A structurally dynamic N-terminal helix is a key functional determinant in staphylococcal complement inhibitor (SCIN)...

Area of Science:

• Immunology
• Structural Biology
• Microbial Pathogenesis

Background:

• The complement system is crucial for innate immunity, identifying and clearing microbial invaders.
• Staphylococcus aureus employs secreted proteins, including Staphylococcal complement inhibitors (SCINs), to evade complement-mediated attack.
• SCINs target the alternative pathway C3 convertase, a critical complex involving C3b.

Purpose of the Study:

• To elucidate the structural basis for the reduced binding affinity of N-terminal truncation mutants of SCINs to C3b.
• To investigate the functional importance and context-dependency of the N-terminal SCIN region.
• To understand how SCINs contribute to Staphylococcus aureus immune evasion strategies.

Main Methods:

• Crystallographic studies and Nuclear Magnetic Resonance (NMR) dynamics of full-length SCINs.
• C3b binding assays and functional experiments.
• Surface Plasmon Resonance (SPR) to characterize inhibitor-enzyme-substrate complexes.

Main Results:

• Full-length SCINs possess conformationally dynamic N-terminal helical motifs.
• The N-terminal region of SCINs is functionally critical and its importance is context-dependent.
• SCINs form inhibitor·enzyme·substrate complexes ((SCIN·C3bBb)·C3) that block complement receptor interaction with C3b, directly inhibiting phagocytosis.

Conclusions:

• SCINs utilize dynamic N-terminal domains to effectively inhibit the complement cascade.
• The formation of ternary complexes provides a dual mechanism of immune evasion: convertase inhibition and phagocytosis blocking.
• Tethering multi-host protein complexes by small bacterial inhibitors represents a potential broad immune evasion strategy by Staphylococcus aureus.