Role of the beta-strand insert in the central domain of the fibrinogen gamma-module.

The crystal structure of the fibrinogen γ-module (residues γ143−411) [Yee, V. C., et al. (1997) Structure 5, 125−138] revealed an unusual feature. Namely, residues γ381−390 in the functionally important COOH-terminal region form a β-strand that is inserted into an antiparallel β-sheet of the central domain (γ192−286), while the rest (γ393−411) seems to be flexible. To clarify the structural and functional importance of this β-strand insert, we analyzed the folding status of the plasmin-derived fibrinogen fragment D3 and several truncated variants of the γ-module expressed in Escherichia coli. It was found that D3, in which most of the COOH-terminal domain of the γ-module (γ287−379) is removed proteolytically, retains a γ374−405 peptide that seems to be associated noncovalently with the bulk of the molecule via its β-strand insert region. A study of the denaturation−renaturation process of D3 suggested that without this peptide its truncated γ-module remains folded but is destabilized. This was confirmed d...