Contribution of the c1q polypeptide chains in the recognition of crp

C1q, the recognition subunit of the classical complement pathway, is a multifunctional protein involved in a great number of immunological processes. The molecular basis of the functional diversity of C1q is its ability to recognize a broad range of ligands via the gC1q region. One of the most important ligands of C1q is the human C-reactive protein (CRP), a major acute phase protein, which plays a crucial role in keeping the tolerance to self-structures. The aim of the present study is to evaluate the contribution of the C1q globular head fragments rghA, rghB and rghC to the recognition of CRP. The obtained results indicate that all three C1q-chains are involved in the CRP-binding, and that rghB is the most important one. The pH-dependence of the interactions between rghA/B/C and CRP demonstrates that different amino acid residues from the three chains take part in the formation of the CRP-binding site. The analysis of the CRP-binding activities of biotinylated and nonbiotinylated rghB indicates that Tyr residues from the ghB are the most likely to play a crucial role in the interaction with CRP and that probably only one Lys residue from the rghB is involved as well.