The receptor for C1q, gC1q-R is a ubiquitous, highly anionic cellular protein of 33 kDa that was identified by us and characterized as a molecule that binds to the globular heads of C1q (gC1q). Known alternatively as p33, and sometimes as p32 or HABP (hyaluronic acid binding protein), it is also - and in fact predominantly - a protein of the mitochondrial matrix. In addition, however, it is distributed in several other cellular compartments, including the ER, the nucleus and on the cell surface. It is its surface expression that has been the primary focus of our laboratory since gC1q-R/p33 has been shown to modulate the function of numerous extracellular ligands including viral and bacterial proteins. What is extraordinary however, is that although gC1q-R fulfills the role of a typical transmembrane protein in that it can induce cellular responses upon ligand binding, neither its crystal structure nor its amino acid sequence reveals a motif for the presence of a transmembrane segment. Furthermore, although experiments show that it does not possess a GPI (glycosylphophatidylinositol) anchor; isolated gC1q-R can rebind to cells in a specific and saturable manner. The present studies were therefore undertaken to answer the following major question: How is gC1q-R anchored to the cell surface and able to interact with extracellular ligands? To address this question four prominent peptides predicted from the crystal structure to play a role in membrane binding and/or ligand binding were synthesized, purified, and tested by competitive binding using Raji and Molt-4 cells attached to microtiter plate wells. Two of these peptides, SF-1 and SF-2 correspond to residues 144-161 and 190-202 respectively and were from the solution face (S face) of the homotrimetric doughnut-shaped protein; whereas the other two, designated MF-1 and MF-2, are from the membrane face (M face) and correspond to residues (174-180 and 212-223) of the molecule respectively. The results show that only MF-1 (n=5) inhibits the binding of biotinylated-gC1q-R in a manner that was specific and dose-dependent. This conclusion was further corroborated by experiments in which binding of gC1q-R to the cells was inhibited in the presence of anti-MF-1 antibody, but species-and isotope matched antibody. In conclusion, the results of our experiments together with the location of MF-1 in the crystal structure and its highly polar nature argue in favor of its being involved in the interaction with the cell surface, perhaps through specific interaction with particular ligand of the membrane, which is yet to be identified.

This research was supported by the Simons Foundation and by a generous gift from Larry and Sheila Dalzell.