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The phosphocholine and the polycation-binding sites on rabbit C-reactive protein are structurally and functionally
Steven Black1, Alok Agrawal, David Samols
1Department of Biochemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA.
Molecular Immunology
|May 17, 2003
Summary
Transgenic mice revealed distinct binding sites for phosphocholine and polycations on C-reactive protein (CRP). These sites influence CRP's ability to activate the complement system, crucial for immune responses.
Area of Science:
- Immunology
- Biochemistry
- Molecular Biology
Background:
- C-reactive protein (CRP) is a key acute phase protein involved in innate immunity.
- CRP binds various ligands, including phosphocholine (PCh), histones, and polycations.
- Ligand-bound CRP can activate the classical complement pathway, a critical part of the immune system.
Purpose of the Study:
- To investigate the distinct binding sites of CRP for PCh and polycations.
- To understand how these binding interactions affect complement activation.
- To elucidate the structural basis of CRP-mediated complement activation.
Main Methods:
- Generation of transgenic mice expressing variant forms of rabbit CRP (rbCRP) with specific mutations (F66Y/E81K and Y175A).
- In vitro analysis of rbCRP variants' binding affinities to PCh, polycations, and histones.
- Assessment of complement activation by rbCRP variants using pneumococcal C-polysaccharide (PnC) and lysine polymers.
- Measurement of C1q binding to immobilized rbCRP variants.
Main Results:
- The F66Y/E81K rbCRP variant showed significantly reduced PCh binding but retained and enhanced polycation and histone binding.
- The Y175A rbCRP variant could not activate complement with PnC but did so with lysine polymers or when adsorbed to surfaces.
- All tested rbCRP forms (wild type, F66Y/E81K, Y175A) bound C1q when adsorbed to a solid phase, indicating classical pathway activation.
- CRP-mediated complement activation is dependent on the nature and location of the bound ligand.
Conclusions:
- The PCh-binding site and the polycation-binding site on rbCRP are distinct, though potentially overlapping.
- Ligand-specific conformational changes in CRP are critical for initiating complement activation via the classical pathway.
- Mutagenesis studies provide insights into the structure-function relationship of CRP in immune complex recognition and complement engagement.