Molecular basis for complement recognition by integrin αXβ2

Integrin alpha(X)beta(2) functions as complement receptor for iC3b and mediates recognition and phagocytosis of pathogens. We used negative-stain EM to examine the alpha(X)beta(2) interaction with iC3b. EM class averages of alpha(X)beta(2) in complex with iC3b define the binding sites on both the integrin and iC3b. iC3b contains C3c and thioester domain moieties linked by a long flexible linker. The binding site is on the key ring of the C3c moiety, at the interface between the MG3 and MG4 domains. Similar complexes are seen between alpha(X)beta(2) and the C3c fragment. alpha(X)beta(2) binds through the aX aI domain, on the face known to bear the metal ion-dependent adhesion site, at the opposite end of the aI domain from its site of insertion in the beta-propeller domain.