Modular organization of the carboxyl-terminal, globular head region of human C1q A, B, and C chains.
The first step in the activation of the classical complement pathway, by immune complexes, involves the binding of the globular heads of C1q to the Fc regions of aggregated IgG or IgM. Located C-terminal to the collagen region, each globular head is composed of the C-terminal halves of one A (ghA), one B (ghB), and one C chain (ghC). To dissect their structural and functional autonomy, we have expressed ghA, ghB, and ghC in Escherichia coli as soluble proteins linked to maltose-binding protein (MBP). The affinity-purified fusion proteins (MBP-ghA, -ghB, and -ghC) bound differentially to heat-aggregated IgG and IgM, and also to three known C1q-binding peptides, derived from HIV-1, HTLV-I, and beta-amyloid. In the ELISAs, the MBP-ghA bound to heat-aggregated IgG and IgM as well as to the HIV-1 gp41 peptide; the MBP-ghB bound preferentially to IgG rather than IgM, in addition to binding beta-amyloid peptide, whereas the MBP-ghC showed a preference for IgM and the HTLV-I gp21 peptide. Both MBP-ghA and MBP-ghB also inhibited C1q-dependent hemolysis of IgG- and IgM-sensitized sheep erythrocytes. However, for IgM-coated erythrocytes, MBP-ghC was a better inhibitor of C1q than MBP-ghB. The recombinant forms of ghA, ghB, and ghC also bound specifically to apoptotic PBMCs. We conclude that the C1q globular head region is likely to have a modular organization, being composed of three structurally and functionally independent modules, which retains multivalency in the form of a heterotrimer. The heterotrimeric organization thus offers functional flexibility and versatility to the whole C1q molecule.
Kishore
Uday
U
Medical Research Council Immunochemistry Unit, Department of Biochemistry, University of Oxford, Oxford, United Kingdom. ukishore@hammer.imm.ox.ac.uk
Gupta
Sanjeev K
SK
Perdikoulis
Michael V
MV
Kojouharova
Mihaela S
MS
Urban
Britta C
BC
Reid
Kenneth B M
KB
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D016229:Amyloid beta-Peptides / Q000378:metabolism; D017209:Apoptosis / Q000276:immunology; D002352:Carrier Proteins / Q000235:genetics; D015922:Complement C1q / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D003169:Complement Inactivator Proteins / Q000737:chemistry; D015686:Gene Products, env / Q000378:metabolism; D015700:HIV Envelope Protein gp41 / Q000378:metabolism; D006461:Hemolysis / Q000276:immunology; D015368:Human T-lymphotropic virus 1; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D007075:Immunoglobulin M / Q000378:metabolism; D058619:Maltose-Binding Proteins; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding / Q000276:immunology; D017434:Protein Structure, Tertiary / Q000235:genetics; D021122:Protein Subunits / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000235:genetics; D015682:Retroviridae Proteins, Oncogenic / Q000378:metabolism; D012995:Solubility; D054299:env Gene Products, Human Immunodeficiency Virus
D016229:Amyloid beta-Peptides; D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; D015686:Gene Products, env; D015700:HIV Envelope Protein gp41; D007074:Immunoglobulin G; D007075:Immunoglobulin M; D058619:Maltose-Binding Proteins; D010446:Peptide Fragments; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; D015682:Retroviridae Proteins, Oncogenic; D054299:env Gene Products, Human Immunodeficiency Virus; C055265:gp21 protein, Human T-lymphotropic virus 1; D015922:Complement C1q