The Structure-Activity Relationship of the Turgorin PLMF 1 in the Sensitive Plant Mimosa pudica L.: In Vitro Binding of [ 14C-Carboxyl]-PLMF 1to Plasma Membrane Fractions from Mimosa Leaves and Bioassays with PLMF I-Isomeric Compounds

Abstract

Summary Saturable in vitro binding of [ l4 C-carboxyl]-PLMF 1 was found to be present in plasma membrane fractions of leaves from Mimosa pudica L. The chlorophyll-free preparations of plasma membranes were obtained from microsomes by partitioning in an aqueous dextran, polyethylene glycol two-phase system. The plasma membranes were right-side-out orientated and sealed vesicles, as measured by the latent K + ,Mg 2+ -ATPase activity upon addition of Triton X-l00. At least one PLMF 1 binding site was found with a Kn-value of 45 µM. The number of binding sites amounted to 4.4nmol/mg protein. The half-maximal dose-response (EC 50 ) of PLMF 1 to initiate leaflet movement was 5.5 µM. The discrepancy between Kn and EC 50 is discussed using the spare receptor model as well as by a mechanism using signal amplification via a second mediator. The results from bioassays carried out with the PLMF l-enantiomer and the PLMF l-anomer were found to be consistent with the results mentioned above. All the findings point to the existence of a PLMF l-specific receptor in Mimosa pudica L.