MAGNESIUM-IONS PROMOTE ASSEMBLY OF CHANNEL-LIKE STRUCTURES FROM BETICOLIN-0, A NONPEPTIDE FUNGAL TOXIN PURIFIED FROM CERCOSPORA-BETICOLA

Beticolins are toxins produced by the fungus Cercospora beticola. Usin g beticolin 0 (B0), we have produced a strong and Mg2+-dependent incre ase 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 BO (approximately 10 mu M) and in the presenc e of nominal Mg2+. Addition of 50 mu M Mg2+ induced a macroscopic curr ent which could be reversed to single channel current by chelating Mg2 + with EDTA. Both unitary and macroscopic currents were ohmic. The inc rease in conductance of biological membranes triggered by BO is theref ore likely to originate from the ability of this toxin to organize its elf into transmembrane pores in the presence of Mg2+. The pore is poor ly 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 act ivity could be involved in the deleterious biological activity of the toxin, by causing the collapse of ionic and electrical gradients throu gh biological membranes together with Ca2+ influx and scrambling of ce llular signals.