Isolation and Characterization of a Novel, Developmentally Regulated Proteinase Inhibitor I Protein and cDNA from the Fruit of a Wild pecies of Tomato

Abstract

A novel member of the proteinase Inhibitor I family having a trypsin inhibitor specificity was isolated from the fruit of the wild tomato species Lycopersicon peruvianum (L.) Mill. (LA 107) and characterized. The protein is among the isoinhibitors of Inhibitor I that comprise 50% of the soluble proteins in the fruit of this wild species of tomato. A cDNA corresponding to the inhibitor protein and mRNA was isolated and characterized. The Inhibitor I mRNA represented 0.06% of the poly(A) RNA and gene copy number reconstruction experiments gave an estimate of two to four genes/haploid genome. The open reading frame of the cDNA codes for a protein of 111 amino acids having a 42-amino acid prepropolypeptide. The NH2-terminal sequence of the first 21 amino acids of the purified Inhibitor I protein confirmed that the cDNA was identical to the protein. The amino acid sequence of the L. peruvianum fruit Inhibitor I exhibits 74% identity with the wound-inducible Inhibitor I from tomato leaves. Whereas all previously identified members of the Inhibitor I family have either Met, Leu, or Asp at the P1 site and can inhibit enzymes such as chymotrypsin, subtilisin, and elastase, the fruit Inhibitor I possesses Lys at the P1 position. Thus, this is the first member of the extensive Inhibitor I family from plants and animals that exhibits trypsin inhibitory specificity. The presence of this inhibitor in wild tomato fruit may reflect a functional role to protect the tissues against herbivory.