Structure-activity of deleted and substituted systemin, an 18-amino acid polypeptide inducer of plant defensive genes

The primary structure-activity relationships of systemin, an 18-amino acid polypeptide from tomato leaves that regulates the expression of two wound-inducible proteinase inhibitor genes in tomato and potato plants, were investigated. Analogs of systemin, the only example of a polypeptide signal from plants, were synthesized with progressive deletions of amino acids from both the NH2 terminus and COOH terminus and assayed in young excised tomato plants. All of the analogs exhibited severely decreased proteinase inhibitor-inducing activities, indicating that the entire 18-amino acid sequence is necessary for maximal activity. Deletion of the COOH-terminal Asp abolished inducing activity. Progressive replacement of each amino acid of the entire polypeptide with Ala revealed two regions, near residues Pro13, where Ala substitution reduced activity to less than 0.2%, and Thr17, which totally inactivated the analog. Other replacements with Ala had little or only moderate effects on activity. The two inactive analogs, des-Asp18 systemin and Ala17 systemin, were potent inhibitors of the inducing activity of the native systemin. These analogs, therefore, contain structural conformations sufficient for competition with systemin, but they are not competent for proteinase inhibitor gene induction. A synthetic COOH-terminal tetrapeptide, Met-Gln-Thr-Asp, retained low proteinase inhibitor inducing activity, but virtually any replacements with other amino acids either eliminated activity or reduced the activity to very low or nearly undetectable levels. These results indicate that residues near the COOH terminus of systemin are necessary for activity, possibly involving a phosphorylation at Thr17, and that other regions of the systemin sequence are important for interacting with a receptor(s) but are not sufficient to activate proteinase inhibitor gene expression.