The Plant Wound Hormone Systemin Binds with the N-Terminal Part to Its Receptor but Needs the C-Terminal Part to Activate It

Correspondence to: Georg Felix, felix{at}fmi.ch (E-mail), 41-61-697-45-27 (fax).

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-¹²⁵I-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 ~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 syst–Ala-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.

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