Mac-1 (CD11b/CD18), a leukocyte-restricted integrin receptor, mediates neutrophil/monocyte adhesion to vascular endothelium and phagocytosis of complement-opsonized particles. Recent studies have shown that Mac-1 also functions as a receptor for fibrinogen in a reaction linked to fibrin deposition on the monocyte surface. In this study, we have used extended proteolytic digestion of fibrinogen to identify the region of this molecule that interacts with Mac-1. We found that an Mr approximately

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... ,000 plasmic fragment D of fibrinogen (D30) produced dose-dependent inhibition (IC50 = 1.6 microM) of the interaction of intact 125I-fibrinogen with stimulated neutrophils and monocytes. 125I-D30 bound saturably to these cells with specific association of 136,200 +/- 15,000 molecules/cell in a reaction inhibited by OKM1 and M1/70, monoclonal antibodies specific for the alpha subunit of Mac-1. Direct microsequence analysis and an epitope-mapped monoclonal antibody showed that D30 lacks the COOH-terminal dodecapeptide of the gamma chain as well as the Arg-Gly-Asp sequences in the A alpha chain. We conclude that fibrinogen interacts with the leukocyte integrin Mac-1 through a novel recognition site that is not shared with other known integrins that function as fibrinogen receptors.