Structural Insights Into Complement Inhibition: Visualizing Distinct Binding Modes of C4b-Binding Protein Complexes...

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• C4b-binding protein (C4BP) is a key innate immune regulator in serum.
• Human C4BP forms complex spider-like higher-order structures (HOS) via disulfide-linked C4BPα and C4BPβ chains, binding protein S.
• C4BP HOS can assemble into larger complexes with other proteins, influencing its function.

Purpose of the Study:

• To structurally and compositionally characterize the interactions of C4BP with its native binders, C4b and serum amyloid P component (SAP).
• To elucidate the distinct binding modes of C4BP when interacting with C4b and SAP.

Main Methods:

• Mass photometry
• High-speed atomic force microscopy
• Cross-linking mass spectrometry

Main Results:

• Two distinct binding modes for C4BP were identified when bound to C4b versus SAP.
• C4b binds to the N-terminal region of a single C4BPα leg, allowing multiple C4b molecules to attach to the C4BP HOS.
• SAP binds to the C4BPα-C4BPβ oligomerization core, with C4BPα legs wrapping around it, engaging the entire spider-like HOS.

Conclusions:

• C4BP exhibits differential binding strategies for C4b and SAP, crucial for its complement inhibitory and other roles.
• Understanding these distinct binding modes provides insights into the structural basis of C4BP function in innate immunity and associated conditions like amyloidosis.