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Distal recognition site for classical pathway convertase located in the C345C/netrin module of complement component

A Sandoval1, R Ai, J M Ostresh

  • 1Torrey Pines Institute for Molecular Studies, San Diego, CA 92121, USA.

Journal of Immunology (Baltimore, Md. : 1950)
|July 6, 2000
PubMed
Summary

Researchers identified a novel binding site on complement component 5 (C5) critical for classical pathway (CP) activation. This KYNFSF sequence is essential for CP convertase activity but not for cobra venom factor (CVF) convertase.

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Area of Science:

  • Biochemistry
  • Immunology
  • Molecular Biology

Background:

  • Previous studies identified potential protein-protein interaction sites in complement C3 and C5.
  • Indels in the complement C345 protein family provided initial clues to these interaction sites.

Purpose of the Study:

  • To investigate a specific protein-protein interaction site near the C terminus of the C5 alpha-chain.
  • To determine the role of the KYNFSF sequence in complement C5 convertase activity.

Main Methods:

  • Alanine-scanning mutagenesis was performed on 16 residues within the KEALQIKYNFSF RYIYPLD sequence of C5.
  • Mutant C5 activities were assessed for classical pathway (CP) and cobra venom factor (CVF)-dependent convertase activation.
  • A synthetic peptide corresponding to the KYNFSF sequence was used to validate findings.

Main Results:

  • Alanine substitutions in the KYNFSF sequence significantly reduced C5 activity, with K, Y, and F residues having the most pronounced effects.
  • Mutants with reduced activity were resistant to CP C5 convertase but showed normal susceptibility to CVF-dependent convertase.
  • The synthetic peptide mimicked these effects, inhibiting CP but not CVF convertase activation.

Conclusions:

  • The KYNFSF sequence represents a novel, distal binding site for the classical pathway (CP) C5 convertase.
  • This binding site is specific to CP activation and is not involved in CVF-dependent convertase activity.
  • The identified module (C345C/NTR) containing this site is found in various proteins, suggesting broader functional relevance.