ROLE OF THE ALPHA-C DOMAINS OF FIBRIN IN CLOT FORMATION

The role of the carboxyl-terminal portion of the alpha chains of fibri n (alpha C domains) in clot formation was investigated by transmission and scanning electron microscopy and turbidity studies of clots made from preparations of molecules missing one or both of these domains. H ighly purified and entirely clottable preparations of bovine fragment X monomer, one containing primarily molecules missing a single alpha C domain (fragment X(1)) and the other consisting of molecules missing both alpha C domains (fragment X(2)), were used for these experiments. These preparations were characterized by various methods, including t he complete determination of the amino- and carboxyl-termini of all pe ptides and fragments. These preparations formed clots on dilution to n eutral pH. In all cases, clots observed by either scanning or transmis sion electron microscopy were made up of a branched network of fibers, similar to those formed by thrombin treatment of intact fibrinogen, s uggesting that the alpha C domains are not necessary for protofibril a nd fiber formation or branching. However, both the fiber and clot stru cture varied with the different fractions, indicating that the alpha C domains do participate in polymerization. The rate of assembly, as in dicated by the lag period and maximum rate of turbidity increase, as w ell as the final turbidity, was decreased with removal of the alpha C domains, suggesting that they accelerate polymerization. Preparations of isolated alpha C fragment added to fibrin monomer have striking eff ects on the turbidity curves, showing a decrease in the rate of polyme rization in a dose-dependent manner but not complete inhibition. Elect ron microscopy of fibrin monomer desA molecules at neutral pH showed t hat most of the alpha C domains, like those in fibrinogen, remain asso ciated with the central region. Thus, it appears that normally with th rombin cleavage of fibrinogen the effects of the interactions of alpha C domains observed here will be most significant for lateral aggregat ion.