Multiple Binding Sites in Fibrinogen for Integrin αMβ2 (Mac-1)*

The leukocyte integrin αMβ2 (Mac-1) is a multiligand receptor that mediates a range of adhesive reactions of leukocytes during the inflammatory response. This integrin binds the coagulation protein fibrinogen providing a key link between thrombosis and inflammation. However, the mechanism by which αMβ2 binds fibrinogen remains unknown. Previous studies indicated that a model in which two fibrinogen γC domain sequences, P1 (γ190–202) and P2 (γ377–395), serve as the αMβ2 binding sites cannot fully account for recognition of fibrinogen by integrin. Here, using surface plasmon resonance, we examined the interaction of the ligand binding αMI-domain of αMβ2 with the D fragment of fibrinogen and showed that this ligand is capable of associating with several αMI-domain molecules. To localize the alternative αMI-domain binding sites, we screened peptide libraries covering the complete sequences of the γC and βC domains, comprising the majority of the D fragment structure, for αMI-domain binding. In addition to the P2 and P1 peptides, the αMI-domain bound to many other sequences in the γC and βC scans. Similar to P1 and P2, synthetic peptides derived from γC and βC were efficient inhibitors of αMβ2-mediated cell adhesion and were able to directly support adhesion suggesting that they contain identical recognition information. Analyses of recognition specificity using substitutional peptide libraries demonstrated that the αMI-domain binding depends on basic and hydrophobic residues. These findings establish a new model of αMβ2 binding in which the αMI-domain interacts with multiple sites in fibrinogen and has the potential to recognize numerous sequences. This paradigm may have implications for mechanisms of promiscuity in ligand binding exhibited by integrin αMβ2.