Moieties of Complement iC3b Recognized by the I-domain of Integrin αXβ2

Complement fragment iC3b serves as a major opsonin for facilitating phagocytosis via its interaction with complement receptors CR3 and CR4, also known by their leukocyte integrin family names, alpha M beta 2 and alpha X beta 2, respectively. Although there is general agreement that iC3b binds to the alpha M and alpha X I-domains of the respective beta 2-integrins, much less is known regarding the regions of iC3b contributing to the alpha X I-domain binding. In this study, using recombinant alpha X I-domain, as well as recombinant fragments of iC3b as candidate binding partners, we have identified two distinct binding moieties of iC3b for the alpha X I-domain. They are the C3 convertase-generated N-terminal segment of the C3b alpha'-chain (alpha'NT) and the factor I cleavage-generated N-terminal segment in the CUBf region of alpha-chain. Additionally, we have found that the CUBf segment is a novel binding moiety of iC3b for the alpha M I-domain. The CUBf segment shows about a 2-fold higher binding activity than the alpha'NT for alpha X I-domain. We also have shown the involvement of crucial acidic residues on the iC3b side of the interface and basic residues on the I-domain side.