title: The interaction of protein A and Fc fragment of rabbit immunoglobulin G as probed by complement-fixation and nuclear-magnetic-resonance studies.
abstract: Protein-A-Fc-fragment complexes were observed in sedimentation-velocity experiments by ultracentrifugation. The interaction was studied by protein-fluorescence-quenching titrations of the Fc fragment with protein A, allowing the dissociation constant to be determined under a variety of conditions. The first component of the complement pathway, C1, is activated by complexes of protein A with rabbit IgG (immunoglobulin G), and the structural basis for this interaction was studied by using n.m.r. (nuclear magnetic resonance). The four Fc-fragment binding sites on protein A were shown to contain aromatic amino acids, and to be connected by mobile hydrophilic regions. Neither n.m.r. nor proton-relaxation-enhancement studies show evidence of a large conformational change of the Fc fragment on binding protein A, and this suggests that the cross-linking of the Fc fragments may be primarily responsible for the activation of component C1. This is supported by the inability of a univalent tryptic fragment of protein A to activate complement fixation by rabbit IgG.
mesh_terms: D000818:Animals; D001666:Binding Sites, Antibody; D055598:Chemical Phenomena; D002621:Chemistry; D003168:Complement Fixation Tests; D003165:Complement System Proteins / Q000378:metabolism; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D013205:Staphylococcal Protein A / Q000276:immunology*; D014461:Ultracentrifugation
chemical_list: D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D013205:Staphylococcal Protein A; D003165:Complement System Proteins