Ultracentrifuge studies of the binding of IgG of different subclasses to the Clq subunit of the first component of complement

Abstract

Normal Ig[immunoglobulin]G and myeloma proteins of the IgG1, 2, 3, and 4 subclasses were mixed with human C1q [q fragment of the 1st complement component] and studied in the analytical ultracentrifuge for complex formation. Binding of IgG to C1q is apparent from the enlargement of area and from the increase in sedimentation rate of the well-separated schlieren peak of C1q. The accurate determination of binding parameters requires that sedimentation rates by corrected for hydrodynamic interaction, and area measurements corrected for the Johnston-Ogston effect. At the highest Ig concentrations employed in these studies, more than 10 IgG molecules are bound to each C1q. If it is assumed that the number of binding sites must be an integral multiple of 6, then the data support a 12 binding site model, although an 18 site model cannot be ruled out. Myeloma IgG proteins of all subclasses bind to C1q, with affinities decreasing in the order G3 > G1 > G2 > G4. No binding of IgA to C1q could be detected.