Interactions mediated by the N-terminus of fibrinogen's Bβ chain

Specific molecular interactions mediated by the N-terminus of fibrinogen's B beta chain were revealed using laser tweezers-based force spectroscopy. We examined interactions between fibrinogen fragments representing the center of the molecule, NDSK, desA-NDSK, and desAB-NDSK, and two recombinant fibrinogens, gamma D364H and gamma D364A, which have nonfunctional gamma-chain polymerization sites to prevent the dominant knob-hole binding. Interactions between desA-NDSK, where the N-terminus of the B beta chain is present, and the fibrinogen variants showed a complex spectrum of rupture forces which disappeared with desAB-NDSK, lacking both FpA and FpB. The interactions between desA-NDSK and gamma D364H or gamma D364A were inhibited by addition of soluble FpB, but not FpA or the polymerization inhibitor peptides GPRP and GHRP. When gamma D364H fibrinogen was replaced with its X-fragment lacking alpha C-domains or with fragment D, the strongest component of the rupture force spectrum disappeared, suggesting interactions between the uncleaved FpB and the alpha C-domain. Electron microscopy confirmed the binding of desA-NDSK to either D or E regions of fibrinogen as well as to alpha C-domains. The data demonstrate the existence of weak transient interactions within and between fibrin molecules mediated by the N-terminus of the fibrinogen B beta chain.