Structural requirements for the intracellular subunit polymerization of the complement inhibitor C4b-binding protein

Biochemistry. 2002 Jul 30;41(30):9349-57. doi: 10.1021/bi025980+.


C4b-binding protein (C4BP), an important inhibitor of complement activation, has a unique spider-like shape. It is composed of six to seven identical alpha-chains with or without a single beta-chain, the chains being linked by disulfide bridges in their C-terminal parts. To elucidate the structural requirements for the assembly of the alpha-chains, recombinant C4BP was expressed in HEK 293 cells. The expressed C4BP was found to contain six disulfide-linked alpha-chains. Pulse-chase analysis demonstrated that the recombinant C4BP was rapidly synthesized in the cells and the polymerized C4BP appeared in the medium after 40 min. The alpha-chains were polymerized in the endoplasmic reticulum (ER) already after 5 min chase. The polymerization process was unaffected by blockage of the transport from the ER to the Golgi mediated by brefeldin A or low temperature (10 degrees C). The C-terminal part of the alpha-chain (57 amino acids), containing 2 cysteine residues and an amphiphatic alpha-helix region, was required for the polymerization. We constructed and expressed several mutants of C4BP that lacked the cysteine residues and/or were truncated at various positions in the C-terminal region. Gel filtration analysis of these variants demonstrated the whole alpha-helix region to be required for the formation of stable polymers of C4BP, which were further stabilized by the formation of disulfide bonds.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Biopolymers / chemistry*
  • Cell Line
  • Chymotrypsin / metabolism
  • Complement Inactivator Proteins*
  • DNA Primers
  • Glycoproteins*
  • Humans
  • Hydrolysis
  • Protein Conformation
  • Receptors, Complement / chemistry*
  • Receptors, Complement / genetics
  • Receptors, Complement / isolation & purification
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / isolation & purification
  • Spectroscopy, Fourier Transform Infrared


  • Biopolymers
  • Complement Inactivator Proteins
  • DNA Primers
  • Glycoproteins
  • Receptors, Complement
  • Recombinant Proteins
  • Chymotrypsin