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

Thomas Meindl, Thomas Boller and Georg Felix
The Plant Cell
Vol. 10, No. 9 (Sep., 1998), pp. 1561-1570
DOI: 10.2307/3870619
Stable URL: http://www.jstor.org/stable/3870619
Page Count: 10
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The Plant Wound Hormone Systemin Binds with the N-Terminal Part to Its Receptor but Needs the C-Terminal Part to Activate It
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Abstract

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 $({\rm syst}^{1-14})$ or the C-terminal four amino acids $({\rm syst}^{15-18})$, added singly or in combination, were inactive as inducers of these responses. ${\rm Syst}^{1-14}$ but not ${\rm syst}^{15-18}$ antagonized activity of intact systemin in a competitive manner. Likewise, intact systemin showed stimulatory, ${\rm syst}^{1-14}$ antagonistic activity, and ${\rm syst}^{15-18}$ 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 $\text{systemin-}{}^{125}{\rm I}\text{-iodotyrosine}$. 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 ${\rm syst}^{15-18}$ or structurally unrelated peptides. Binding was also competed by the systemin antagonists ${\rm syst}^{1-14}$ 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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