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Importance of the alpha 3-fragment of complement C4 for the binding with C4b-binding protein
M Hessing1, C van 't Veer, T M Hackeng
1Department of Biology, Faculty of Science, Kyushu University, Fukuoka, Japan.
Insights
Researchers identified the specific region on complement component C4b that binds to C4b-binding protein (C4BP). This finding is crucial for understanding the regulation of the complement system and developing targeted therapies.
Area of Science:
- Immunology
- Biochemistry
Background:
- C4b-binding protein (C4BP) regulates the classical complement pathway.
- C4BP acts as a cofactor for factor I in C4b degradation and accelerates C4b2a complex decay.
- Previous studies showed that antibodies against C4b's alpha'-chain inhibit C4b-C4BP binding.
Purpose of the Study:
- To pinpoint the exact structural domain of C4b responsible for binding to C4BP.
- To elucidate the molecular interactions governing complement regulation.
Main Methods:
- Generation of proteolytic fragments of C4 using trypsin and Staphylococcus aureus V8 protease.
- Analysis of fragments using SDS-PAGE, immunoblotting, and amino acid sequencing.
- Utilized monoclonal antibodies against C4b to identify binding sites.
Main Results:
- Identified specific residues Ala738-Arg826 within the alpha 3-fragment of C4b as critical for C4BP interaction.
- Demonstrated that this region is essential for the binding of C4b to C4BP.
Conclusions:
- The alpha 3-fragment, specifically residues Ala738-Arg826, is the key binding site for C4BP on C4b.
- This precise localization advances our understanding of complement system regulation by C4BP.
Abstract:
The human regulatory complement component C4b-binding protein (C4BP) is a multimeric plasma protein, which regulates the classical pathway of the complement system. C4BP functions as a cofactor to factor 1 in the degradation of C4b and accelerates the decay rate of the C4b2a complex. Previously, we have demonstrated that monoclonal antibodies (C4-2 and 9) directed against the alpha'-chain of C4b inhibit the binding of C4b to C4BP. In order to identify the structural domain of C4b that binds C4BP, proteolytic fragments of C4 were generated with trypsin and Staphylococcus aureus V8 protease. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting and amino acid sequence analysis of the proteolytic fragments reactive with the anti-C4 mAb's revealed that the residues Ala738-Arg826 of the alpha 3-fragment of C4b are important for the interaction with C4BP.