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Prediction from sequence comparisons of residues of factor H involved in the interaction with complement component
1Department of Biochemistry, University of Oxford, U.K.
Insights
Researchers identified the bovine factor H region binding C3b. This protein is crucial for regulating complement system activity, with specific domains interacting with C3b and factor I.
Area of Science:
- Biochemistry
- Immunology
- Molecular Biology
Background:
- Factor H is a key regulator of the complement system.
- Understanding Factor H's structure and function is vital for immune response research.
Purpose of the Study:
- To derive the amino acid sequence of bovine factor H, focusing on the C3b binding site.
- To compare bovine factor H with human and mouse counterparts.
- To identify key residues involved in C3b binding and factor I cofactor activity.
Main Methods:
- Sequencing of overlapping cDNA clones to determine amino acid sequence.
- Analysis of conserved complement protein (CP) modules.
- Multiple sequence alignments of homologous Factor H proteins.
- Structure-based prediction of residue function.
Main Results:
- A cDNA sequence encoding 669 amino acids of bovine factor H was obtained, comprising CPs 2-12.
- Bovine factor H demonstrated binding to human C3(NH3) and cofactor activity with factor I.
- Conserved residues in CPs 2-4 were identified, suggesting their importance in C3b interaction and orientation.
Conclusions:
- Bovine factor H shares structural and functional similarities with human and mouse Factor H.
- CPs 3 and 4 are likely directly involved in C3b binding, while CPs 2 and 5 facilitate proper orientation.
- This study provides insights into the molecular mechanisms of complement regulation.
Abstract:
The amino acid sequence of the region of bovine factor H containing the C3b binding site has been derived from sequencing overlapping cDNA clones. A cDNA sequence encoding 669 amino acids was obtained. Like human and mouse factor H the sequence can be arranged into a number of internally homologous units (CPs), each of which is about 60 amino acids long and is based on a framework of four conserved cysteine residues. Bovine factor H is of the same molecular mass as human and mouse factor H, and is therefore likely to be composed of 20 contiguous CPs. Comparisons with human and mouse factor H indicate that the partial bovine sequence encodes CPs 2-12 inclusive of bovine factor H. Bovine factor H binds to human ammonia-treated C3 (causing thiolester cleavage) [C3(NH3)] and promotes the cleavage of human C3(NH3) in the presence of bovine factor I. Other studies indicate that CPs 2-5 of human factor H encompass the C3b binding and factor I cofactor activity site. Multiple sequence alignments of human factor H, mouse factor H (which also interacts with human C3b) and bovine factor H with CP modules whose structures have been determined experimentally, have been used to predict residues in the hypervariable loops of CPs 2-5 and to identify residues of potential importance in human C3 binding and factor I cofactor activity. Leu-17 and Gly-20 of CP 2, Ser-17, Ala-19, Glu-21, Asp-23 and Glu-25 of CP 3 and Lys-18 of CP 4 are all conserved between the three species. It may be that CPs 3 and 4 interact with C3(NH3) directly, whilst CPs 2 and 5 maintain the correct orientation for CPs 3 and 4 to interact.