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Updated: Sep 13, 2025

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

Tereza Kadavá1, Jürgen Strasser2, Maryam Marefat2

  • 1Biomolecular Mass Spectrometry and Proteomics, Bijvoet Center for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, University of Utrecht, Utrecht, the Netherlands; Netherlands Proteomics Center, Utrecht, the Netherlands.

Molecular & Cellular Proteomics : MCP
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Summary

C4b-binding protein (C4BP) forms unique spider-like structures. These structures interact differently with complement protein C4b and serum amyloid P component (SAP), revealing distinct binding mechanisms for immune regulation.

Keywords:
C4bC4b-binding proteincomplement regulationcross-linking mass spectrometryhigh-speed atomic force microscopymass photometryserum amyloid P component

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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.