Protonophoric Activity of Eutypine, a Toxin fromEutypa lata,in Plant Mitochondria

Eutypine is a toxin produced byEutypa lata,the causal agent of the dying-arm disease of grapevine. We have previously shown that this toxin behaves as a lipophylic weak acid (pK= 6.2) and induces drastic changes in both the respiration and energy balance of grapevine cells. In the present study, the molecular mode of action of eutypine at the mitochondrial level, using methyl-eutypine, the unprotonable derivative of the toxin, has been investigated. The effects of these molecules on mitochondrial respiration and membrane potential were compared using isolated mitochondria from grapevine cells in suspension cultures or potato tuber mitochondria. Eutypine induces marked stimulation of oxygen consumption and a depolarizing effect, while methyl-eutypine exhibits a very small effect on both the rate of oxygen uptake and membrane potential. For high eutypine concentrations, a mixed effect corresponding to a direct inhibition of electron transport and uncoupling can be observed. In addition, below 200 μM, eutypine displays a linear relationship between oxidation rate and membrane potential similar to that of the classical protonophore carbonyl cyanide-m-chlorophenylhydrazone (CCCP). However, unlike CCCP, eutypine induces a potential-dependent proton conductance that can be due to the potential-dependent migration of the dissociated form of the toxin across the membrane. It is concluded that eutypine uncouples mitochondrial oxidative phosphorylation and decreases the ADP/O ratio in grapevine cells by increasing the proton leaks via a cyclic protonophore mechanism. The physiological aspects of these results are discussed.