Apr 16, 2020

Complement receptor 3 forms a compact high affinity complex with iC3b

BioRxiv : the Preprint Server for Biology
R. K. JensenGregers Rom Andersen

Abstract

Complement receptor 3 (CR3, also known as Mac-1, integrin M2, or CD11b/CD18) is expressed on a subset of myeloid and certain activated lymphoid cells. CR3 is essential for the phagocytosis of complement-opsonized particles such as pathogens and apoptotic or necrotic cells. The receptor recognizes cells opsonized with the complement fragment iC3b and to a lesser extend C3dg. While the interaction between the iC3b thioester domain and the ligand binding CR3 M I-domain is now structurally well characterized, additional CR3-iC3b interactions lack structural insight. Using an integrated structural biology approach, we analyze the interaction between iC3b and the headpiece fragment of the CR3 ectodomain. Surface plasmon resonance experiments found an affinity of 30 nM of CR3 for iC3b compared to 515 nM for the iC3b thioester domain. The iC3b1 intermediate formed during factor I degradation is shown to be a CR3 headpiece ligand in addition to iC3b and C3dg. Small angle x-ray scattering analysis reveals that in solution the iC3b-CR3 complex is more compact than either of the individual proteins and prior models of the complex derived by electron microscopy. Overall, the data suggest that the iC3b-CR3 complex is structurally ordered and...Continue Reading

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