title: Evidence that the two C1q binding membrane proteins, gC1q-R and cC1q-R, associate to form a complex.
abstract: Two types of widely coexpressed, highly acidic, cell membrane binding proteins that display preferential domain specificity for C1q have been described: a 60-kDa calreticulin homologue, designated cC1q-R, that binds to the collagen-like "stalk" and a 33-kDa glycoprotein with affinity for the globular "heads" (gC1q-R). Although the two molecules are known to be coexpressed on all cell types examined to date and often coelute during purification, there is no direct evidence showing that they associate with each other either on the membrane or when examined in a purified system. In this report we present the first evidence that 1) biotinylated cC1q-R binds to recombinant as well as native gC1q-R, as assessed by solid phase ELISA; 2) binding sites for cC1q-R are located within N-terminal residues 76 through 93 of the mature form of gC1q-R and within residues 204 through 218; 3) this interaction is inhibited by two mAbs, 60.11 and 46.23, that recognize primarily epitopes within the N terminus of gC1q-R corresponding to residues 74 through 96 and by mAb 74.5.2 that recognizes epitopes within residues 204 through 218; and 4) biotinylated cC1q-R binds to microtiter-fixed Raji and K562 cells, and this interaction is inhibited by mAb 60.11. Furthermore, coimmunoprecipitation analysis of Raji cell membranes with anti-gC1q-R mAbs showed the presence of cC1q-R in addition to gC1q-R. Taken together, the evidence suggests that cC1q-R is able to form a complex with gC1q-R and may associate with gC1q-R on the cell surface.
authors.affiliation: Department of Medicine, State University of New York, Stony Brook 11794, USA. berhane@epo.som.sunysb.edu
authors.forename: B
authors.initials: B
authors.lastname: Ghebrehiwet
authors.forename: P D
authors.initials: PD
authors.lastname: Lu
authors.forename: W
authors.initials: W
authors.lastname: Zhang
authors.forename: S A
authors.initials: SA
authors.lastname: Keilbaugh
authors.forename: L E
authors.initials: LE
authors.lastname: Leigh
authors.forename: P
authors.initials: P
authors.lastname: Eggleton
authors.forename: K B
authors.initials: KB
authors.lastname: Reid
authors.forename: E I
authors.initials: EI
authors.lastname: Peerschke
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D001667:Binding, Competitive / Q000276:immunology; D001704:Biopolymers / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D001710:Biotin / Q000378:metabolism; D002352:Carrier Proteins; D015922:Complement C1q / Q000378:metabolism*; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D018960:Hyaluronan Receptors; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D024101:Mitochondrial Proteins; D008969:Molecular Sequence Data; D010716:Phosphatidylinositols / Q000378:metabolism; D011485:Protein Binding / Q000276:immunology; D011951:Receptors, Complement / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D013347:Subcellular Fractions / Q000276:immunology / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D000911:Antibodies, Monoclonal; D001704:Biopolymers; C087299:C1QBP protein, human; D002352:Carrier Proteins; D018960:Hyaluronan Receptors; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D010716:Phosphatidylinositols; D011951:Receptors, Complement; C032402:complement 1q receptor; D001710:Biotin; D015922:Complement C1q