TRYPSIN-INHIBITOR POLYMORPHISM - MULTIGENE FAMILY EXPRESSION AND POSTTRANSLATIONAL MODIFICATION

Trypsin inhibitors from winter pea seeds (c.v. Frilene) have been puri fied and shown to consist of six protease inhibitors (PSTI I, II, III, IVa, IVb, and V). Based on amino acid composition, molecular mass, an d N-terminal sequence, the six inhibitors are closely related to one a nother and belong to the Bowman-Birk family of inhibitors. To define t he relations among them, molecular mass and amino acid composition of peptides obtained from digestion with trypsin were determined. The seq uence and the biosynthetic mechanism of the isoform formation have bee n partially resolved for four major isoforms. Two isoinhibitor forms ( PSTI IVa, IVb) in pea seeds are due to expression of two distinct gene s; PSTI IVa has four amino acid replacements when its sequence is comp ared with the sequence of PSTI IVb. Two others (PSTI I, II) result fro m posttranslational proteolytic cleavage of nine C-terminal residues o f forms PSTI IVa and IVb, respectively.