Trypsin inhibitor polymorphism: multigene family expression and posttranslational modification.

Trypsin inhibitors from winter pea seeds (c.v. Frilene) have been purified and shown to consist of six protease inhibitors (PSTI I, II, III, IVa, IVb, and V). Based on amino acid composition, molecular mass, and N-terminal sequence, the six inhibitors are closely related to one another and belong to the Bowman-Birk family of inhibitors. To define the relations among them, molecular mass and amino acid composition of peptides obtained from digestion with trypsin were determined. The sequence and the biosynthetic mechanism of the isoform formation have been partially resolved for four major isoforms. Two isoinhibitor forms (PSTI IVa, IVb) in pea seeds are due to expression of two distinct genes; PSTI IVa has four amino acid replacements when its sequence is compared with the sequence of PSTI IVb. Two others (PSTI I, II) result from posttranslational proteolytic cleavage of nine C-terminal residues of forms PSTI IVa and IVb, respectively.