A 160-kD systemin receptor on the surface of Lycopersicon peruvianum suspension-cultured cells

Systemin, an 18-amino acid polypeptide wound signal, activates defense gene s in leaves of young tomato plants and induces rapid alkalinization of medi a containing suspension-cultured Lycopersicon peruvianum cells. a monoiodin ated form of a systemin analog synthesized with Tyr-2 and Ala-15 (Tyr-2,Ala -15-systemin) likewise exhibits similar biological activities. (125I)-Tyr-2 ,Ala-15-systemin rapidly, reversibly, and saturably bound to suspension-cul tured 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 syste min analogs and was found to correlate with their respective biological act ivities. Treatment of suspension-cultured cells with methyl jasmonate incre ased the total binding of I-125-Tyr-2,Ala-15-systemin more than threefold, suggesting that methyl jasmonate was activating transcription of the gene e ncoding 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 photoaffin ity systemin analog, N-(4-[p-azidosalicylamido]butyl)3'-(2' -Cys-3,Ala-15-s ystemindithiol)propionamide specifically labeled a 160-kD cell surface prot ein, and the labeling was completely inhibited by a 20-fold excess of unlab eled systemin. These data indicate that a 160-kD protein may be the physiol ogical receptor for systemin in suspension-cultured cells.