Aluminum fluoride and magnesium, activators of heterotrimeric GTP-binding proteins, affect high-affinity binding of the fungal toxin fusicoccin to the fusicoccin-binding protein in oat root plasma membranes.

The fusicoccin-binding protein was solubilised from purified oat root plasma membranes. The solubilised protein retained full binding activity, provided that protease inhibitors were included. Sodium fluoride reduced the high-affinity [3H]fusicoccin binding to almost zero in a concentration-dependent way, with an optimum at approximately 20 mM sodium fluoride. The presence of magnesium (> 100 microM) was required for the inhibitory action of fluoride, whereas addition of low amounts of aluminum (25 microM) shifted the fluoride optimum to lower concentrations. Fluoride changes the biochemical properties of the binding protein in a reversible manner, because the inhibition was both prevented and reversed by 1 M ammonium sulphate. The combined effects of aluminium, fluoride and magnesium are reminiscent of the action of activated GTP-binding proteins. Since no functional assay for GTP-binding-protein activity in plants is available yet, GTP-binding-protein activation by fluoride and magnesium was deduced from competition with binding of [gamma-35S]GTP[S] to purified plasma membranes. Indeed, fluoride (20 mM) completely blocked the specific binding of [gamma-35S]GTP[S]. It is concluded that the inhibitory effect of fluoride upon the binding of fusicoccin is indirect and mediated through activated GTP-binding proteins. A hypothesis on the mechanism of fusicoccin action is presented wherein the fusicoccin-binding protein is one component of a signal-transduction chain, two or more steps downstream of a heterotrimeric GTP-binding protein.