C1q receptor

Clq is a collagen-like, structurally complex, cationic glycoprotein consisting of 18 similar but distinct polypeptide chains. Each chain consists of an 81-amino-acid-long, collagen-like N-terminal portion and a C-terminal globular region of 136 amino acids. In normal plasma Clq circulates as a subunit of the first component of complement, C1, which is a Ca²⁺-dependent pentamolecular complex with a structural formula of ClqClr₂C1s₂. The chapter describes assay methods of Clq. Most of the assays for ClqR used in laboratory are essentially based on a single parameter, i.e., the ability of the receptor to bind to the collagenous region of the Clq molecule to which Clr₂Cl s₂complex is also believed to bind. Such a binding invariably inhibits the capacity of Clq to mediate the following functions: rosette formation, association with Clr₂Cls₂to form C1 and C1q-dependent cellular cytotoxicity. Another property of the CIqR, which is similar to that of the plasma CIq inhibitor, is its ability to precipitate Clq on solid or liquid medium in a manner comparable to that of antigen-antibody interaction. These properties have been exploited to develop simple assays, some of which are described in the chapter.