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Conversion of Fibrinogen to Fibrin: Mechanism of Exposure of tPA- and Plasminogen-Binding Sites

2000; American Chemical Society; Volume: 39; Issue: 51 Linguagem: Inglês

10.1021/bi001847a

1943-295X

Sergiy Yakovlev, Makogonenko Em, Natalia Kurochkina, W. Nieuwenhuizen, Kenneth C. Ingham, Leonid Medved,

Cell Adhesion Molecules Research

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 Aα148−160 and γ312−324 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 E1 fragment, D−D·E1). Neither D nor D−D bound tPA, plasminogen, or anti-Aα148−160 and anti-γ312−324 monoclonal antibodies, indicating that their fibrin-specific epitopes were inaccessible. The Aα148−160 epitope became exposed only upon proteolytic removal of the β- and γ-modules from D. At the same time, both epitopes were accessible in the D−D·E1 complex, indicating that the DD·E interaction resulted in their exposure. This exposure was reversible since the dissociation of the D−D·E1 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.