Cross-linking of
fibrinogen at its COOH-terminal gamma chain cross-linking site occurs in the presence of
factor XIIIa due to self-association at a constitutive D domain site ("gammaXL"). We investigated the contribution of COOH-terminal regions of
fibrinogen Aalpha chains to the gammaXL site by comparing the gamma chain cross-linking rate of intact
fibrinogen (fraction I-2) with that of plasma fraction I-9, plasmic fraction I-9D, and plasmic fragment D1, which lack COOH-terminal Aalpha chain regions comprising approximately 100, approximately 390, and 413 residues, respectively. The cross-linking rates were I-2 > I-9 > 1-9D = D1, and indicated that the terminal 100 or more Aalpha chain residues enhance gammaXL site association.
Fibrinogen Dusart, whose structural abnormality is in the COOH-terminal "alphaC" region of its Aalpha chain (Aalpha R554C-albumin), is associated with
thrombophilia ("Dusart Syndrome"), and is characterized functionally by defective
fibrin polymerization and clot structure, and reduced
plasminogen binding and tPA-induced fibrinolysis. In the presence of XIIIa, the Dusart
fibrinogen gamma chain cross-linking rate was about twice that of normal, but was normalized in proteolytic
fibrinogen derivatives lacking the Aalpha chain abnormality, as was reduced
plasminogen binding. Electron microscopy showed that
albumin-bound Dusart
fibrinogen "alphaC" regions were located in the vicinity of D domains, rather than at their expected tethered location near the
fibrinogen E domain. In addition, there was considerable
fibrinogen aggregation that was attributable to increased intermolecular COOH-terminal Aalpha chain associations promoted by untethered Dusart
fibrinogen aC domains. We conclude that enhanced Dusart
fibrinogen self-assembly is mediated through its abnormal alphaC domains, leads to increased gammaXL self-association and gamma chain cross-linking potential, and contributes to the
thrombophilia that characterizes the "Dusart Syndrome."