Characteristics of fibrinogen binding to the domain of CD11c, an alpha subunit of p150,95

beta 2 integrins on leukocytes play important roles on cell-cell or cell-ma trix adhesion through their ability to bind multiple ligands. The alpha sub units of leukocyte CD11/CD18 integrins contain an approximately 200-amino-a cid inserted domain (I-domain) which is implicated in ligand binding functi on. To understand the characteristics of ligand binding to the ct subunit o f beta 2 integrin p150,95 (CD11c/CD18), a recombinant form of the I-domain of CD11c was generated and analyzed for the interaction with fibrinogen, on e of the ligands of p150,95. It was found that the CD11c I-domain bound fib rinogen specifically. Fibrinogen binding to the CD11c I-domain was inhibite d by a molar excess of fragment E, a central domain of fibrinogen, and not by that of fragment D, a distal domain of fibrinogen, suggesting that CD11c /CD18 recognizes a central domain of fibrinogen. Divalent cations such as M g2+ and Mn2+ were required for fibrinogen binding to the CD11c I-domain. Al so alanine substitutions on the putative metal binding sites of the CD11c I -domain such as Asp(242) and Tyr(209) reduced its ability to bind fibrinoge n. These data reinforce the fact that the divalent cation is a prerequisite for ligand binding of the CD11c I-domain. (C) 1999 Academic Press.