A revised mechanism for the activation of complement C3 to C3b: a molecular explanation of a disease-associated...

Area of Science:

• Immunology
• Biochemistry
• Structural Biology

Background:

• The complement system is a critical part of innate immunity, involved in host defense and inflammation.
• Complement C3b is a central protein in complement activation, and its structure dictates its function.
• Understanding C3b's conformational dynamics is key to comprehending complement regulation and associated diseases.

Purpose of the Study:

• To elucidate the solution structure and conformational variability of complement C3b and its analogs (C3u, C3d).
• To investigate the role of the Arg(102)-Glu(1032) salt bridge in C3b structure and function.
• To explain the clinical associations of different C3b allotypes.

Main Methods:

• Analytical ultracentrifugation and X-ray/neutron scattering were employed to study C3, C3b, C3u, C3c, and C3d.
• Atomistic scattering modeling was used to determine structures in different salt concentrations.
• Surface plasmon resonance was utilized to assess the binding affinity of C3d variants to C3c.

Main Results:

• C3b and C3u adopt compact structures in low salt (50 mM NaCl) and extended structures in physiological salt (137 mM NaCl).
• The Arg(102)-Glu(1032) salt bridge connects the thioester-containing domain (TED) and macroglobulin 1 (MG1) domains in compact C3b.
• Disruption of this salt bridge abolished C3d binding to C3c, highlighting its functional importance.
• The conformational flexibility of TED in C3b suggests higher reactivity than previously recognized.
• The study experimentally explains the clinical relevance of C3S (Arg(102)) and C3F (Gly(102)) allotypes.

Conclusions:

• Complement C3b exhibits significant conformational plasticity, adapting its structure based on ionic strength.
• The Arg(102)-Glu(1032) salt bridge is crucial for maintaining C3b structure and its interaction with Factor H.
• The findings provide a structural basis for the differential reactivity of C3b and explain allotype-associated disease risks.