The Binding Protein for Globular Heads of Complement C1q, gC1qR

Abstract

A binding protein for the globular head domains of complement component C1q, designated gC1qR, recently described to be present on vascular and blood cells (Ghebrehiwet, B., Lim, B.-L., Peerschke, E. I. B., Willis, A. C., and Reid, K. B. M. (1994) J. Exp. Med. 179, 1809 -1821 was expressed in recombinant form in bacteria to investigate its functional and structural properties. The recombinant gC1qR was found to be functional because tetramerization of the 24.3-kDa polypeptide occurred as described for the native protein, and the binding of the ligand C1q by recombinant gC1qR was indistinguishable from binding shown by gC1qR isolated from Raji cells. Recombinant gC1qR immobilized to microspheres was used to search for additional binding proteins unrelated to C1q. Surprisingly, it was found that vitronectin or complexes containing vitronectin were retained from plasma or serum, and subsequent analysis revealed the specific binding of the ternary vitronectin-thrombin-antithrombin complex to gC1qR. Because the thrombin-antithrombin complex was unable to interact with gC1qR, direct binding with vitronectin was investigated in a purified system. The heparin binding multimeric form of vitronectin but not the plasma form of vitronectin was found to bind specifically to gC1qR isolated from Raji cell membrane as well as to recombinant gC1qR. This interaction was saturable (K D ϳ20 nM) and inhibitable by glycosaminoglycans such as heparin but not by chondroitin sulfate. C1q and vitronectin did not compete with each other for binding to gC1qR, and both ligands seem to interact with different parts of the gC1qR because a truncated version of recombinant gC1qR lacking the N-terminal 22-amino acid portion hardly interacted with vitronectin but bound C1q as well as the intact gC1qR. These findings establish gC1qR as a novel vitronectin-binding protein that may participate in the clearance of vitronectincontaining complexes or opsonized particles or cooperate with vitronectin in the inhibition of complementmediated cytolysis.

Crystal structure of human p32, a doughnut-shaped acidic mitochondrial matrix protein

“…The negatively charged side of p32 may potentially bind to the inner mitochondrial membrane in the presence of divalent metal ions, as negatively charged phospholipids can chelate divalent cations. Indeed, circumstantial evidence suggests that p32 is capable of associating with membranes (10,13,14,18). If p32 is indeed localized proximally to the inner mitochondrial membrane, the central channel in the p32 trimer may have important functions.…”

“…For p32 to interact with the globular head domain of C1q (10), H-kininogen (13), vitronectin (14), and hyaluronic acid (15), it should be located on the extracellular side of the plasma membrane or in the extracellular matrix. Although p32 has been detected in the supernatants of several cultured cells and in normal human and rat sera (42), there is no direct evidence that it is specifically targeted to the cell surface or secreted.…”

“…It has also been reported that p32 interacts with the globular head domain of the plasma complement component C1q, whose hemolytic activity is inhibited as a result (10). Binding of p32 to transcription factor IIB (11), the lamin B receptor (12), high molecular-weight kininogen and factor XII (13), vitronectin (14), and hyaluronic acid (15) has also been reported.…”

C1q-mediated chemotaxis by human neutrophils: involvement of gClqR and G-protein signalling mechanisms

“…The precise binding proteins on the neutrophil cell surface involved in C1q-cell mediated chemotaxis are still unknown. Although C1q, kininogen and factor XII are bound with high affinity by purified gC1qBP\gC1qR [38][39][40], the gC1qBP cannot be expected to serve as a surface receptor for these molecules since it is located intracellularly on normal endothelial cells [41] and this is also probably the case in other cell types. However, it is possible that when cells are stressed, or undergoing apoptosis, that gC1qR may be found on the cell surface and thus be able to bind C1q and modulate binding of C1q to the cell surface.…”

The Human gC1qR/p32 Gene, C1qBP

“…Numerous reports have claimed that gC1qR and its homologue could be isolated or identified in various cellular compartments, including plasma membrane, cytoplasm, mitochondria and nucleus. gC1qR isolated from the plasma membrane was originally characterized as a high affinity C1q-binding protein (2), and later many reports showed that gC1qR could interact with several proteins of the intrinsic coagulation/bradykinin-forming cascade, including high molecular weight kininogen (3), Factor XII (4), fibrinogen (5), and multimeric vitronectin (6). Conversely, intracellular gC1qR was shown to interact and subsequently down-regulate the surface expression of the ␣ 1b -adrenergic receptor (7) as well as bind to the kinase domain of protein kinase C and thus prevent its substrate phosphorylation activity (8).…”