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Identification of three physically and functionally distinct binding sites for C3b in human complement factor H by
1Department of Biochemistry, University of Texas Health Science Center, Tyler 75710-2003, USA.
Summary
Human complement factor H has multiple binding sites for C3b, with distinct roles in controlling complement activation. These findings clarify factor H
Area of Science:
- Immunology
- Complement System Biology
Background:
- Human complement factor H regulates complement activation and distinguishes host cells.
- Factor H is a large protein with 20 short consensus repeat (SCR) domains.
- The biological roles of domains beyond the known C3b binding site (SCR 1-4) are significant.
Purpose of the Study:
- To investigate the functional roles of different domains of human complement factor H.
- To identify and characterize additional C3b and heparin binding sites on factor H.
- To elucidate the contribution of these sites to complement regulation.
Main Methods:
- Full-length human factor H cDNA was expressed in the baculovirus system.
- Recombinant factor H and its mutants were purified.
- Cofactor activity, C3b binding, and heparin binding were assessed.
Main Results:
- Factor H possesses at least three distinct C3b binding sites: in SCR 1-4, SCR 6-10, and SCR 16-20.
- A second heparin binding site was identified in the SCR 6-10 region, alongside one near SCR 13.
- Only the SCR 1-4 C3b binding site exhibited factor I cofactor activity.
- Mutants lacking individual C3b binding sites showed 6- to 8-fold reduced affinity for C3b on erythrocytes.
Conclusions:
- Multiple C3b binding sites on factor H contribute to complement control on erythrocytes.
- The distinct binding sites explain previously observed behaviors of factor H, such as heterogeneous binding and differential regulation.
- This research clarifies the complex functional landscape of complement factor H.