Analysis of binding sites on complement factor I that are required for its activity

Area of Science:

• Immunology
• Biochemistry

Background:

• Factor I (FI) is a central serine protease inhibitor of the complement system.
• FI degrades activated complement fragments C4b and C3b using various cofactors.
• FI comprises a light chain (serine protease domain) and a heavy chain (FIMAC, CD5, LDLr1, LDLr2 domains).

Purpose of the Study:

• To elucidate the functional role of Factor I domains in complement inhibition.
• To identify specific binding sites within FI responsible for C4b and C3b degradation.

Main Methods:

• Homology modeling was used to predict potential binding sites in FI domains.
• Site-directed mutagenesis was employed to create 16 FI mutants.
• Functional analyses and binding assays were performed on purified FI mutants.

Main Results:

• Mutations in the FIMAC domain significantly impaired FI's ability to degrade C4b and C3b.
• Mutations in CD5 and LDLr1/2 domains showed a lesser effect on degradation.
• Mutants with impaired degradation also exhibited reduced binding to C3met.

Conclusions:

• The FIMAC domain of Factor I contains critical binding sites for C4b and C3b degradation.
• FI's inhibitory function is primarily mediated through its FIMAC domain.