003166:Complement Activating Enzymes