Binding of human C4 to C-reactive protein-pneumococcal C-polysaccharide complexes during activation of the classical complement pathway.

Sequential interaction of CRP-PnC aggregates, made at slight CRP excess, with purified human C1, C4 and C2oxy resulted in formation of an effective C3-convertase, indicating the binding of C1, C4 and C2 on the aggregates. Immunoprecipitation experiments demonstrated that, following cleavage of 125I-C4 by CRP-PnC-C1 complexes, approximately 3% of the 125I-C4 was bound to CRP while a lower percentage was bound to PnC, CRP-C4 complexes could also be demonstrated by substituting 125I-CRP for 125I-C4. The nature of the CRP-C4 bond was examined by electrophoretic analysis. Complexes of 125I-C4-CRP prepared as earlier were incubated at 100 degrees C for 2 min in buffer containing 2% SDS and 5% beta-mercaptoethanol and subjected to electrophoresis in SDS-containing polyacrylamide gradient slab gels. Autoradiography of the dried gels revealed the presence of high mol. wt bands containing the alpha'-chain of C4b. CRP could also be demonstrated in these high mol. wt bands which apparently represented covalent complexes between the alpha'-chain of C4b and CRP monomers. Since CRP contains no detectable carbohydrate, it seems likely that an amide bond is formed between the two proteins.