Fusicoccin Binding to Its Plasma Membrane Receptor and the Activation of the Plasma Membrane H+-ATPase

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Abstract
The characteristics of fusicoccin binding were investigated in microsomes from 24-h-old radish (Raphanus sativus L.) seedlings. The time course of fusicoccin binding depended on fusicoccin concentration: equilibrium was reached much faster at 10 nanomolar fusicoccin than at 0.3 nanomolar fusicoccin. Scatchard analysis of equilibrium binding as a function of fusicoccin concentration indicated a single class of receptor sites with a K(d) of 1.8 nanomolar and a site density of 6.3 picomoles per milligram protein. Similar values (K(d) 1.7 nanomolar and site density 7 picomoles per milligram protein) were obtained from the analysis of the dependence of equilibrium binding on membrane concentration at fixed fusicoccin concentrations. Fusicoccin binding comigrated with the plasma membrane H(+)-ATPase in an equilibrium sucrose density gradient: both activities formed a sharp peak (1.18 grams per milliliter) clearly distinct from that of markers of other membranes which all peaked at lower densities. The saturation profiles of fusicoccin binding and of fusicoccin-induced activation of the plasma membrane H(+)-ATPase, measured under identical conditions, were similar, supporting the view that fusicoccin-induced activation of the plasma membrane H(+)-ATPase is mediated by fusicoccin binding to its plasma membrane receptor.