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

The crystal structure of the fibrinogen gamma -module (residues gamma 143-4 11) [Yee, V. C., et al. (1997) Structure 5, 125-138] revealed an unusual fe ature. Namely, residues gamma 381-390 in the functionally important COOH-te rminal region form a beta -strand that is inserted into an antiparallel bet a -sheet of the central domain (gamma 192-286), while the rest (gamma 393-4 11) seems to be flexible. To clarify the structural and functional importan ce of this beta -strand insert, we analyzed the folding status of the plasm in-derived fibrinogen fragment D-3 and several truncated variants of the ga mma -module expressed in Escherichia coli. It was found that D-3, in which most of the COOH-terminal domain of the gamma -module (gamma 287-379) is re moved proteolytically, retains a gamma 374-405 peptide that seems to be ass ociated noncovalently with the bulk of the molecule via its beta -strand in sert region. A study of the denaturation-renaturation process of D-3 sugges ted that without this peptide its truncated gamma -module remains folded bu t is destabilized. This was confirmed directly with the truncated recombina nt variants of the gamma -module, including residues gamma 148-392, gamma 1 48-373, and gamma 148-286. They all were folded, but those devoid of the be ta -strand insert were destabilized. The results indicate that although the beta -strand insert contributes to the stabilization of the gamma -module, it can be removed without destroying the compact structure of the latter. On the basis of this finding and some other observations, we propose a mech anism for the function-related conformational changes in the fibrin(ogen) g amma -modules.