C1q, the key component of the classical pathway of the Complement system, is known for its vast functional activity including clearance of apoptotic cells. The binding of C1q to apoptotic blebs occurs via an interaction with the phosphatidylserine externalized on the cell surface. In this study, we characterized the interaction between C1q and phosphatidylserine, with emphasis on the structure of the phosphatidylserine-binding site within the globular domains of C1q and the nature of binding of C1q with phosphatidylserine, using both in vitro and in silico methods. We established that all three globular fragments, forming one C1q globular domain, bound phosphatidylserine with the leading role of the phosphatidylserine-binding site pertaining to the A chain of the globular fragment of C1q. We also determined the closest-contact amino acids of C1q participating in the interaction with phosphatidylserine. An important role is suggested for the glycosylated Asn124 residue in the A chain of the globular fragment.

Background and Aims: C1q is an autoantigen in different autoimmune disorders, Systemic Lupus Erythematosus (SLE) and Lupus Nephritis (LN) among them. The two functional domains of C1q, the collagen-like region (CLR) and the globular head region (gC1q), are frequently recognized by autoantibodies in SLE and LN when C1q is immobilized. We studied whether autoantibodies to C1q in SLE and LN patients recognized C1q as a soluble autoantigen and whether the act of immobilization was a prerequisite for the recognition of C1q autoepitopes localized on gC1q domains. Methods: The interaction of soluble C1q and its globular fragments ghA, ghB, and ghC with immobilized IgG autoantibodies (and vice versa) from sera of 48 patients with SLE and LN was studied with ELISA. Data were compared using Spearman correlation coefficient. Fluorescence spectroscopy was used to study the interaction between C1q and LN IgG autoantibodies both presented in solution. Results: We found that anti-C1q autoantibodies from SLE and LN patients specifically bound C1q and gC1q fragments, ghA, ghB, and ghC, both as immobilized and soluble antigens. Correlation analysis indicated a negative correlation between the levels of autoantibodies against immobilized and soluble C1q and immobilized and soluble gC1q fragments which indicates different epitopes when these proteins were recognized as autoantigens in soluble and immobilized conformations. Conclusions: Serum C1q in patients with SLE is a target molecule for binding from anti-C1q autoantibodies. The gC1q region undergoes a conformational change in an immobilized and a soluble form, thus exposing different epitope-binding sites.