Here we report the crystal structure at approximate to 4-Angstrom resolutio
n of a selectively proteolyzed bovine fibrinogen. This key component in hem
ostasis is an elongated 340-kDa glycoprotein in the plasma that upon activa
tion by thrombin self-assembles to form the fibrin clot. The crystals are u
nusual because they are made up of end-to-end bonded molecules that form fl
exible filaments. We have visualized the entire coiled-coil region of the m
olecule, which has a planar sigmoidal shape, The primary polymerization rec
eptor pockets at the ends of the molecule face the same way throughout the
end-to-end bonded filaments, and based on this conformation, we have develo
ped an improved model of the two-stranded protofibril that is the basic bui
lding block in fibrin. Near the middle of the coiled-coil region, the plasm
in-sensitive segment is a hinge about which the molecule adopts different c
onformations. This segment also includes the boundary between the three- an
d four-stranded portions of the coiled coil, indicating the location on the
backbone that anchors the extended flexible A alpha arm. We suggest that a
flexible branch point in the molecule may help accommodate variability in
the structure of the fibrin clot.