The plant wound hormone systemin binds with the N-terminal part to its receptor but needs the C-terminal part to activate it

Suspension-cultured cells of Lycopersicon peruvianum respond with rapid medium alkalinization and a strong increase of a MAP kinase-like activity when treated with subnanomolar concentrations of the plant wound hormone systemin. Systemin fragments comprising the N-terminal 14 amino acids (syst(1-14)) or the C-terminal four amino acids (syst(15-18)), added singly or in combination, were inactive as inducers of these responses. Syst(1-14) but not syst(15-18) antagonized activity of intact systemin in a competitive manner. Likewise, intact systemin showed stimulatory, syst(1-14) antagonistic activity, and syst(15-18) showed no activity in leaf pieces of tomato (L. esculentum) plants assayed for the induction of ethylene biosynthesis. To study the molecular basis of perception, we extended the C-terminal end of systemin by a tyrosine residue and radioiodinated it to yield systemin-(125)l-iodotyrosine. In membrane preparations of L. peruvianum, this radioligand exhibited rapid, saturable, and reversible binding to a single class of binding sites. Binding showed a dissociation constant of similar to 1 nM, and binding of radioligand was efficiently competed by unlabeled systemin but not by syst(15-18) Or structurally unrelated peptides, Binding was also competed by the systemin antagonists syst(1-14) and systAla-17 (IC50 of 500 and 1000 nM, respectively). Thus, this binding site exhibits the characteristics expected for a functional systemin receptor. Based on these results, we propose a two-step mechanism for systemin action, with binding of the N-terminal part to the receptor as the first step and activation of responses with the C-terminal part as the second step.