Conversion of fibrinogen to fibrin: Mechanism of exposure of tPA- and plasminogen-binding sites

Conversion of fibrinogen into fibrin results in the exposure of cryptic interaction sites and modulation of various activities. To elucidate the mechanism of this exposure, we tested the accessibility of the A alpha 148-160 and gamma 312-324 fibrin-specific epitopes that are involved in binding of plasminogen and its activator tPA, in several fragments derived from fibrinogen (fragment D and its subfragments) and fibrin (cross-linked D-D fragment and its noncovalent complex with the E-1 fragment, D-D.E-1). Neither D nor D-D bound tPA, plasminogen, or anti-A alpha 148-160 and anti-gamma 312-324 monoclonal antibodies, indicating that their fibrin-specific epitopes were inaccessible. The A alpha 148-160 epitope became exposed only upon proteolytic removal of the beta- and gamma -modules from D. At the same time, both epitopes were accessible in the D-D.E-1 complex, indicating that the DD.E interaction resulted in their exposure. This exposure was reversible since the dissociation of the D-D.E-1 complex made the sites unavailable, while reconstitution of the complex made them exposed. The results indicate that upon fibrin assembly, driven primarily by the interaction between complementary sites of the D and E regions, the D regions undergo conformational changes that cause the exposure of their plasminogen- and tPA-binding sites. These changes may be involved in the regulation of fibrin assembly and fibrinolysis.