Inhibition of C9 polymerization within the SC5b-9 complex of complement by S-protein

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• The complement system is crucial for innate and adaptive immunity.
• The membrane attack complex (MAC) is a key effector of complement-mediated cell lysis.
• Understanding the assembly and regulation of MAC is vital for comprehending immune responses.

Purpose of the Study:

• To investigate the role of S-protein in the polymerization of C9 during C5b-9 complex assembly.
• To elucidate the structural differences between SC5b-9 and the MAC.
• To determine the mechanism by which S-protein regulates C9 polymerization.

Main Methods:

• SDS-PAGE to quantify tubular poly C9.
• Electron microscopy to visualize SC5b-9 structure.
• Biochemical assays using biotinylated S-protein and avidin-coated gold particles.

Main Results:

• Poly C9 formation was observed during C5b-9 assembly.
• S-protein addition dose-dependently blocked poly C9 formation, yielding SC5b-9.
• SC5b-9 appeared as a wedge-shaped structure lacking tubular poly C9, with S-protein located at the wide end.
• S-protein was found to block the membrane site of C5b-7 and inhibit C9 polymerization by SC5b-8.

Conclusions:

• S-protein possesses a dual function in SC5b-9 assembly: blocking the C5b-7 membrane site and inhibiting C9 polymerization.
• The absence of tubular poly C9 is the primary structural distinction between SC5b-9 and the MAC.
• S-protein acts as a critical regulator in the complement cascade, modulating MAC formation.