Fibrinogen contains cryptic PAI-1 binding sites that are exposed on binding to solid surfaces or limited proteolysis

Objective - In this work, we identified the fibrinogen sequence that on exposure serves as the primary binding site for functionally active PAI-1 and to a lesser extent for its latent form. In contrast, this site only weakly interacts with PAI-1 substrate. Methods and Results - The binding site is located in the N-terminal alpha(20-88) segment of fibrinogen, in the region exposed on (1) adsorption of fibrinogen to solid surfaces; (2) the release of fibrinopeptide A during thrombin conversion of fibrinogen to fibrin; and (3) plasmin degradation of fibrinogen. This region was first identified by the yeast 2-hybrid system, then its binding characteristics were evaluated using the recombinant alpha(16-120) fragment and its shorter version, the alpha(20-88) fragment, in a solid phase binding assay and plasmon surface resonance measurements. Because fibrinogen fragment E does not bind PAI-1, it suggests that sequences of A alpha chain interacting with PAI-1 are located in the N-terminal part of the alpha(20-88) segment. Conclusions - Therefore, PAI-1 directly bound to the alpha(20-88) and thus concentrated in fibrinogen/fibrin, particularly at sites of injury and inflammation, may account for the recent observations that both its active and latent forms stimulate cell migration and wound healing.