ANALYSIS OF A-ALPHA-251 FIBRINOGEN - THE ALPHA-C DOMAIN HAS A ROLE INPOLYMERIZATION, ALBEIT MORE SUBTLE THAN ANTICIPATED FROM THE ANALOGOUS PROTEOLYTIC FRAGMENT-X

Numerous experiments have demonstrated that the C-terminal domain of t he fibrinogen A alpha-chain, the alpha C domain, has a role in polymer ization, To further examine the role of this domain, we synthesized a recombinant fibrinogen, A alpha 251 fibrinogen, that lacks the alpha C domain. We examined thrombin-catalyzed fibrinopeptide release and fou nd that the rate of FpB release from A alpha 251 fibrinogen was 2.5-fo ld slower than FpB release from normal fibrinogen, while the rate of F pA release was the same for both proteins. We examined thrombin-cataly zed polymerization and found that the rates of protofibril formation a nd lateral aggregation were similar for both proteins, although discer nible differences in lateral aggregation were clear. The rate of proto fibril formation for A alpha 251 fibrinogen was never less than 85% of normal fibrinogen, while the rate of lateral aggregation for A alpha 251 fibrinogen varied from 64 to 74% of normal, We examined polymeriza tion of fibrin monomers and found that polymerization of A alpha 251 f ibrin was similar to normal fibrin at 0.4 M NaCl, but clearly differen t from normal at 0.05 M NaCl. These results indicate that the alpha C domain has a role in lateral aggregation, but this role is more subtle than anticipated from previous experiments, particularly those with f ibrinogen fragment X, We interpret this unanticipated finding as indic ative of an important contribution from the N-terminus of the beta-cha in, such that protein heterogeneity that includes small amounts of fib rin lacking that N-terminus of the beta-chain leads to markedly altere d lateral aggregation.