Interaction of C1q with DNA.

With the aim of clarifying the relationship between the activation of the first component of complement (C1) by immunoglobulin and by polyanions, the mode of interaction of C1q with DNA was investigated by structural and inhibition studies. DNA inhibits C1q binding to IgG immune complexes (ICs) through binding to C1q rather than to IgG, as seen from two lines of evidence. Firstly, DNA does not bind to ICs under conditions where full binding to C1q is observed. Secondly, at I - 0.15, DNA inhibits more strongly when mixed first with C1q rather than with ICs. The inhibition of C1q-IgG binding by DNA is subject to kinetic factors. Firstly, DNA is not an effective inhibitor if added after C1q has bound to ICs. This at least in part reflects a portion of the IgG-bound C1q that exchanges only very slowly with free C1q. Secondly, the relative rates of association of C1q to DNA and ICs at different ionic strengths are important in determining whether inhibition is observed. The existence of a kinetic effect in the inhibition by DNA means that inhibition experiments cannot be used to establish whether DNa binds to the same site on C1q as IgG. This question was therefore approached by structural studies. Precipitation of C1q with DNA was greatly diminished by heat or pH 4.45 denaturation of C1q, by pepsin digestion to remove the globular heads, and by limited modification with 1, 2-cyclohexanedione. In contrast, extensive modification with methyl acetimidate only had a limited effect. In these respects the structural requirements for C1q-DNA precipitation were similar to those for C1q-Igg binding, as would be consistent with binding of DNA and IgG to nearby or overlapping sites on C1q. In view of residual DNA-precipitating activity in the pepsin fragment preparation of C1q, there is the possibility that there are additional DNA sites on the collagenous tails.