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

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