The C-terminal domain of Staphylococcus aureus Efb recruits FHR-2 to C3b, synergistically inhibiting the terminal...

Area of Science:

• Immunology and Microbiology
• Complement System
• Protein-Protein Interactions

Background:

• Extracellular fibrinogen-binding protein (Efb) from Staphylococcus aureus inhibits complement activation.
• Efb's C-terminal domain (Efb-C) binds C3b's thioester-containing domain (TED/C3d), blocking C3 proconvertase formation.
• The functional consequences of Efb-C binding to C3b beyond this initial blockade are not fully understood.

Purpose of the Study:

• To investigate the interaction between Efb-C, C3b, and the complement regulatory protein FHR2 (factor H-related protein 2).
• To determine how Efb-C influences the binding of FHR2 to C3b.
• To elucidate the functional impact of the Efb-C/C3b/FHR2 complex on complement activation.

Main Methods:

• X-ray crystallography to study the structure of the Efb-C/C3b/FHR2 complex.
• Binding assays to assess the interaction between Efb-C, C3b, and FHR2 (full-length and FHR2[3-4]).
• Functional assays to evaluate complement activation rates, decay acceleration, cofactor activity, and downstream inhibition.

Main Results:

• A novel interaction was identified between Efb-C, C3b, and FHR2.
• Efb-C significantly enhances the binding of FHR2 (full-length and FHR2[3-4]) to C3b, likely by increasing the accessibility of the FHR2-binding site.
• Efb-C and FHR2 exhibit potent, synergistic inhibition of complement activation downstream of C5, an effect not observed with Efb-C and FHR2[3-4].

Conclusions:

• Efb-C binding to C3b has additional inhibitory effects on the complement system beyond blocking C3 proconvertase formation.
• Efb-C modulates the interaction between C3b and FHR2, leading to enhanced FHR2 binding.
• The combined action of Efb-C and full-length FHR2 provides potent synergistic inhibition of late-stage complement activation.