The structure of complement C3b provides insights into complement activation and regulation.

Area of Science:

• Immunology
• Biochemistry
• Structural Biology

Background:

• The human complement system is crucial for innate immunity, aiding pathogen elimination.
• Dysregulated complement activation contributes to inflammatory and autoimmune diseases.
• Complement component 3 (C3) is central to complement activation, generating C3b.

Purpose of the Study:

• To elucidate the molecular structure of complement component 3b (C3b).
• To understand the structural basis for C3b's activation and function in the complement cascade.
• To reveal how C3b's structure facilitates interactions with regulatory proteins and target surfaces.

Main Methods:

• X-ray crystallography was used to determine the structure of C3b.
• Comparative structural analysis of C3, C3c, and C3b was performed.
• Molecular modeling and analysis of conformational changes were employed.

Main Results:

• The C3b structure reveals significant secondary structure loss in the CUB domain.
• This structural change repositions the thioester domain, enabling conformational changes.
• The altered structure enhances accessibility of ligand-binding sites and creates a cavity shielding factor I cleavage sites.

Conclusions:

• The determined C3b structure provides a molecular basis for its activation and function.
• Structural insights explain how C3b covalently attaches to surfaces and interacts with regulatory factors.
• This work advances understanding of complement system regulation and its role in disease.