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 (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 (16-120) fragment and its shorter version, the (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 chain interacting with PAI-1 are located in the N-terminal part of the (20-88) segment.

Conclusions--Therefore, PAI-1 directly bound to the (20-, 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.