Magnesium ions promote assembly of channel-like structures from beticolin 0, a non-peptide fungal toxin purified from Cercospora beticola

Abstract

Beticolins are toxins produced by the fungus Cercospora beticola. Using beticolin 0 (B0), we have produced a strong and Mg2+-dependent increase in the membrane conductance of Arabidopsis protoplasts and Xenopus oocytes. In protein-free artificial bilayers, discrete deflexions of current were observed (12 pS unitary conductance in symmetrical 100 mM KCl) in the presence of B0 (approximately 10 μM) and in the presence of nominal Mg2+. Addition of 50 μM Mg2+ induced e macroscopic current which could be reversed to single channel current by chelating Mg2+ with EDTA. Both unitary and macroscopic currents were ohmic. The increase in conductance of biological membranes triggered by B0 is therefore likely to originate from the ability of this toxin to organize itself into transmembrane pores in the presence of Mg2+. The pore is poorly selective, displaying permeability ratios P(Cl)/P(K), P(Na)/P(K) and P(Ca)/P(K) close to 0.3, 0.65 and 0.4, respectively. Such channel-like activity could be involved in the deleterious biological activity of the toxin, by causing the collapse of ionic and electrical gradients through biological membranes together with Ca2+ influx and scrambling of cellular signals.