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Decay-accelerating factor (DAF), complement receptor 1 (CR1), and factor H dissociate the complement AP C3 convertase
Dennis E Hourcade1, Lynne Mitchell, Lisa A Kuttner-Kondo
1Washington University School of Medicine, Department of Medicine, Division of Rheumatology, St. Louis, Missouri 63110, USA. dhourcad@im.wustl.edu
Insights
Researchers mapped factor B (fB) sites involved in complement system regulation. Specific mutations revealed how decay accelerating factors DAF, CR1, and factor H interact with C3bBb convertase.
Area of Science:
- Immunology
- Complement System
- Protein Interactions
Background:
- The AP C3 convertase, C3bBb(Mg2+), is crucial in the complement cascade.
- Its stability is regulated by decay accelerating factors: DAF, CR1, and factor H.
- Understanding these interactions is key to modulating complement activity.
Purpose of the Study:
- To map the specific sites on factor B (fB) involved in interactions with decay accelerating factors.
- To elucidate the differential binding sites of DAF, CR1, and factor H on the C3bBb complex.
Main Methods:
- Generated a panel of factor B (fB) mutants, focusing on the type A domain.
- Assembled C3bBb complexes with mutant fB.
- Assessed decay acceleration resistance mediated by DAF, CR1, and factor H.
Main Results:
- Mutations at fB alpha helices 4 and 5 conferred resistance to DAF and CR1 but not factor H.
- Mutations at the alpha 1 helix (e.g., D254G) conferred resistance to all three regulators and increased C3b-binding.
- These findings suggest distinct interaction sites for DAF/CR1 versus factor H on C3bBb.
Conclusions:
- DAF and CR1 likely interact with C3bBb at factor B's alpha 4/5 helices.
- Factor H may interact at a different site, potentially on the C3b subunit.
- Mutations at the C3b.Bb interface disrupt regulator-mediated and spontaneous C3b dissociation from Bb.
Abstract:
The AP C3 convertase, C3bBb(Mg(2+)), is subject to irreversible dissociation (decay acceleration) by three proteins: DAF, CR1, and factor H. We have begun to map the factor B (fB) sites critical to these interactions. We generated a panel of fB mutations, focusing on the type A domain because it carries divalent cation and C3b-binding elements. C3bBb complexes were assembled with the mutants and subjected to decay acceleration. Two critical fB sites were identified with a structural model. 1) Several mutations centered at adjacent alpha helices 4 and 5 (Gln-335, Tyr-338, Ser-339, Asp-382) caused substantial resistance to DAF and CR1-mediated decay acceleration but not factor H. 2) Several mutations centered at the alpha 1 helix and adjoining loops (especially D254G) caused resistance to decay acceleration mediated by all three regulators and also increased C3b-binding affinity and C3bBb stability. In the simplest interpretation of these results, DAF and CR1 directly interact with C3bBb at alpha 4/5; factor H likely interacts at some other location, possibly on the C3b subunit. Mutations at the C3b.Bb interface interfere with the normal dissociation of C3b from Bb, whether it is spontaneous or promoted by DAF, CR1, or factor H.