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Visualization of human C4b-binding protein and its complexes with vitamin K-dependent protein S and complement
Insights
C4b-binding protein (C4bp) regulates complement C3 convertase by binding C4b. Protein S forms a complex with C4bp, and electron microscopy reveals C4bp
Area of Science:
- Complement system biology
- Protein structure and function
- Immunology
Background:
- C4b-binding protein (C4bp) is crucial for regulating the classical complement pathway's C3 convertase.
- C4bp accelerates enzyme decay and promotes C4b degradation by factor I.
- C4bp is a large plasma protein composed of disulfide-linked subunits.
Purpose of the Study:
- To investigate the structural and functional interactions between C4bp, C4b, and protein S.
- To elucidate the binding mechanisms and stoichiometry of these protein complexes.
- To visualize the molecular architecture of C4bp and its interactions using electron microscopy.
Main Methods:
- Purification and characterization of C4bp, C4b, and protein S.
- Biochemical assays to determine binding affinities and functional effects.
- Electron microscopy with negative staining for structural visualization.
Main Results:
- C4bp forms a complex with protein S, a vitamin K-dependent protein, with distinct binding sites.
- Electron microscopy revealed C4bp's spider-like structure with seven flexible subunits radiating from a central body.
- Protein S binds to a unique short subunit of C4bp, while C4b interacts with the peripheral ends of the elongated subunits, indicating seven C4b-binding sites per C4bp molecule.
Conclusions:
- C4bp possesses a unique multi-subunit structure facilitating its regulatory role in complement.
- The distinct binding sites for C4b and protein S allow for independent or cooperative interactions.
- The structural data provides insights into the mechanism of complement regulation by C4bp and its interaction with protein S.
Abstract:
C4b-binding protein (C4bp) participates in the regulation of the C3 convertase of the classical pathway of complement. By binding to C4b, which is one of the structural subunits of this enzyme, C4bp accelerates the decay-dissociation of the enzyme and renders C4b susceptible to degradation by factor I (C3b inactivator). C4bp is a high molecular weight plasma protein (Mr = 570,000) composed of apparently identical subunits (Mr = 70,000) linked by disulfide bonds. In plasma and in purified form C4bp also forms a bimolecular complex (Kd = 0.9 X 10(-7) M) with protein S, a recently identified vitamin K-dependent plasma protein. The binding sites on C4bp for protein S and C4b are distinct and noncompetitive and protein S does not influence the function of C4bp as a regulator of the C3 convertase. C4bp, C4b, and protein S were visualized by electron microscopy by negative staining. C4bp was found to have an unusual spider-like structure. It is composed of seven thin (30 A), elongated (330 A), and flexible subunits that are linked to a small central body. Protein S exhibited two globular domains of equal size with a center-to-center distance of approximately equal to 50 A. Protein S was found to bind to the C4bp through only one of its domains by attaching to a short subunit that is distinct from the other seven subunits. C4b imaged as an irregular, relatively compact molecule. It was found to interact with the peripheral ends of the elongated subunits, suggesting seven C4b-binding sites per molecule of C4bp.