Systemin, an 18-amino acid polypeptide wound signal, activates defense genes in leaves of young tomato plants and induces rapid alkalinization of media containing suspension-cultured Lycopersicon peruvianum cells. A monoiodinated form of a systemin analog synthesized with Tyr-2 and Ala-15 (Tyr-2,Ala-15-systemin) likewise exhibits similar biological activities. (125)I-Tyr-2,Ala-15-systemin rapidly, reversibly, and saturably bound to suspension-cultured L. peruvianum cells with a K(d) of 0.17 nM and a Hill coefficient of 0. 92. The specificity of binding was assessed with alanine-substituted systemin analogs and was found to correlate with their respective biological activities. Treatment of suspension-cultured cells with methyl jasmonate increased the total binding of (125)I-Tyr-2, Ala-15-systemin more than threefold, suggesting that methyl jasmonate was activating transcription of the gene encoding the binding protein. Treatment of cells with cycloheximide markedly decreased binding of iodinated systemin to the cells, indicating that the binding protein was constantly being synthesized and degraded. A photoaffinity systemin analog, N-(4-[p-azidosalicylamido]butyl)-3'(2'-Cys-3, Ala-15-systemindithiol)propionamide, specifically labeled a 160-kD cell surface protein, and the labeling was completely inhibited by a 20-fold excess of unlabeled systemin. These data indicate that a 160-kD protein may be the physiological receptor for systemin in suspension-cultured cells.