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 (syst1-14) or the C-terminal four amino acids (syst15-18), added singly or in combination, were inactive as inducers of these responses. Syst1-14 but not syst15-18 antagonized activity of intact systemin in a competitive manner. Likewise, intact systemin showed stimulatory, syst1-14 antagonistic activity, and syst15-18 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 approximately 1 nM, and binding of radioligand was efficiently competed by unlabeled systemin but not by syst15-18 or structurally unrelated peptides. Binding was also competed by the systemin antagonists syst1-14 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.
Friedrich Miescher-Institute, P.O. Box 2543, CH-4002 Basel, Switzerland.
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Meindl
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Boller
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Felix
D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000096:biosynthesis; D007700:Kinetics; D018551:Solanum lycopersicum / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D008861:Microsomes / Q000378:metabolism; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D010455:Peptides / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D010937:Plant Growth Regulators / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D011869:Radioligand Assay; D011956:Receptors, Cell Surface / Q000378:metabolism
D010446:Peptide Fragments; D010455:Peptides; D010937:Plant Growth Regulators; D011956:Receptors, Cell Surface; C073704:systemin; D017871:Calcium-Calmodulin-Dependent Protein Kinases