Isolation, cDNA cloning, and overexpression of a 33-kD cell surface glycoprotein that binds to the globular "heads" of C1q

J Exp Med. 1994 Jun 1;179(6):1809-21. doi: 10.1084/jem.179.6.1809.


This work describes the functional characterization, cDNA cloning, and expression of a novel cell surface protein. This protein designated gC1q-R, was first isolated from Raji cells and was found to bind to the globular "heads" of C1q molecules, at physiological ionic strength, and also to inhibit complement-mediated lysis of sheep erythrocytes by human serum. The NH2-terminal amino acid sequence of the first 24 residues of the C1q-binding protein was determined and this information allowed the synthesis of two degenerate polymerase chain reaction primers for use in the preparation of a probe in the screening of a B cell cDNA library. The cDNA isolated, using this probe, was found to encode a pre-pro protein of 282 residues. The NH2 terminus of the protein isolated from Raji cells started at residue 74 of the predicted pre-pro sequence. The cDNA sequence shows that the purified protein has three potential N-glycosylation residues and is a highly charged, acidic molecule. Hence, its binding to C1q may be primarily but not exclusively due to ionic interactions. The "mature" protein, corresponding to amino acid residues 74-282 of the predicted pre-pro sequence, was overexpressed in Escherichia coli and was purified to homogeneity. This recombinant protein was also able to inhibit the complement-mediated lysis of sheep erythrocytes by human serum and was shown to be a tetramer by gel filtration in nondissociating conditions. Northern blot and RT-PCR studies showed that the C1q-binding protein is expressed at high levels in Raji and Daudi cell lines, at moderate levels in U937, Molt-4, and HepG2 cell lines, and at a very low level in the HL60 cell line. However, it is not expressed in the K562 cell line. Comparison of gC1q-R NH2-terminal sequence with that of the receptor for the collagen-like domain of C1q (cC1q-R) showed no similarity. Furthermore, antibodies to gC1q-R or an 18-amino acid residue-long NH2-terminal synthetic gC1q-R peptide did not cross-react with antibodies to cC1q-R. Anti-gC1q-R immunoblotted a 33-kD Raji cell membrane protein, whereas anti cC1q-R recognized a molecule of approximately 60 kD. The NH2-terminal sequence of gC1g-R appears to be displayed extracellularly since anti-gC1g-R peptide reacted with surface molecules on lymphocytes, polymorphonuclear leukocytes, and platelets, as assessed by flow cytometric and confocal laser scanning microscopic analyses.(ABSTRACT TRUNCATED AT 400 WORDS)

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Base Sequence
  • Binding Sites
  • Carrier Proteins
  • Cell Line
  • Chromatography, Affinity
  • Chromatography, Ion Exchange
  • Cloning, Molecular
  • Complement C1q / metabolism*
  • DNA Primers
  • DNA, Complementary / biosynthesis
  • DNA, Complementary / isolation & purification
  • Electrophoresis, Polyacrylamide Gel
  • Erythrocytes / physiology
  • Gene Expression
  • Hemolysis
  • Humans
  • Hyaluronan Receptors*
  • Kinetics
  • Membrane Glycoproteins / biosynthesis
  • Membrane Glycoproteins / isolation & purification
  • Membrane Glycoproteins / metabolism*
  • Mitochondrial Proteins
  • Molecular Sequence Data
  • Molecular Weight
  • Polymerase Chain Reaction
  • Receptors, Complement / biosynthesis*
  • Receptors, Complement / isolation & purification
  • Receptors, Complement / metabolism*
  • Recombinant Proteins / biosynthesis
  • Recombinant Proteins / isolation & purification
  • Recombinant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Sheep
  • Tumor Cells, Cultured


  • C1QBP protein, human
  • Carrier Proteins
  • DNA Primers
  • DNA, Complementary
  • Hyaluronan Receptors
  • Membrane Glycoproteins
  • Mitochondrial Proteins
  • Receptors, Complement
  • Recombinant Proteins
  • complement 1q receptor
  • Complement C1q

Associated data

  • GENBANK/X75913