RESUMEN
It was shown previously that in sheep calcium-dependent anti-GAT there is a subpopulation which reacts with and can be precipitated with poly G [Liberti (1975) Immunochemistry 12, 303-310]. This entire subpopulation was also found to react with the cross-reacting polypeptides GA and GT. Furthermore, from hydrogen exchange experiments, it was found that only the immunizing antigen GAT completely filled the combining sites of these antibodies whereas poly G was shown to occupy an average of 47% of all sites, and GA and GT, 75 and 85% respectively. Since precipitins formed with this subpopulation and each of these antigens should have reasonably similar densities and orientations of 'aggregated' IgG but differing combining sites occupancies, we have used this system to explore the relative role of Fc aggregation and/or IgG distortion vs combining site-transmitted effects on the binding of Clq to antibody. For two preparations of this subpopulation (one of high avidity, the other obtained via poly G-Sepharose and of lower avidity) there is only a 5% difference in the delta G (10.2-10.8 kcal/mole) of Clq-IgG interaction for a change in combining site occupancy of 47-100%. For the high-avidity preparation there is a correlation between delta G and degree of ligand occupancy of combining site. This could reflect combining site-transmitted effects or may be related to small differences in the molecular architecture of these precipitins. Clq saturation curves support the latter notion. In view of the very moderate effect of combining site filling (from 47 to 100%) on Clq-IgG interaction for the high-avidity preparation and the absence of any correlation for the lower-avidity preparation. It appears that an allosteric model for antibody initiation of complement is untenable. Unless combining site-originating contributions are completed when less than 50% of an antibody-binding site is occupied by ligand, it would seem that Clq binding to immune complexes must be governed either by enhanced interactions resulting from Fc clustering which occurs via antibody interactions with antigen or by distortion of the antibody molecular upon ligand binding or some combination thereof.