DIAGRAM 1.

The F877 fragment. The amino acid sequence of the fragment F877 is shown in the upper panel. All the mutations are labeled on top of the sequence, and the two Gly → Ser substitution sites for fragments G901S and G913S are also underlined. The 14-residue Pro-free region (see text) is underlined, and the chymotrypsin digestion site Phe-935 is marked by the arrow. The KGE sequence (position 918-920, see text) is in italics. The residues introduced to form the Cys-knot are shown in italics. The gene expression construct of the fragments is shown in the bottom panel. The His-tagged thioredoxin was removed during the last purification procedure by thrombin digestion at the cleavage site APR marked by the triangle.