Evidence that Clq binds specifically to CH2-like immunoglobulin γ motifs present in the autoantigen calreticulin and interferes with complement activation

Calreticulin (CRT) is located predominantly in the endoplasmic reticulum (ER) of cells, where it functions as a quality control controller of protein folding. However, CRT is also a prevalent autoantigen in patients with systemic lupus erythematosus (SLE), where its release from the cell may arise as a results of dysfunctional apoptosis and inefficient removal of ER vesicles, which are an abundant source of CRT and other autoantigens, Indicative of this is the presence of autoantibodies against CRT in the sera of 40-60% of all SLE patients. Once released into the circulation, CRT might bind directly to Clq and we have suggested that this association may result in a defect in Clq-mediated clearance of antigen-antibody complexes. It has been previously shown that CRT under physiological salt conditions binds to the globular head of Clq. It is known that the globular head region of Clq binds to the C(H)2 domain in the Fc portion of immunoglobulin gamma (IgG). The N-terminal half of CRT contains a number of short regions of 7-10 amino acids that show sequence similarity to the putative Clq binding region in the C(H)2 domain of IgG. By use of a series of 92 overlapping CRT synthetic peptides, a number of Clq binding sites on the CRT molecule have been identified, including several containing a C(H)2 -like motif similar to the ExKxKx Clq binding motif found in the C(H)2 domain of IgG. A number of these peptides were shown to inhibit binding of Clq to IgG and reduce binding of native CRT to Clq. Moreover, several of the peptides were capable of inhibiting the classical pathway of complement activation. These studies have identified specific binding sites on the CRT molecule for Clq and lend support to the hypothesis that interaction of CRT with Clq may interfere with the ability of Clq to associate with immune complexes in autoimmune-related disorders.