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The structure of OMCI, a novel lipocalin inhibitor of the complement system

Pietro Roversi1, Olga Lissina, Steven Johnson

  • 1Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, England, UK.

Journal of Molecular Biology
|April 21, 2007
PubMed

Insights

The complement system protein OmCI from ticks binds C5, preventing its activation and parasite defense. This protein, OmCI, may also sequester inflammatory fatty acids, dampening host responses to tick bites.

Area of Science:

  • Immunology
  • Parasitology
  • Structural Biology

Background:

  • The complement (C) system is crucial for innate immunity against parasites.
  • Tick saliva contains proteins that modulate host immune responses.

Purpose of the Study:

  • To characterize OmCI, a tick protein that inhibits complement activation.
  • To elucidate the mechanism by which OmCI interferes with the complement system.

Main Methods:

  • Protein expression and purification of OmCI.
  • X-ray crystallography to determine OmCI structure.
  • Mass spectrometry to identify bound molecules.
  • Biochemical assays to assess complement inhibition.

Main Results:

  • OmCI, a 16 kDa protein from Ornithodoros moubata, specifically binds complement C5.
  • OmCI possesses a lipocalin fold and binds ricinoleic acid, a fatty acid derivative.
  • OmCI prevents C5 convertase interaction, rather than blocking the C5a cleavage site.

Conclusions:

  • OmCI inhibits complement activation by preventing C5 interaction with the C5 convertase.
  • OmCI may modulate host inflammatory responses by sequestering fatty acid derivatives.
  • OmCI represents a novel strategy employed by ticks to evade host immunity.

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