The molecule is an unusually acidic, single chain protein with an apparent molecular weight of 33 kDa. It does not possess a conventional sequence motif compatible with a membrane spanning segment nor a consensus site for a GPI anchor. gC1q-R migrates as a single chain under both reducing and non-reducing conditions, but it behaves as an oligomer on gel-filtration in non-dissociating conditions. Its multimer formation may be a critical process by which the gC1q-R molecule increases its affinity for multivalent ligands such as C1q. gC1q-R has been shown to inhibit complement activation by preventing the binding of C1q to antibodies, suggesting that the binding site for gC1q-R and the binding site for immune complexes, which are present on the C1q globular 'heads', may be located at the same position. gC1q-R is capable of interacting with several proteins involved in blood clotting, namely, thrombin, prothrombin, the heparinbinding form of vitronectin, the ternary complex, vitronectin-thrombin-antithrombin, as well as high-molecular-weight kininogen and Hageman factor. Besides its role in the complement pathway, gC1q-R participates in enhancement of Fc-receptor and CR1-mediated phagocytosis, procoagulant activity on platelets, and chemotactic activity on mast cells, eosinophils, neutrophils, and fibroblasts. gC1q-R is expressed on a wide variety of cells and can serve as a binding site for plasma and microbial proteins. Its antigenic sites may be cryptic on cells in suspension but become exposed when the cells are fixed by bifunctional cross-linkers. Probably it is also expressed on the cell membrane as a tetramer. Crosslinking or activation may thus bring about a tetrameric assembly of gC1q-R followed by a conformational change within the molecule, thereby exposing epitopes which are otherwise hidden. A form of GC1q-R is also found inside the cell. Intracellular gC1q-R has been shown to bind the cytoplasmic tail of the α1B-adrenergic receptor and to PKCμ.Synonyms: Complement component 1 Q subcomponent-binding protein, GC1QBP, GC1q-R protein, GC1q-R protein, Glycoprotein gC1qBP, HABP1, Hyaluronan-binding protein 1, Mitochondrial matrix protein p32, SF2P32, p32, p33, p33