Identification of a urokinase receptor-integrin interaction site - Promiscuous regulator of integrin function

Adhesion and signaling by integrins require their dynamic association with nonintegrin membrane proteins. One such protein, the glycolipid-anchored ur okinase receptor (uPAR), associates with and modifies the function of the b eta(2)-integrin Mac-1 (CD11b/CD18). In this study, a critical non-I-domain binding site for uPAR on CD11b (M25; residues 424-440) is identified by hom ology with a phage display peptide known to bind uPAR, recombinant soluble uPAR and cells expressing uPAR bound to immobilized M25, binding being prom oted by urokinase and blocked by soluble M25, but not a scrambled control o r homologous peptides from other beta(2)-associated alpha-chains. Mac-1, bu t not a mutated Mac-1 in which M25 was replaced with the homologous sequenc e of CD11c, co-precipitated with uPAR, In the beta-propeller model of alpha -chain folding, M25 spans an exposed loop on the ligand-binding, upper surf ace of alpha M, identifying uPAR as an atypical alpha M ligand. Although no t blocking ligand binding to Mac-1, M25 (25-100 mu M) inhibited leukocyte a dhesion to fibrinogen, vitronectin, and cytokine-stimulated endothelial cel ls. M25 also blocked the association of uPAR with beta(1)-integrins and imp aired beta(1)-integrin-dependent spreading and migration of human vascular smooth muscle cells on fibronectin and collagen. These observations indicat e that uPAR associates with integrins directly and that disruption of this association broadly impairs integrin function, suggesting a novel strategy for regulation of integrins in the settings of inflammation and tumor progr ession.