The fourth blade within the beta-propeller is involved specifically in C3bi recognition by integrin alpha M beta 2

Interactions between the complement degradation product C3bi and leukocyte integrin alpha M beta 2 are critical to phagocytosis of opsonized particles in host defense against foreign pathogens and certain malignant cells. Previous studies have mapped critical residues for C3bi binding to the I-domains of the alpha M and the beta2 subunits. However, the role of the alpha M beta-propeller in ligand binding remains less well defined, and the functional residues are still unknown. In the present study, we studied the function of the alpha M beta-propeller in specific ligand recognition by alpha M beta 2 using a number of different approaches, and we report four major findings. 1) Substitution of five individual segments (Asp398-Ala402, Leu412-Leu419, Tyr426-Met434, Phe435-Glu443, and Ser444-Thr451) within the W4 blade of the beta-propeller with their homologous counterparts in integrin alpha2 abrogated C3bi binding, whereas substitution of eight other segments outside this blade had no effect. 2) These five mutants defective in C3bi binding supported strong alpha M beta 2-mediated and cation-dependent cell adhesion to fibrinogen, suggesting that the conformations of these five defective mutants were intact. 3) Polyclonal antibodies recognizing sequences within the W4 blade significantly blocked C3bi binding by wild-type alpha M beta 2. 4) A synthetic peptide corresponding to Gln424-Gly440 within W4 interacted directly with C3bi. In conclusion, our data demonstrate that the W4 blade (residues Asp398 to Thr451) is involved specifically in C3bi but not fibrinogen binding to alpha M beta 2. Altogether, our study supports a model in which three separate domains of alpha M beta 2 (the alpha MI-domain, the alpha M beta-propeller, and the beta 2I-domain) function together and contribute to the formation of the C3bi-binding site.