Two distinct conformations of factor H regulate discrete complement-binding functions in the fluid phase and at cell...

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Factor H (FH) is a key regulator of the complement system's alternative pathway.
• FH polymorphism (Y402H) is linked to age-related macular degeneration.
• Understanding FH structure and function is vital for immunity and disease research.

Purpose of the Study:

• To elucidate the solution structures of Factor H (FH) allotypes (His-402 and Tyr-402).
• To investigate FH self-association and its role in complement regulation.
• To determine how FH structures mediate interactions with complement proteins and cell surfaces.

Main Methods:

• Analytical ultracentrifugation to study FH self-association.
• Small-angle X-ray scattering (SAXS) for structural analysis of monomeric FH.
• Mass spectrometry (MS) and surface plasmon resonance (SPR) for validation.
• Monte Carlo methods to fit SAXS data and determine atomistic FH structures.
• Crystallography to visualize FH complexes with C3b and C3dg.

Main Results:

• FH exhibits self-association (up to 12%) in solution.
• SAXS and computational modeling defined monomeric FH dimensions (Rg: 7.2-7.8 nm, length: 25 nm).
• Two distinct, accurate atomistic structures of full-length glycosylated FH were determined: one with an extended N-terminus and folded C-terminus, the other with an extended C-terminus and folded N-terminus.
• Crystal structures revealed specific FH conformations for C3b/C3d binding and glycosaminoglycan interaction.

Conclusions:

• The study provides the most accurate structures to date for full-length glycosylated FH.
• Distinct FH conformations are functionally specialized for different roles in complement regulation and host protection.
• These findings deepen the understanding of FH's multifaceted roles in immunity and its association with diseases like AMD.