biostudies-literatureKadava TDutch Research Council (NWO)101046https://www.ebi.ac.uk/biostudies/studies/S-EPMC12419116biostudies-literatureUnknown24(9)C4b-binding protein (C4BP) is an innate immune inhibitor found in serum. Human C4BP adopts spider-like higher-order structures (HOS) formed by disulfide-linked C4BPα and C4BPβ chains that non-covalently bind vitamin K-dependent protein S (ProS). These spider-like structures can form even larger complexes as C4BP interacts with other, mostly complement-related, proteins. The complement inhibitory role of C4BP is primarily mediated through its interaction with C4b. C4BP also binds with high affinity to serum amyloid P component (SAP), a pentraxin family member associated with amyloidosis conditions. Here, we structurally and compositionally characterize C4BP interactions with these two natively occurring binders. To achieve this, we combine mass photometry, high-speed atomic force microscopyMolecular & cellular proteomics : MCPStructural Insights Into Complement Inhibition: Visualizing Distinct Binding Modes of C4b-Binding Protein Complexes With C4b and SAP.PMC12419116SPI.2017.028Strasser JHeck AJRKadava TYin VCPreiner JTrouw LAMarefat MfalseStructural Insights Into Complement Inhibition: Visualizing Distinct Binding Modes of C4b-Binding Protein Complexes With C4b and SAP.C4b-binding protein (C4BP) is an innate immune inhibitor found in serum. Human C4BP adopts spider-like higher-order structures (HOS) formed by disulfide-linked C4BPα and C4BPβ chains that non-covalently bind vitamin K-dependent protein S (ProS). These spider-like structures can form even larger complexes as C4BP interacts with other, mostly complement-related, proteins. The complement inhibitory role of C4BP is primarily mediated through its interaction with C4b. C4BP also binds with high affinity to serum amyloid P component (SAP), a pentraxin family member associated with amyloidosis conditions. Here, we structurally and compositionally characterize C4BP interactions with these two natively occurring binders. To achieve this, we combine mass photometry, high-speed atomic force microscopy2025-01-01T00:00:00Z2025 Aug2026-06-02T17:45:44.705Z2026-05-27T03:07:42.691ZS-EPMC124191164075414010.1016/j.mcpro.2025.101046