Biological functions of C1q expressed by conformational changes

Of all the serum proteins C1q is one of the most positively charged. Two dimensional-isoelectric focussing studies revealed an isoelectric point of 9.3. Due to its unique character C1q is able to interact directly with highly negatively charged molecules (e.g. mucopolysaccharides, MPS.) This interaction is ionic in nature as demonstrated by electro-immunoassay. Binding of C1q to immune complexes induces conformational changes. The inhibitory effect of MPS on C1q, as measured by hemolytic assay, provides positive evidence for such conformational changes. The detection of neoantigens further supports this observation. These in vitro studies suggest that the observed autoantibodies against the collagenous portion of C1q in certain SLE patients are induced by long-acting conformational changes within the C1q-immune complex.