Abstract
Fibrin polymerizes through the interaction of sites exposed by the thrombin-mediated cleavage of fibrinopeptides in the central E region of the protein and complementary sites near the ends of the molecules, open in the D regions of both fibrinogen and fibrin. A preparation of fragment E, containing the central domain and part of the coiled-coil regions of fibrin, was used in mixtures with fibrinogen in this electron microscopy study to investigate the formation of fibrillar structures. At short times, linearly ordered oligomers of fibrinogen were observed with an additional mass of E fragments at the end-to-end junctions. At later times, long flexible polymers made up of 30 or more fibrinogen and fragment E units, with a tendency for lateral aggregation and tangle formation, were seen. These single-stranded assemblies could be readily dissociated in dilute acetic acid into their fibrinogen and fragment E components. However, if the aggregates were treated with factor XIIIa so that all gamma chains became ligated by Nepsilon(gamma-glutamyl)lysine linkages, the polymers could no longer be taken apart. Because the only gamma chains in the preparation are present in the fibrinogen molecules interacting end-to-end, the findings show that the factor XIIIa-induced cross-linking of gamma chains in the clotting of fibrinogen or fibrin must occur between molecules that are longitudinal (or end-to-end) rather than transverse (or half-staggered).